Experimental Transient Behaviour Characterisation of ...769611/FULLTEXT01.pdf · Experimental...

DEGREE PROJECT, IN , SECOND LEVELCELTE

STOCKHOLM, SWEDEN 2014

Experimental Transient BehaviourCharacterisation of Induction Motorfed by Variable Frequency Drives forPump Applications

AMER HALILOVIC

KTH ROYAL INSTITUTE OF TECHNOLOGY

ELECTRICAL ENGINEERING

Experimental Transient Behaviour Characterisation of Induction Motor fed by Variable Frequency Drives

for Pump Applications

by

Amer Halilović

Master Thesis in Electrical Machines and Drives

Royal Institute of Technology

School of Electrical Engineering Department of Electrical Energy Conversion

Supervisor: Martin Zetterquist, Xylem

Examiner: Juliette Soulard, KTH

Stockholm, Sweden, 2014 XR-EE-E2C 2014:008

Abstract

The use of variable frequency drives in centrifugal pump applicationshas raised the question of how to select a drive. Clogging obstacles inwaste water applications create unknown transient loads for the pumpsystem. A sudden load increase occurrence can clog the pump if thedrive cannot supply enough current to reach the motor’s torque demand.In order to select a suitable drive, an empirical approach has been im-plemented, investigating three different drives. Results have shown thatselecting a drive with the highest possible overload capabilities, evenif for a short time is most suitable. Operation in vector speed controlgives the most reliable operation if an automatic parameter tuning isperformed by the drive.

Keywords: Variable Frequency Drive, Centrifugal Pump,Induction Motor, Volts-per-Hertz Control, Speed Control, Overdi-mensioning

Sammanfattning

Användningen av frekvensomriktare i centrifugalpumpar har väcktfrågan om hur en omriktare skall väljas. Igensättande objekt i avlopps-vatten kan ge upphov till transienta laster i pumpsystemen. En oförut-sedd lastökning kan sätta igen pumpen om frekvensomriktaren inte kanförse motorn tillräckligt med ström för att möta momentbehovet. Föratt välja en lämplig omriktare har ett empiriskt tillvägagångssätt valtsi en undersökning av tre olika omriktare. Resultat har visat att det ärlämpligast att välja en omriktare med högst överbelastningskapacitet,även om under en kort tid. Vektor hastighetskontroll är metoden somger stabil körning om omriktaren fått automatiskt ställa in motorpara-metrarna.

Nyckelord: Frekvensomriktare, centrifugalpump, asynkron-motor, skalärkontroll, hastighetskontroll, överdimensionering

Acknowledgement

I would like to thank my supervisor Martin Zetterquist, Development Engineer atXylemWater Solutions AB along with the R&DManager for Electrical Componentsand Systems Jürgen Mökander for the opportunity and the support I received. Iwould also like to thank my examiner at KTH, Juliette Soulard, that has been aninspirational source and guided me with feedback throughout the project. I wouldalso like to acknowledge the participants in the reference group who contributedwith valuable feedback. A special thanks goes to Per Miskas who assisted me in thelab.

List of Symbols

I Current [A]T Torque [T]T/I Torque-over-current-ratio [Nm/A]f0 Frequency [Hz]fsw Switching frequency [kHz]Vref Reference voltage [V]Vcarrier Carrier voltage [V]p Pulse number [-]M Modulation index [-]

Contents

Contents

1 Introduction 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Aim and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Structure of the Report . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Theory Behind Variable Frequency Drives 52.1 Overview of a Variable Frequency Drive . . . . . . . . . . . . . . . . 52.2 Power Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3 Output Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.4 Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.4.1 Volts-per-Hertz Control . . . . . . . . . . . . . . . . . . . . . 102.4.2 Vector Control . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.5 Drive Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.6 Direct On-Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.7 DC-link . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3 Tested Drives and Motor 153.1 Tested Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.1.1 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.1.2 Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.1.3 Waveforms and Spectra - Measurements and Analysis . . . . 173.1.4 Waveforms and Spectra - Discussion . . . . . . . . . . . . . . 27

3.2 Test Motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4 Test Bench Methodology 334.1 Materials: The Test Bench, Measurements and Post Analysis . . . . 334.2 Methodology: Test Procedures . . . . . . . . . . . . . . . . . . . . . 34

5 Test Bench Results 375.1 Drive A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.1.1 Locked Rotor Test . . . . . . . . . . . . . . . . . . . . . . . . 375.1.2 Start Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.1.3 Maximum Torque Test . . . . . . . . . . . . . . . . . . . . . . 415.1.4 Speed Step Test . . . . . . . . . . . . . . . . . . . . . . . . . 425.1.5 Summary of Results . . . . . . . . . . . . . . . . . . . . . . . 44

5.2 Drive B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455.2.1 Locked Rotor Test . . . . . . . . . . . . . . . . . . . . . . . . 455.2.2 Start Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465.2.3 Maximum Torque Test . . . . . . . . . . . . . . . . . . . . . . 475.2.4 Speed Step test . . . . . . . . . . . . . . . . . . . . . . . . . . 485.2.5 Summary of Results . . . . . . . . . . . . . . . . . . . . . . . 49

5.3 Drive C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505.3.1 Locked Rotor Test . . . . . . . . . . . . . . . . . . . . . . . . 505.3.2 Start Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515.3.3 Load Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525.3.4 Speed Step Test . . . . . . . . . . . . . . . . . . . . . . . . . 535.3.5 Summary of Results . . . . . . . . . . . . . . . . . . . . . . . 56

5.4 Drive Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

6 Analysis and Discussion 596.1 Test Bench Analysis and Discussion . . . . . . . . . . . . . . . . . . 59

6.1.1 Drive Size Selection . . . . . . . . . . . . . . . . . . . . . . . 596.1.2 A Novel Evaluation Method . . . . . . . . . . . . . . . . . . . 596.1.3 The Test Results . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 606.2.1 Drive Selection . . . . . . . . . . . . . . . . . . . . . . . . . . 606.2.2 The Topic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616.2.3 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

7 Conclusion and Future Work 637.1 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

7.1.1 Drive Selection Recomendation . . . . . . . . . . . . . . . . . 647.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Appendices 65

A Complementary Results 67A.1 Drive A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

A.1.1 Locked Rotor Test . . . . . . . . . . . . . . . . . . . . . . . . 67A.1.2 Start Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68A.1.3 Maximum Torque Test . . . . . . . . . . . . . . . . . . . . . . 69A.1.4 Speed Step Test . . . . . . . . . . . . . . . . . . . . . . . . . 72

A.2 Drive B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73A.2.1 Locked Rotor Test . . . . . . . . . . . . . . . . . . . . . . . . 73A.2.2 Start Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75A.2.3 Maximum Torque Test . . . . . . . . . . . . . . . . . . . . . . 78

A.2.4 Speed Step test . . . . . . . . . . . . . . . . . . . . . . . . . . 81A.3 Drive C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

A.3.1 Locked Rotor Test . . . . . . . . . . . . . . . . . . . . . . . . 83A.3.2 Start Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85A.3.3 Maximum Torque Test . . . . . . . . . . . . . . . . . . . . . . 89A.3.4 Speed Step Test . . . . . . . . . . . . . . . . . . . . . . . . . 91

References 93

Chapter 1

Introduction

1.1 Background

Centrifugal pumps are commonly used to transport liquid fluids with low viscosity,such as water and waste water for example. An electric motor powers a shaft-mounted impeller which allows, by the use of the liquids centrifugal force, to createa difference in pressure sucking the water through the pump, see figure 1.1. Theload is typically proportional to the square of the speed in pump applications, whichallows the electric motor to be connected to the three phase electricity network, alsoknown as directly on-line, DOL. With the development of technology, Variable Fre-quency Drives, or motor drives, have opened up for the opportunity to control thespeed of the motor, and hence the pump. This has led to that new desirable oper-ational demands can be met by means of controlling the motor. Since for example,the flow of the pumped liquid is linearly proportional to speed, the flow can bemore efficiently regulated by controlling the speed. Previously, valves have beenused to regulate the flow. By the means of speed regulation in order to alter theflow, significant percentage of energy savings can be achieved [1].

Figure 1.1. Cross-section of a centrifugal pump displaying the electric motor andthe impeller [2].

1

CHAPTER 1. INTRODUCTION

Although it is beneficial to use a Variable Frequency Drive, VFD, it does notprovide a perfectly engineered system for all applications. Considerable thought hasto be put in the centrifugal pump selection. By having a pump design suited fora certain application, energy usage can be optimised along with preventing inter-nal damage and increasing the life span of the pump. A well defined technologicalprocess has to be outlined when selecting a pump. Parameters such as pressure,flow and temperature as well as the pumping service and the load curve should beknown in order to define the working parameters of the pump, such as the head,capacity, suction and discharge pressures [3].

The most important aspect when selecting any component for a system is to beaware what it is intended to be used for. In pump applications, the power is pro-portional to the cube of the speed. Everything from impeller to electric motor canbe designed to suit the demands. If the load curve is well defined, a VFD can ac-cordingly be selected to match the motor. Centrifugal pumps are usually consideredto have non varying load characteristics. That is, the load at a certain operatingpoint is usually always the same at that point [4]. However, the load is not alwaysbehaving ideally. The load can unexpectedly increase instantaneously if an objectis sucked in towards the pump. When the object hits the impeller, it can either getstuck or in some cases get through, depending not only on the size of the objectbut also on the strength of the system. If the electric motor is strong enough it canovercome a sudden load increase as long as the required current is not too high.An electric motor can usually be overloaded for a short period of time, perfectly toovercome such a load increase. The load will momentarily increase for the VFD aswell, and a problem is encountered if the VFD cannot be overloaded as much as themotor. This raises a constraint on the VFD. In order to overcome the sudden loadincrease, the VFD has to be overdimensioned in regards to the connected motorin order to be able to supply enough current when the motor is running shortlyin overload mode. Since price goes up with drive size, it is important to defineprecisely the level of overdimension for the VFD required by the application.

VFDs are still a technology under development. There are numerous hardwareand software approaches to overcome encountered problems. Research papers havebeen addressing the issues on selecting a drive. Each manufacturer has respectiveguides on how to select the right drive for a certain application. Recommendationson how to take into consideration the name plate data of the motor and ambient ofthe VFD can be found, among others, in NEMA [5]. However, the answer to thequestion about how closely a VFD can be matched to a motor for pump applicationswith possible clogging was not found.

In this work, a method of investigating how to choose a drive for a centrifugalpump is presented. The problem is difficult to address considering all the varyingaspects and system complexity. Therefore, empirical investigations of which factorsthat can influence the torque production at a given current in an electric motor for

2

1.2. AIM AND SCOPE

centrifugal pumps are carried out.As mentioned, the load characteristics are familiar for these applications but it

is a question of whether the VFD can supply enough current to overcome suddentransients. The dynamical aspect is unknown, and it is uncertain how often andto what extent transients can occur due to objects that are sucked in towards theimpeller. The characteristics of how the exact load curve looks like is of crucialimportance to be able to address the problem to full extent, and would be of greatvalue if it was known. This new approach of empirically addressing the problem isexpected open up for new ideas and narrow down the directions to take in order togain a complete answer to the question.

1.2 Aim and ScopeThe aim of this report is to present the investigations leading to the dimensioningof a variable frequency drive to an induction motor. Three drives were investigated,two from the same manufacturer, one with electrolytic capacitor bank and the otherwith a film capacitor bank. The third drive is from another manufacturer with anelectrolytic capacitor bank. All drives were of same size with similar functionalities.The functionalities such as energy optimising functions, acceleration ramp up times,voltage boosts, switching frequencies and scalar and vector speed control modulationmethods were varied throughout the investigation. The test motor was a squirrelcage induction motor close to the power rating of the chosen drives.

1.3 Structure of the ReportThe plan of this report is as follows. Chapter 2 describes the theory behind electricdrives, from the power converters to control methods. Pump theory and theorybehind induction motors and drive operated motors are as well included in chapter2. In chapter 3, the tested drives are introduced, their properties and functionalities,along with the tested induction motor. At the end of the chapter, an analysis ofthe input and output waveforms of the drive is performed. The methodology ofthe performed tests is found in chapter 4 and the results in the following chapter,chapter 5. Finally, chapter 6 has further analysis following with discussion andconcluding remarks in chapter 7.

3

Chapter 2

Theory Behind Variable FrequencyDrives

2.1 Overview of a Variable Frequency DriveA variable frequency drive for AC motors comprises different power electronic con-verters connected between the grid and the electrical machine. The basic principleis to convert the ac network voltage to dc in a rectifier in order to change it backto three-phase ac voltage with a variable frequency in an inverter [6], as illustratedin figure 2.1.

Figure 2.1. Simple block diagram of a variable frequency drive.

The dc-link block in figure 2.1 provides a stiff input to the inverter.

2.2 Power ElectronicsRectifiers, or ac-dc converters, typically convert a single-phase or a three-phase acvoltage to a dc voltage.

The input to an induction motor in centrifugal pumps is usually a three-phasesupply since the motor commonly has three phases, and therefore the inputs andoutputs of the VFD have as well three phases. Furthermore, there are a numerousadvantages of using a three-phase rectifier over a single phase. A single-phase rec-

5

CHAPTER 2. THEORY BEHIND VARIABLE FREQUENCY DRIVES

tifier has a much poorer power factor due to the high distortion in the line current.The dc-current will as well contain more ripple in a single-phase opening for theopportunity for the three-phase to have smaller capacitors at the output in certaincases.

The investigated drives have three phases and diode rectifiers. The main differ-ence between a diode and a thyristor rectifier is that diode rectifiers do not havethe possibility to change the dc output level and additionally, a thyristor rectifiercan work as a line-commutated inverter [6, 7].

Inverters, or dc-ac converters, convert dc voltage into ac voltage and are com-mon for motor drives where the desire is to be able to control both frequency andmagnitude of a sinusoidal output. The input of the converter is connected to dc-link and the output to the motor. Since the capacitors in the dc-link stabilisesthe voltage, the inverter is voltage stiff, hence said to be a Voltage Stiff Inverter,VSI. The drives tested in this report have all voltage stiff inverters. Current StiffInverters, CSI, have on the other hand an inductive dc link making it current stiff.A drawback with current stiff converters is that the current cannot instantaneouslychange making it undesirable in certain pump applications [6, 7, 8].

A simple circuit diagram of a three-phase inverter is shown in figure 2.2.

Figure 2.2. Three-phase inverter [8].

The inverter in figure 2.2 is a 6-pulse inverter which the converters in the chosendrives are as well. A three-phase full-bridge converter will produce six pulses. Iftwo six-pulse converters would be connected in series or parallel, a 12-pulse con-verter would be formed which can achieve higher voltage or current ratings. The12-pulse converter would have a better harmonic performance but could be alike tohaving a six-pulse converter with an LC-filter connected to the output reducing theripple in the output current. The cost of the above mentioned configurations areapproximately the same, in no favour to any of the converters. 12-pulse converters

6

2.3. OUTPUT WAVEFORMS

are more common for power ranges above 1000 kW, such as in HVDC applicationsand traction drives [9].

2.3 Output Waveforms

The output level of the inverter in figure 2.2 varies as each switch pair is closed insequence at a time. The pulses can be allowed to vary in length yielding to thetechniques of Pulse-Width Modulation, PWM. One way is to compare a sinusoidalreference wave to a sawtooth, or triangular, carrier wave. The generated wave,see figure 2.3, will have a sinusoidal fundamental that can vary in amplitude andfrequency.

Figure 2.3. PWM switching technique [6].

Each generated wave, using specific techniques, will have their correspondingharmonic spectra. Depending on the application, the unwanted harmonics can beeliminated or significantly reduced using different techniques [8].

Figure 2.4 shows an example of a harmonic spectra for a carrier based modula-tion method.

7


Figure 2.4. An example of a harmonic spectra for a carrier based modulationmethod where m and n are orders of carrier and sideband harmonics, respectively[10].

The pulse number, p, and the modulation index, M , are defined as follows,

p = fsw

f0(2.1)

where fsw is the switching frequency and f0 is the frequency of the carrierwaveform, and

M = Vref

Vcarrier(2.2)

where Vref is the amplitude of the reference waveform while Vcarrier refers tothe amplitude of the carrier waveform.

The first bar in figure 2.4 shows the fundamental at an order of 1. The peakvalue is in this case 0.8 p.u. and corresponds to the modulation index. The secondharmonic present in the figure is the base band harmonic of the fundamental at order3. Depending on the modulation technique, this harmonic can either be absent oreven injected. The figure illustrates the different orders of harmonics as carrier andbase band harmonics with indexesm and n, respectively. The first carrier harmonic,i.e. m = 1, is in this example at order 21. The first carrier corresponds to the pulsenumber p, referring to equation 2.1, where the order of the harmonic of the carrierharmonic is the pulse number. In this case, the switching frequency would then be:

fsw = p · f0 = 21 · 50 = 1050 Hz. (2.3)assuming that the reference frequency is 50 Hz, which implies that the funda-

mental adapts this frequency. The second order carrier, which is situated at twicethe order of the first, i.e. at harmonic order 42, is not present as seen in this ex-ample since only odd carrier multiples are present. Furthermore, each carrier issurrounded by side bands, where only even side bands are present around odd car-riers while odd side bands are only present around even carriers. Figure 2.4 is a

8

2.4. CONTROL

typical example of a sinusoidal reference waveform with a triangular carrier and athird-harmonic injection in the phase voltage.

By injecting a third harmonic in the phase voltage operation in the overmodu-lation region is allowed. Operating in the overmodulation region can give a higherfundamental output waveform. The injected harmonic will be eliminated in theline-to-line voltage, but it will let M increase up to 1.15. Other methods such asspace vector modulation rather than the carrier PWM can be used to choose thezero vector placement in order to enter the overmodulation region safely in samemanners. A quasi-square wave can give the highest value of M in the overmodula-tion region reaching M = 1.273 [8, 10].

Space vector modulation, SVM, is based on that the possible switch combina-tions for an inverter leads to discrete output voltages that can be represented asvectors. The reference vector is represented by the reference voltage generatingthrough this technique a combination of state vectors in order to obtain an averageoutput voltage [11].

The difference between PWM and SVM is that the latter uses a vector presen-tation of the voltages while the PWM technique uses carrier and reference waves inorder to generate switching of the inverter. The switching sequence in SVM can becontrolled manually in order to improve current ripple, minimise switching lossesand eliminate common-mode voltages, making it more suitable to use over PWMin certain cases. Although, SVM is merely a vector presentation of PWM witha third-harmonic injection [12]. In PWM, the modulation strategy defines whenthe switching will occur. Despite that it is shown in [8] that switching in betweenhalf carrier periods improves the VSI maximum output voltage and the harmonicperformance, it cannot be achieved by this technique. Therefore, SVM opens up tothe opportunity of better harmonic performance and higher output voltages. SVMperforms better than conventional PWM in the higher modulation index region [13].The main benefit of SVM is that it has the additional degree of freedom of choosingthe switches which can be used to improve the harmonic performance [8].

2.4 Control

There are several control strategies in order to operate a motor with a variablefrequency drive. Basically, the strategies can be divided into two groups, scalarbased controllers and vector based controllers. The drives of choice have a scalarVolts-per-Hertz, V/f -control, a Direct Torque Control, DTC, and a Speed Control,all sensorless. The two investigated strategies will be the V/f -control and the speedcontrol since they are commonly used in pump applications and there is no clearbenefit in controlling the torque over the speed.

9


2.4.1 Volts-per-Hertz Control

V/f -control is a traditional scalar method of controlling inverter-fed induction mo-tors. The magnitude of the stator voltage and the excitation frequency varies lin-early with each other. How a V/f -control works is illustrated with a block diagramin figure 2.5.

Figure 2.5. V/f -control block diagram.

The motor current is monitored in the current limit block. The value of thefrequency is changed depending on the value of the current. The output goes toa block that adjusts the volts-per-hertz ratio defining the voltage magnitude sentto the voltage control block. Successively, the current to the motor is determinedby the voltage control block that indicates the position of the flux with regards tothe current. It is crucial that this angle is correct, otherwise the motor might notbe in stable operation, as which can happen at low speeds or sporadic operations.In order to keep the speed close to the desired speed, a block that alters the fre-quency reference when the load changes can be introduced, often referred as slipcompensation [14].

2.4.2 Vector Control

Vector control is based on relations valid for dynamic states on the contrary tothe steady state based relationships in scalar control. Instantaneous positions ofvoltage, current and flux space vectors can be controlled in addition to magnitudeand frequency which was the only option in scalar control. In vector control, theangle between the voltage and current is known. The motor current is controlled bythis angle that now opens up for improved torque control and low speed operation.Torque current estimations can give a better slip approximation which gives a bettercontrol of the speed [14].

10

2.5. DRIVE REQUIREMENTS

In Field-Orientated Control, FOC, the motor flux and torque can be controlledindependently. The knowledge of the exact orientation of the space vectors can helpin overcoming control problems [14, 15]. With field-orientated control of inductionmachines, the torque can almost right away reach the torque demand [16, 17].

FOC can be rotor flux orientated or stator flux orientated where the main ideais to have a linear relationship between the control variables and torque. Therelationship is achieved by transforming the motor equations in a coordinate systemthat rotates with the stator or rotor flux vectors, respectively [15]. The direct andindirect rotor flux orientated control were first introduced by Blaschke [18] andHasse [19].

2.5 Drive RequirementsChoosing the right power electronics and modulation methods depends on the ap-plication and the choice of the motor drive. If the current rises above its ratingsin the machine, the machine will heat up. An electric motor can be overloaded forshort periods of time without damaging the motor. But on the other hand, thedrive cannot withstand an overload in percentage as large as the motor. Therefore,the drive is more prone to overheat. Depending on the application, it can be de-sirable to overdimension the electronics in order to be able to overload the motor [6].

Furthermore, the switching frequency of the power converter can reduce thecurrent ripple in the motor without having a large inductance in the motor. Byincreasing the switching frequency the losses in the converter increase linearly [6].

Considerations needs to be payed when choosing converters and modulationmethods to that the machine is probably designed for a certain frequency from thesupply. The stray and eddy current losses will increase with the use of convertersbecause of the introduction of harmonics of different orders in the system. Theintroduced harmonics can lead to torque pulsations and can have an effect for lowerfrequencies. Depending on the motor design, the harmonic losses can be higher witha PWM inverter compared to a square-wave inverter [6] for example.

A VFD ramps up the frequency from 0 to the designated value. The currentdemand is relatively low if the slip can be held small. If the designated value is 50Hz, as when connected Direct On-Line, the start current would be high.

2.6 Direct On-LineDirect On-Line Start (DOL) is a starting method where the motor is directly con-nected to the three-phase electric power supply. The motor will draw a high currentand operate at locked-rotor torque during the acceleration only to decrease the cur-rent drawn at reached speed. It is a much simpler method of starting a machine

11


but will have to have a stiff power supply as it can have an impact on the wholesystem. During the start up, the motor will build up heat which can lead to a breakdown if the motor is shut down and started repeatedly in an insufficient period oftime [20].

In pump applications, where the load has quadratic characteristics, the acceler-ation time will be longer exposing the machine to a longer overheating time [21].

Another disadvantage of direct-on-line starting is the high torque oscillations inthe initial stage of the starting process which reduces the life time of mechanicalcouplings and hence the pump [22].

2.7 DC-link

In order to provide a stiff input to the inverter in the drive system, a dc-link com-prised of capacitors is used that smooths the ripple in the dc output voltage andbalances the instantaneous power difference between the input and the output [23].The dc link is highlighted in figure 2.6.

Figure 2.6. The DC link in a variable frequency drive.

The operating conditions vary with capacitor type. Electrolytic capacitor tech-nology has been used for years and is now gradually being replaced by film capacitorsthat is now able to match the low cost. The foremost important parameters to con-sider in power electronic applications is the current and voltage ripple. A goodoutput profile is of outmost importance in order to get a well performing inverterproviding the desired output ac wave.

Electrolytic capacitors have more capacitance per volume compared to film ca-pacitors. The fact yields to that more film capacitors, in number, have to be usedin order to match the size of the capacitance of electrolytic type. Film capacitorsare therefore charged and discharged more frequently per a 20 ms period. Thiscan create more ripple in the dc voltage. A high ripple can create difficulties inmodulating the rectified voltage. One of the reason why film capacitors are not ascommon as electrolytic capacitors on the market.

There is a trade-off when replacing electrolytic capacitors with film. The numberof film capacitors needs to increase in order to keep the ripple down to the samelevel and be able to remain at the same switching frequency.

12

2.7. DC-LINK

The benefit of using film capacitors is that they can achieve a higher powerfactor and a higher cosφ compared to electrolytic capacitors.

The power factor is defined as the ratio of active power over apparent power.With the introduction of harmonics, the power factor will decrease. More currentwill be needed to deliver the same amount of active power in that case. Although,cosφ can still remain the same. cosφ is the angle between the voltage and thecurrent. The difficulty is to maintain a power factor as close to 1 as possible.

13

Chapter 3

Tested Drives and Motor

3.1 Tested DrivesThree drives were investigated. The selection of the drives was mainly based onwhat was already in use with induction motors in pump applications. All threedrives were of same nominal ratings. Two were from the same manufacturer withsame functionalities but with different hardware, named here Drive A and Drive B,respectively. The third drive was from a different manufacturer, named Drive C inthis report.

3.1.1 Hardware

Drive A

Output Power: 15 kWRated Voltage: 400 VRated Current: 30 ADC-link Capacitors: ElectrolyticOverload Capability: 200% for 4 seconds

Drive B

Output Power: 15 kWRated Voltage: 400 VRated Current: 30 ADC-link Capacitors: FilmOverload Capability: 200% for 4 seconds

Drive C

Output Power: 15 kWRated Voltage: 400 VRated Current: 30 A

15

CHAPTER 3. TESTED DRIVES AND MOTOR

DC-link Capacitors: ElectrolyticOverload Capability: 110% for 60 seconds

3.1.2 Functionality

Drive A and B, from the same manufacturer, have the same functions. Everythingfrom available switching frequency to control methods is similar. The third driveshares some of the functions found in drive A and B. They are though designed inother manners since the drive is from another manufacturer.

Following is a list of drive parameter settings for each drive and description oftheir functionalities.

Automatic Parameter Tune - Drive A, B and C

The electric parameters of the connected motor are determined by initiating thisfunction.

Energy Optimising Function - Drive A, B and C

Drive A and B’s energy optimising function works in the way that the out-put voltage applied to the motor is reduced in order to reduce the overall energyconsumption. The function is intended to be used when the motor is running atconstant speed and when it is not heavily loaded.

For Drive C, the function is described as to make the output voltage suit thecurrent load situation. The nominal value of cosφ has to be set correctly in orderfor the function to work optimally. It does as well reduce the applied voltage at agiven low load.

Acceleration Ramp Up Time - Drive A, B and C

The parameter determines how long time it will take for the frequency to accel-erate from 0 to its designated value in Hertz.

The lowest value that can be set for drive A and B is 0.0 seconds. 1 second isthe lowest value for drive C.

Switching Frequency - Drive A, B and C

The switching frequency for Drive A and B can be set from 4 to 24 kHz whilein drive C, from 2 to 16 kHz.

Control - Drive A and B

The control methods for Drive A and B comprises of a scalar Voltz-per-Hertz,

16

3.1. TESTED DRIVES

V/f -control, and a vector speed control.

V/f Voltage Boost - Drive A and B

The function increases the applied motor voltage at low frequencies. The func-tion only works in V/f -control. The purpose is to make motor starts easier.

Switching Pattern - Drive C

Two different switching patterns are available within drive C: SFAVM, StatorFlux Asynchronous Vector Modulation, and 60 AVM, 60 Degree Asynchronous Vec-tor Modulation. 60 AVM is suitable for low speed performance and SFAVM suitshigh speed efficiency and at full motor output.

Overmodulation - Drive C

Drive C offers the possibility whether to allow the VFD to enter the overmodu-lation region or not.

Modulation Random - Drive C

Drive C has a function that reduces acoustic motor switching noise by changingthe synchronism of the PWM.

3.1.3 Waveforms and Spectra - Measurements and Analysis

The following section analyses waveforms and harmonic spectra measured on theinput and outputs of the three drives.

The drives were connected to induction motors with name plate data:

Motor 1:3-phase, 50 Hz, 3.1 kW, 1445 rpm, 400 V, 6.7 A, cosφ=0.80

Motor 2:3-phase, 50 Hz, 15 kW, 2915 rpm, 400 V, 27 A, cosφ=0.88

Both motors were run at no load, as the objective was to study the shape of thewaveforms and the harmonic spectra.

The input current waveform of Drive A connected to Motor 1 is shown in figure3.1. The drive was set to operate in V/f -control and the switching frequency wasset to 4 kHz.

17


Figure 3.1. Drive A connected to motor 1, input phase current waveform, operatedin V/f -control with fsw = 4 kHz.

The shape of the current in figure 3.1 indicates that Drive A has electrolyticcapacitors in the dc-bank. The switching frequency or the modulation method didnot have an impact on the waveform nor the corresponding harmonic spectrum inthe input phase current.

The output phase voltage and the corresponding harmonic spectrum of drive Aconnected to motor 1 is shown in figure 3.2 when the drive was running in V/f -control. The switching frequency was set to 4 kHz.

18

3.1. TESTED DRIVES

Figure 3.2. Drive A connected to motor 1, output phase voltage waveform andcorresponding harmonic spectrum, V/f -control, fsw = 4 kHz.

Figure 3.2 shows the phase voltage waveform and the corresponding spectrum.It is visible that only odd carrier harmonics are present and only even side bandsaround odd carriers and odd side bands around even carriers. There is also a thirdharmonic present with a 1/6th amplitude of the fundamental. The switching fre-quency can be calculated by the use of equation 2.1 giving:

Drive settings: fsw = 4 kHz, calculated: fsw = p · f0 = 40 · 50 = 2 kHz

When the switching frequency was later set to 16 and 24 kHz, the pulse numberwas at an order of 160 and 240, respectively, giving:



The above results show that the actual switching frequency is only half of theswitching frequency set in the drive parameter settings.

19


Figure 3.3 shows the line-to-line output voltage waveform and correspondingharmonic spectrum of drive A connected to motor 1.

Figure 3.3. Drive A connected to motor 1, output line-to-line voltage waveform andcorresponding harmonic spectrum, V/f -control, fsw = 4 kHz.

The third harmonic is cancelled in the line-to-line voltage shown in figure 3.3yielding to a smaller harmonic distortion, in accordance to theory. As before, evenside bands are present around the odd carrier, while odd side bands are presentaround even carriers. Both the first odd and the first even carrier are suppressed.Similar to before, the pulse number in figure 3.3 is 40 giving half the switchingfrequency of the stated by the drive manufacturer. As the switching frequency wasincreased to other values, half of what was set was obtained as for previous cases.

Figure 3.4 shows the input current waveform of the film capacitor drive, DriveB, connected to motor 2.

20

3.1. TESTED DRIVES

Figure 3.4. Drive B connected to motor 2, input phase current waveform andcorresponding harmonic spectrum, V/f -control, fsw = 4 kHz.

The waveform of the output current shown in figure 3.4 suggests that the dc-bank is of film capacitor type. The red line shows the level of the RMS value.

Drive B connected to motor 2 was running in V/f -control. The switching fre-quency was set to 4 kHz. The output waveform of voltage and corresponding har-monic spectrum is shown in figure 3.5.

21


Figure 3.5. Drive B connected to motor 2, output phase voltage waveform andcorresponding harmonic spectrum, V/f -control, fsw = 4 kHz.

The figure, 3.5, showing the phase voltage shows similar results to as beforewith around odd carriers odd side bands are suppressed and around even carrierseven side bands are suppressed. In this case, there was no third harmonic injection.What is interesting to note is that the indicated pulse number, here p = 100, differsfrom before:

Drive was set to fsw = 4 kHz, calculated: fsw = p · f0 = 100 · 50 = 5 kHz

Figure 3.6 shows the line-to-line voltage of drive B connected to motor 2.

22

3.1. TESTED DRIVES

Figure 3.6. Drive B connected to motor 2, output line-to-line voltage waveform andcorresponding harmonic spectrum, V/f -control, fsw = 4 kHz.

The result of the switching frequency mentioned above was confirmed as well inthe line-to-line output, figure 3.6.

Figure 3.7 shows the phase voltage and corresponding spectrum when the switch-ing frequency was set to 16 kHz in drive B, connected to motor 2.

23


Figure 3.7. Drive B connected to motor 2, output phase voltage waveform andcorresponding harmonic spectrum, V/f -control, fsw = 16 kHz.

Figure 3.7 shows that now there was a third harmonic injected in this case. Thecalculated switching frequency from the harmonic spectrum tells:

Drive was set to fsw = 16 kHz, calculated: fsw = p · f0 = 160 · 50 = 8 kHz

The actual switching frequency was only the half of what was set in the drive,at 16 kHz.

The input phase current waveform of Drive C connected to motor 2 is shown infigure 3.8. The switching frequency was set to 4 kHz and the switching pattern was60 AVM.

24

3.1. TESTED DRIVES

Figure 3.8. Drive C connected to motor2, input phase current waveform, fsw = 4kHz.

The current shown in figure 3.8 shows typical characteristics of electrolytic ca-pacitors in the dc-bank.

The output waveform and corresponding spectra of the phase and line-to-linevoltage waveform is displayed in figure 3.9 and 3.10, respectively, for drive C con-nected to motor 2. The switching pattern was 60 AVM and the switching frequency2 kHz.

25


Figure 3.9. Drive C, output phase voltage waveform and corresponding harmonicspectrum, fsw = 2 kHz.

26

3.1. TESTED DRIVES

Figure 3.10. Drive C, output line-to-line voltage waveform and corresponding har-monic spectrum, fsw = 2 kHz.

Figure 3.9 and 3.10 show the phase and line-to-line voltage, respectively, whenthe switching frequency was set to 2 kHz. The harmonic spectra for both waveformsindicate a pulse number of 40 yielding to a switching frequency of 2 kHz. The thirdharmonic is higher in amplitude than the fundamental in the phase voltage butis cancelled out, as before, in the line-to-line voltage. More low order harmonicsappear in the phase voltage compared to the other drives but are suppressed in theline-to-line voltage. As for previous drives, the phase voltage has only odd carrierharmonics. The odd side bands around odd carriers are suppressed while the evensidebands are suppressed around even carriers. Furthermore, in the line-to-line volt-age, the first carrier is suppressed.

3.1.4 Waveforms and Spectra - DiscussionA drive manufacturer keeps secret how the output waveforms are modulated. Withthat in mind, it is clear that it is quite difficult to figure out the modulation tech-niques. What can be done is to evaluate the harmonic spectrum, the harmonicdistortion and to look at the waveforms, the frequency of the fundamental, if it isnear what is promised, and the amplitudes. As a customer, what is interesting to

27


know is how to operate the drive, which functions will do what and what will bethe performance. Another important thing for the user is to understand the impactof the output on their application.

The method chosen in this report to evaluate the drive performance was to lookat the waveform, the shape and amplitude and the harmonic spectrum. The rawdata of the output was sampled with an oscilloscope to a USB which was then postprocessed in Matlab in order to generate the harmonic spectrum. A few pointsmight get lost in this process considering that the oscilloscope can only save 1000points onto a USB-stick. Since the output frequency on the VFD’s was chosen to50 Hz, the scope of one period on the oscilloscope was 20 ms, which is just belowthe boundary of possible aliasing effects, considering the fast switching frequencyin the kHz-range.

The results from the output waveforms of Drive A were surprising. The switch-ing frequency indicated by the harmonic spectrum showed always half of what wasconfigured in the drive. It is strange that there would be harmonic groups in orderslower than the first carrier group of harmonics.

Drive B, which is the same type and from the same manufacturer as Drive Abut with film capacitors instead of electrolytic capacitors in the dc-bank, showedalso a deviating switching frequency compared to the configured value in the drive.For a low frequency of 4 kHz set in the drive the switching frequency was actually 1kHz higher, while for a higher switching frequency of 16 kHz, the obtained switchingfrequency was 8 kHz. A 1 kHz higher switching frequency might not matter in someapplications, but half the switching frequency of the promised can lead to undesiredperformances or unwanted noise, depending on the application.

Since both Drive A and B are from the same manufacturer it could be thatthey use the same modulation techniques for both drives, but since the dc bank isdifferent the power factor might be better for one of the drives. Nevertheless, thepulse number indicated by the harmonic spectrum should still correspond to theswitching frequency since it is defined in that manner.

The results obtained by Drive C, which indicate that the switching frequencyconfigured in the drive corresponds to the obtained by the generated harmonicspectra in Matlab allows to conclude that the measurement method and the postprocessing is done accurately.

The harmonic spectra indicate that the modulation techniques will for instanceallow an entering in the overmodulation region, as can be seen with third-harmonicinjections, yielding to a higher voltage output. With the possibility of higher outputvalues, the motor can be allowed to produce more torque.

28

3.2. TEST MOTOR

3.2 Test Motor

The selected motor was an induction motor with a squirrel cage rotor. The name-plate info and the motor parameters are found in table 3.1.

Table 3.1. Motor parameters and nameplate data of the induction motor.

Frequency 50 Hz Rs 0.799 ΩVoltage 3×400 V Rr 27.3 ΩCurrent 27 A Xs 1.80 Ω

No. of Phases 3 Xr 1.63 ΩSpeed 2915 rpm Xm 63.9 Ω

Power Factor 0.88Input Power 16.6 kW Torque 49 NmShaft Power 15 kW Rotor Inertia 0.019 kgm2

Theoretical calculated torque and current versus speed curves are found in fig-ures 3.11 and 3.12, respectively.

Figure 3.11. Theoretically calculated torque-speed curve according to given motorparameters in table 3.1.

29


Figure 3.12. Theoretically calculated current-speed curve according to given motorparameters in table 3.1.

The curves shown in figure 3.11 and 3.12 are elaborated to study a runningmotor. The theoretical curves are designed to replicate the behaviour from thespeed at the pull-out torque, 2250 rpm, to synchronous speed, 3000 rpm. Therefore,it does not reflect the starting behaviour truly.

According to figure 3.11 and 3.12, the starting torque, at speed 0 rpm, is 105Nm drawing 215 A. The maximum torque occurs at 2250 rpm with a current of150 A giving 175 Nm. At a 100 % load the speed is 2915 rpm and the torque andcurrent are 49 Nm and 27 A, respectively. At no load the current is 10 A.

A summary of the Direct On-Line, DOL, values are found in table 3.2. Thevalues are obtained from running the motor directly connected to the grid. Theshaft was connected to a test bench with a relatively high inertia compared to acentrifugal pump when started at standstill in water. When the motor is runningwith minimum load it has the additional inertia from the test bench. It is thereforeconsidered to be loaded with the least possible load.

30

3.2. TEST MOTOR

Table 3.2. DOL performance values of the tested induction motor.

Run Voltage [V] cosφ [-] Speed [rpm] Torque [Nm] Current [A]Locked Rotor - - 0 137.5 229.7125% Load 400.7 0.9214 2864.6 75.01 40.24100% Load 400.0 0.9131 2899.2 59.29 31.8975% Load 400.9 0.8827 2928.7 44.03 24.2650% Load 399.8 0.8114 2951.7 29.12 17.6025% Load 399.5 0.5991 2976.4 14.45 12.24Min. Load 398.9 0.3213 2993.5 5.36 9.96

The locked rotor test values in table 3.2 are extrapolated at rated voltage, 400V, from tests performed at lower ratings. The tests are carried out in that way inorder to prevent too high currents forces in the test bench. Values in table 3.2 is inaccordance with figures 3.11 3.12 between the two points, 2250 and 3000 rpm.

31

Chapter 4

Test Bench Methodology

4.1 Materials: The Test Bench, Measurements and PostAnalysis

In the following section the equipment used to perform the tests are listed andbriefly described.

Drives and Motor The drives described in section 3.1 are the test objects andthe motor described in section 3.2 acts as the load of the test objects.

Test Bench The motor, connected to the VFD, was mounted in a test bench.The shaft was connected to a braking motor which applied a load on the motorshaft. The test bench had therefore its own inertia which the induction motor hadto overcome.

Braking Motor The braking motor was a HDP Servomotor from ABB Sace -Italy. The motor had a maximum speed of 3600 rpm and the rated torque 657.6Nm.

Oscilloscope Agilent Technologies DSO7054A Oscilloscope was used to displaytorque and current and to save data to a USB stick which was used to transfer datato a computer for post processing purposes.

Torque Transducer HBM T10F/FS 1000 Nm, a torque transducer was used tomeasure the shaft torque, connected to the oscilloscope.

Ammeter Clamp Agilent 1146A, the clamp was used to measure VFD’s outputphase current, connected to the oscilloscope.

Matlab® was used to post process data and generate torque and current graphs.

33

CHAPTER 4. TEST BENCH METHODOLOGY

4.2 Methodology: Test ProceduresA way to investigate drive behaviour in waste water applications is to study theresponse during transient loads. By instantly increasing the load, the short termoverload capability could be investigated. The amount of load and test time pe-riod could be varied in order to examine how much and for how long the drive canwithstand the excessive load. Such a test would directly correspond to a cloggingscenario where excessive load is applied for a short time. However, the availabletest bench did not have the possibility to instantly increase the load. The possibleload ramp time was long. But it could still provide a feeling of how high the torqueproduction can be and how much current can be delivered by the drive, and for howlong.

The following tests were performed in the test bench.

Locked Rotor Test

The locked rotor test had the objective of studying the current and the startingtorque. The test was performed by blocking the rotor. The initial current andtorque was obtained at 0 rpm.

Start Test

The test was carried out by programming the drive for a ramp from stand-still to50 Hz. The start-up capabilities of the drives were investigated by studying howlow the acceleration ramp time could be programmed yet still be able to acceleratethe motor and how high the torque and current were.

Maximum Torque Test

The purpose of this test was to examine how much torque the motor produces atmaximum load and how much current is drawn at that point from the VFD. Thetest was conducted in the way that the VFD operated the motor at 50 Hz and thenthe test bench brake was applied and increased to full load until the drive reachedits maximum current and tripped, i.e. shuts off in order to protect itself.

Speed Step Test

The output frequency of the VFD was increased from, and to, a fixed value in orderto model a step change in the load. Since it is not possible to suddenly apply ashort load increase by means of the test bench torque, the loading of the motor wassimulated by increasing the speed. The larger the step in frequency and the shorterthe acceleration time was, the heavier the load appeared to be, that is, the greaterthe torque increase would be. How much torque could be produced and how muchcurrent was drawn from the VFD at that point was of interest to examine. The

34

4.2. METHODOLOGY: TEST PROCEDURES

test bench was inactive during the tests, only contributing to the total inertia onthe motor shaft, i.e. the test was run at minimum load.

VFD configurations

The VFD configurations were changed through out the tests in order to study theeffects of the different possible configurations on the drive’s performance. Modu-lation techniques, switching frequency, acceleration ramp time, energy optimisingfunctions and additional available functions were varied.

35

Chapter 5

Test Bench Results

The results of the test bench are divided into three sections for the three drives,respectively. Each section contains results of tests performed with altering drivesettings. At the end of each drive section, a short summary is presented. The lastsection contains a table that summarises chosen results that demonstrated goodperformance. The table acts as an aid in order to make a comparison between thedrives. In Appendix A, complementary results are presented in additional tablesand figures.

5.1 Drive A

The results presented in this section are obtained by operating Drive A in LockedRotor, Start, Maximum Torque and Speed Step Tests when running in V/f andspeed control and altering the VFD settings: Switching Frequency, AccelerationRamp Time and Voltage Boost, as well as running in energy optimisation.

5.1.1 Locked Rotor Test

The highest torque value in the locked rotor test was achieved in V/f -control witha voltage boost of 2.5%. The phase current and torque are shown in figure 5.1.

37

CHAPTER 5. TEST BENCH RESULTS

0.5 1 1.5 2 2.5−80

−60

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0

20

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60

80

Iv

t [s]

[A]

77

0.5 1 1.5 2 2.50

20

40

60

80

100

120

T

t [s]

[Nm

]

133

Figure 5.1. Phase current and torque, locked rotor test, V/f -control, accelerationramp time: 1 s, Voltage boost: 2.5 %, fsw = 24 kHz.

Figure 5.1 shows that the motor gave 133 Nm which is slightly lower than thestarting torque achieved with DOL, 137.5 Nm. On the other hand, the current issignificantly lower 77 A, peak, compared to 229.7 A rms.

The drive was feeding the motor with a current at only 5 Hz in the test shown infigure 5.1, although the drive was set to run in 50 Hz. The torque was also pulsatingat a frequency of around 5 Hz and was damped throughout the test run until thedrive tripped after 3 seconds.

Figure 5.2 shows current and torque of the test that got the highest torque-over-current ratio, 1.66, in speed control.

38

5.1. DRIVE A

0 0.5 1 1.5 2 2.5 3

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0

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Iv

t [s]

[A]

61

0 0.5 1 1.5 2 2.5 30

20

40

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100

T

t [s]

[Nm

]

101

Figure 5.2. Phase current and torque, locked rotor test, speed control, accelerationramp time: 0 s, voltage boost: 0 %, fsw = 12 kHz.

The oscillation, at only 3 Hz, of the torque was lower in amplitude in speedcontrol mode compared to V/f -control, as seen when comparing figures 5.1 and 5.2.The drive supplied the motor with current for over 3 seconds until it tripped.

Comparing the results between V/f -control and speed control, it is noticeablethat the V/f -control configuration for highest torque achieves a 30 % higher torquewith a 25 % higher current demand from the VFD compared to the speed controlconfiguration that gave the highest torque. That is, 133 Nm was the highest torquevalue for 77 A in V/f -control and in speed control, the highest torque was 101 Nmfor 61 A. Hence, the highest torque-over-current ratio was achieved in the V/f -control run, a ratio of 1.72. Moreover, the speed control configuration gave similarresults for different combinations of acceleration time, voltage boost and switchingfrequency while the V/f -control could not deliver any significant torque without aramped acceleration or a voltage boost. The energy efficiency function producedmore torque for less current in speed control mode compared to the V/f -controlmode.

The change in switching frequency in both control modes did not indicate animpact neither on torque production nor current demand.

The results of the performed locked rotor tests can be found in the tables ofsection Appendix A.1.1.

39


5.1.2 Start TestThe start test was conducted by starting the drive from 0 to 50 Hz with differentacceleration times, boost levels and in two different control modes. The switchingfrequency was held constant at 12 kHz for all tests.

The results from V/f -control settings indicated that with a voltage boost theacceleration time could be reduced. The minimum possible acceleration time witha 2.5 % voltage boost was 3.2 s compared to no boost at 3.3 s. If the voltage boostwas chosen to 5 % the drive could not start.

The current and torque graph of a test with an acceleration ramp time set to3.0 s and a voltage boost of 2.5 % is shown in figure 5.3.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−100

−50

0

50

100

Iu

t [s]

[A]

78

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

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40

60

80

T

t [s]

[Nm

]

78

Figure 5.3. Phase current and torque, start test, V/f -control, acceleration ramptime: 3.0 s, voltage boost: 2.5 %, fsw = 12 kHz.

The drive tripped in the test shown in figure 5.3 but still managed to reach 50Hz. The drive was running for just above 3 seconds. The inertia was too large towithstand the current demand at that torque production resulting in a trip. Theother test runs that managed to start the motor without tripping had all around 3.5second runs of accelerating the motor. The torque was again oscillating at around4 Hz, only to stabilise with the increase of speed as seen in figure 5.3.

A test run was conducted when the drive was reset to factory settings. Themode was V/f -control with no voltage boost and an acceleration ramp time set to4.0 s. The drive has no reference values to the connected motor when reset, i.e.motor nameplate data and parameters were not set in the drive. It still managedto start the motor if the acceleration ramp time was 4.0 seconds or above.

The drive was running the motor with a high output current for a high torque

40

5.1. DRIVE A

value for over 4 seconds. The drive was feeding the motor with around 30 A in 0.5 sbefore the motor started spinning. The torque did not oscillate with these settings.The torque-over-current ratio was 1.32 which was 30% higher than the highest valueamong all start tests conducted in V/f -control.

The shortest possible acceleration ramp time in speed control without trippingthe drive was 2.9 seconds. A lower value compared to the shortest time of 3.2seconds in V/f -control with a voltage boost of 2.5%. Figure 5.4 shows the currentand torque of the best performing test.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−80

−60

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0

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Iu

t [s]

[A]

77

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

T

t [s]

[Nm

]

86

Figure 5.4. Phase current and torque, start test, speed control, acceleration ramptime: 2.9 s, fsw = 12 kHz, factory reset.

As seen from figure 5.4 the torque is not oscillating and the drive is supplyingthe motor with 77 A peak value for over 2.5 seconds.

Detailed presentation of results can be found in Appendix A.1.2.

5.1.3 Maximum Torque Test

The motor was running with minimum load at 50 Hz prior the applied load increaseto the point of tripping the drive.

The test was conducted in V/f -control and speed control, respectively, and fordifferent switching frequencies.

The amount of torque produced in both V/f -control and speed control modewas around 100 Nm for all runs. It could be observed that the current stayed ina smaller spread in speed control mode compared to the V/f -control, where thecurrent had values from 70 to 90 A not following the increase of the switchingfrequency. The highest drawn current in the speed control configuration was when

41


the energy optimisation function was active, 88 A. Otherwise, the current stayedat 82-86 A. Why the current was higher when running in energy optimisation wasbecause the applied motor voltage was decreased. The motor was operated in fieldweakening. As the load increased, the drive tried to compensate for the load bymaintaining the torque. Since the voltage could not increase fast enough, morecurrent was consumed.

Figure 5.5 shows the test performed V/f -control.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5

−80

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0

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Iu

t[s]

[A]

71

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.50

20

40

60

80

100

T

t[s]

[Nm

]

100

Figure 5.5. Phase current and torque, maximum torque test performed in V/f -control with fsw = 12 kHz.

Figure 5.5 indicates a linear increase of the torque as the motor was loaded untilthe drive tripped.

A summary of the results obtained can be found in tables in Appendix A.1.3.

5.1.4 Speed Step Test

The speed step was conducted by changing the frequency of the VFD from an initialto a final value.

With a higher frequency step and a shorter acceleration time, the slip was larger,leading to a higher torque production and demanding more current. The shortestacceleration ramp time for steps 45-50 Hz, 40-50 Hz, 31-50 Hz and 31-40 Hz was1.5 seconds without having the drive to trip, while for the step 45-55 Hz, the drivecould withstand 1.0 seconds without tripping. The choice of switching frequencydid not show to have an impact on current demand and torque production. Thedrive could not run if it was set below 31 Hz. The V/f -control could not maintainoperation at low frequencies.

42

5.1. DRIVE A

Figure 5.6 shows the speed step test performed in V/f -control from 40 to 50 Hzwith the acceleration ramp time set to 1.5 s.

Figure 5.6. Phase current and torque, speed step test in V/f -control, fsw = 12 kHz,acceleration time: 1.5 s, 40-50 Hz, rise time: 0.3 s.

The rise time was 0.3 seconds in the test shown in figure 5.6 the current demandwas 79 A. The torque was 99 Nm.

Figure 5.7 shows a speed step from 40 to 50 Hz with the acceleration ramp timeset to 1.4 s giving the rise time from 40 to 50 Hz to be 0.28 seconds.

Figure 5.7. Phase current and torque, speed step test in V/f -control, fsw = 12 kHz,acceleration time: 1.4 s, 40-50 Hz, rise time: 0.28 s.

The test shown in figure 5.7 shows that the drive trips but only after the drivereaches 50 Hz. The rotor was still slipping too much demanding a high current for

43


a long period of time resulting in a trip of the drive.

In speed control, the shortest acceleration ramp time was set to 0.1 seconds forall frequency steps: 45-50, 40-50, 35-50 and 45-55 Hz without forcing the drive to atrip. The torque did not exceed 90 Nm while the current reached up to 78 A. Thetest run that produced the most torque, 90 Nm and had the highest torque-over-current ratio is shown in figure 5.8.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−80

−60

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0

20

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Iu

t [s]

[A]

74

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

T

t [s]

[Nm

]

90

Figure 5.8. Phase current and torque, speed step test in speed control, fsw = 12kHz, acceleration time: 0.1 s, 40-50 Hz.

The performed test shown in figure 5.8 showed a rapid increase in frequency.When the drive reached 50 Hz, the torque slopes down to a steady value just beforethe torque dips because of the rotor exceeding synchronous speed. From both torqueand current curves, it is visible that the control algorithm looks different for speedcontrol compared to V/f -control.

The results from both V/f -control and speed control can be found in tables inAppendix A.1.4.

5.1.5 Summary of Results

Drive A showed better performance when set in speed control. The switching fre-quency did not impact the results. The four tests indicate stable operation and areliable performance. The maximum torque obtained was around 100 Nm whichwas lower than in V/f -control where it reached 133 Nm. The drive could start witha shorter acceleration ramp time in speed control compared to V/f -control. Thedrive showed better performance when an automatic parameter tune was performedcompared to a factory reset. During the speed step test in speed control, the drive

44

5.2. DRIVE B

could be overloaded by 90%. The operation in overload lasted for only 0.5 s due tothe limitation of the performed frequency step.

5.2 Drive BIn this section, the results are presented of the tests performed with Drive B inLocked Rotor, Start, Maximum Torque and Speed Step Tests when running in V/fand speed control and altering the VFD settings: Switching Frequency, AccelerationRamp Time and Voltage Boost, as well as running in energy optimisation.


The first locked rotor tests with Drive B were performed in V/f -control. Theoutput frequency was set to 50 Hz while the switching frequency, voltage boost andacceleration time varied.

The obtained results showed the difficulty in selecting the right parameters toachieve high torque values, especially highlighted when the voltage boost was setto 10%. The drive could not start. The highest torque production was when theacceleration and voltage boost were set to zero shown in figure 5.9. The change inthe switching frequency did not make significant changes in current nor torque.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−100

−80

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0

20

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80

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Iu

t [s]

[A]

86

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

120

T

t [s]

[Nm

]

108

Figure 5.9. Phase current and torque, locked rotor test, V/f -control, accelerationtime: 0 s, voltage boost: 0 %, fsw = 12 kHz.

The best result obtained in V/f -control in the locked rotor test is the test shown

45


in figure 5.9, with a current of 86 A and a torque of 108 Nm oscillating at 3.2 Hzgiving a T/I-ratio of 1.26.

The highest torque value obtained in speed control was obtained with fsw = 12kHz and an acceleration time of 6 s, the current drawn was the lowest, 82 A. Thetorque was oscillating at around 3 Hz. The torque stayed at values around 100 Nmif the acceleration ramp time was not set to zero. Otherwise, the torque was slightlysmaller as can be seen in tables in Appendix A.2.1.

5.2.2 Start TestThe start test was conducted from 0 to 50 Hz with lowest possible acceleration ramptime in V/f and speed control. The switching frequency was set to 12 kHz.

As the acceleration time decreased, the torque increased. The highest torquevalue in V/f -control was reached at around 100 Nm drawing just above 80 A. Theapplied voltage boost of 5 % did not seem to increase the torque nor decrease thecurrent at an acceleration time of 2.5 s. Figure 5.10 shows the current and torquefor the speed step test conducted in V/f -mode with an acceleration ramp time of2.5 seconds.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−100

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Iu

t [s]

[A]

83

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

120

T

t [s]

[Nm

]

102

Figure 5.10. Phase current and torque, speed step test, 0-50 Hz, V/f -control,acceleration ramp time: 2.5 s, voltage boost: 0 %, fsw = 12 kHz.

The current reached 81 A and the torque 102 Nm in the conducted test shownin figure 5.10.

46

5.2. DRIVE B

For all tests conducted in speed control, the current stayed around 80 A and thetorque around 97 Nm.

The shortest acceleration time was 2.2 s without having the drive to trip. Thedrive supplied 82 A with a 97 Nm torque giving the torque-over-current ratio 1.19.

Then the drive was reset to factory defaults. The shortest manageable acceler-ation ramp time was 2.7 s in V/f -control. The current reached 74 A with a 98 Nmtorque giving a 1.33 torque-over-current ratio.

In speed control, the highest torque value, higher than previous test, was whenthe acceleration time was 0.1 s giving 122 Nm with a current of 85 A. The test isdepicted in figure 5.11.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−100

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[A]

85

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

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40

60

80

100

120

140

T

t [s]

[Nm

]

122

Figure 5.11. Phase current and torque, start test, 0-50 Hz, speed control, accelera-tion ramp time: 0.1 s, fsw = 12 kHz, factory reset.

The test depicted in figure 5.11 has the highest value of torque-over-currentratio, 1.44, and the lowest possible acceleration time of the start tests conductedin both V/f -control and speed control with Drive B as can be seen in the tables ofAppendix A.2.2.

5.2.3 Maximum Torque TestMotor load was applied until the drive tripped in the maximum torque test. Thetest was conducted with an automatic motor parameter identification performed.Switching frequency was varied and the initiation of the energy optimising function.

47


In V/f -control, the highest torque was obtained when the energy optimisationfunction was inactive with a switching frequency of 4 kHz. The test conducted whenthe energy optimisation function was active shows a nonlinear torque curve. Witha switching frequency of 24 kHz, most current was drawn, 85 A.

The highest torque produced when the drive was operating in speed control waswith fsw = 24 kHz drawing 88 A. Lowest current drawn, at 83 A, was when theenergy optimisation function was active, still being able to withstand just above100 Nm. Figure 5.12 shows the test.

−5 −4 −3 −2 −1 0 1 2 3 4 5−80

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t [s]

[A]

83

−5 −4 −3 −2 −1 0 1 2 3 4 5−20

0

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40

60

80

100

120

T

t [s]

[Nm

]

103

Figure 5.12. Phase current and torque, load test, speed control, fsw = 4 kHz,Energy Optimisation Function active.

The curve shapes when the energy optimisation was active, figure 5.12, weredifferent in contrast to other test runs. The torque curve was not increasing linearlyand the current was shifting between low and high values. It can clearly be observedthat the drive was trying to manage the load increase, optimising the energy.

More tests are depicted in Appendix A.2.3.

5.2.4 Speed Step testThe speed step tests conducted from an initial to a final frequency value whenoperating in V/f and speed control for different rise times and different steps hadfsw set to 12 kHz.

The drive managed to make a step from 45 to 50 Hz with an acceleration ramptime set to 0 when operating in V/f -control. It was although not the highest current

48

5.2. DRIVE B

drawn nor the highest torque production. For the step 45 to 50 Hz, the current wasaround 50 to 55 A, but could also reach 62 A, while the torque was at around 63Nm.

In speed control, for the step 45 to 50 Hz, the torque was around 64 Nm whilethe current was around 50 to 55 A. As the step increased, 40 to 50 Hz, so didthe torque and current, around 96 Nm and around 82 A, respectively. There wasa slight increase in both current and torque when the step was further increasedto 35-50 Hz. When the drive entered field weakening region, 45-55 Hz steps, thecurrent was around 85 A and the torque up to 90 Nm as shown in figure 5.13.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−100

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Iu

t [s]

[A]

86

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

T

t [s]

[Nm

]

90

Figure 5.13. Phase current and torque, speed step test, 45-55 Hz, speed control,acceleration time: 0.1 s, fsw = 12 kHz.

The minimum possible acceleration ramp time was set to 0.0 s. The drive nevertripped in speed control during the speed step test which can be observed in thecomplementary results of Appendix A.2.4.

5.2.5 Summary of ResultsDrive B could start the motor to 50 Hz with the smallest acceleration ramp timetuned in speed control. The speed control showed reliable operation. When thedrive was reset to factory default it performed well while running but it could notbe turned off without tripping the drive. High torque values, up to 122 Nm, wereobtained with a corresponding peak current of 85 A. The drive did, however, manageto operate the motor without tripping while running. In speed control, the drivecould be overloaded by 105% for one second during the speed step test.

49


5.3 Drive C

The results of the tests performed with Drive C are presented. Locked rotor, Start,Maximum Torque and Speed Step Tests are conducted while varying accelerationtime, switch pattern and switching frequency as well as initiating special functionssuch as energy optimisation.


With the switching pattern set to 60 AVM, the highest torque value and the highesttorque current ratio was obtained when the energy optimising function was active.The torque reached 78 Nm while the current was 47 A. The torque reached morethan 10 Nm higher than when the energy optimising function was inactive. Theswitching frequency did not seem to have a significant impact as summarised intables in Appendix A.3.1.

The figure 5.14 shows a locked rotor test performed with the energy optimisingfunction active.

−8 −6 −4 −2 0 2 4 6 8 10 12−50

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t [s]

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47

−8 −6 −4 −2 0 2 4 6 8 10 12−10

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30

40

50

60

70

80

T

t [s]

[Nm

]

78

Figure 5.14. Phase current and torque, locked rotor test, 60 AVM, accelerationtime: 1.00 s, fsw = 8 kHz, energy optimisation.

The torque curve shows oscillations of larger amplitude in figure 5.14 where theenergy optimisation function was active. All torque curves showed an oscillationat around 3 Hz. It is visible from the figure that the locked rotor test could beperformed for up to 18 seconds without the drive tripping.

50

5.3. DRIVE C

It was hard to tell if the switching frequency has an impact at all when thedrive was working in SFAVM. The highest torque was once again produced whenthe energy optimisation function was active giving 73 Nm for 51 A.

5.3.2 Start TestThe start tests, 0 to 50 Hz, were firstly performed with manually configured param-eters with default torque and current limits at 117 % and 110 %, respectively, in60 AVM. The drive was unable to start the motor for any acceleration ramp time.

After an automatic parameter setting was performed, with same torque andcurrent limit settings, the drive could now start. The highest torque value was 63Nm at 62 A when setting the acceleration ramp time to the minimum value of 1 s.

When the torque and current limits were increased to their respective maximumvalues, 234.7 % and 117.3 %, the drive performed better.

Figure 5.15 shows when the drive was running in energy optimisation mode.

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69

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−10

0

10

20

30

40

50

60

70

T

t [s]

[Nm

]

68

Figure 5.15. Phase current and torque, start test, 60 AVM, acceleration time: 1.5s, fsw = 8 kHz, energy optimisation.

The torque curves for all test show similar pattern except for the test with theenergy optimisation active in figure 5.15. The tests overall showed quite low valuesof torque. The shortest possible acceleration ramp time was set to the minimum, 1second.

The results from when the drive ran in SFAVM mode show that the drive canstart the motor when the acceleration ramp time is set to the minimum value, 1

51


second, if the switching frequency is 10 kHz. The torque reached 66 Nm and thecurrent 66 A.

When the energy optimisation function was active, the drive could only startthe motor if the acceleration ramp time was set to 1.5 seconds or above.

More results can be found in Appendix A.3.2.

5.3.3 Load Test

The load test was performed by loading the motor to the maximum when the drivewas operating at 50 Hz.

The torque for all tests was 70 Nm in 60 AVM except for the one with theenergy optimising function active that reached just a slightly higher value of 71Nm. The value of the switching frequency did not seem to have an impact.

Figure 5.16 shows a load test performed with a switching frequency of 8 kHz.

−3 −2 −1 0 1 2 3 4 5 6 7−80

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t [s]

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63

−3 −2 −1 0 1 2 3 4 5 6 7−10

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30

40

50

60

70

T

t [s]

[Nm

]

70

Figure 5.16. Phase current and torque, load test, 60 AVM, fsw = 8 kHz.

The current in figure 5.16 reached 63 A giving the highest torque-over-currentratio, 1.10, in this mode.

Figure 5.17 shows a test when the energy optimisation function was active.

52

5.3. DRIVE C

−3 −2 −1 0 1 2 3 4 5 6 7−80

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Iu

t [s]

[A]

65

−3 −2 −1 0 1 2 3 4 5 6 7−10

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40

50

60

70

80

T

t [s]

[Nm

]

71

Figure 5.17. Phase current and torque, load test, 60 AVM, fsw = 8 kHz, energyoptimisation.

The torque curve in figure 5.17 looks different compared to the result in figure5.16. In figure 5.16 the drive went into a protection mode when it reached the torquepeak and slowed down the speed in order to be able to keep the motor running untilit tripped. In figure 5.17, the drive tried to adapt the voltage to the current loadsituation which made the run longer before going into protection mode and tripping.

The test results when the drive was set to SFAVM mode indicated that thetorque was around 70 Nm for all tests. The drive went into protection mode afterreaching the highest torque value, 70 Nm.

When the drive was running in energy optimisation, current drawn was 62 Aand the torque 72 Nm. The torque curve indicated that the drive could managethe load increase by adapting to the current load, preventing the drive to go intoprotection mode.

Tables summarising more of the results can be found in Appendix A.3.3.

5.3.4 Speed Step Test

The speed step test was performed by increasing the frequency from 40 to 50 Hz.

Figure 5.18 shows a test performed in 60 AVM with the acceleration ramp timeset to 1.15 s, the lowest manageable value.

53


−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−80

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t [s]

[A]

63

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−10

0

10

20

30

40

50

60

70

T

t [s]

[Nm

]

65

Figure 5.18. Phase current and torque, speed step test, 60 AVM, 40-50Hz,acceleration time: 1.15 s, fsw = 2 kHz.

The torque produced in figure 5.18 was 65 Nm for 63 A. Figure 5.19 shows atest when the acceleration ramp time was set to 1.10 s.

54

5.3. DRIVE C

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−100

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Iu

t [s]

[A]

79

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−10

0

10

20

30

40

50

60

T

t [s]

[Nm

]

52

Figure 5.19. Phase current and torque, speed step test, 60 AVM, 40-50 Hz, accel-eration time: 1.10 s, fsw = 2 kHz.

Figure 5.19 clearly shows how the VFD tries to overcome the load increase.There is a first peak in the torque up to 52 Nm which seems to trigger the Protec-tion Mode after which the drive slowly increases the frequency in order to reach thedesired output.

If the switching frequency was 8 or 10 kHz in SFAVM, the drive managed tostep up the output frequency with the minimum acceleration ramp time possible, 1second.

Figure 5.20 shows a test run with the energy optimising function active.

55


−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−80

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Iu

t [s]

[A]

62

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−10

0

10

20

30

40

50

60

70

T

t [s]

[Nm

]

66

Figure 5.20. Phase current and torque, speed step test, SFAVM, 40-50 Hz, acceler-ation time: 1.00 s, fsw = 8 kHz, energy optimisation.

The result shown in figure 5.20 showed a current of 62 A drawn from the drivewhen the torque was 66 Nm. The torque gradually increased compared to the moresteep increase when the energy optimising function was inactive. Additional figuresand summarising tables of the speed step tests performed with drive C can be foundin Appendix A.3.4.

5.3.5 Summary of Results

Running with energy optimising function active gave the best performance duringall four conducted tests. The drive only operates if the automatic parameter tuningfunction has been carried out. The drive could be overloaded by 60% during thespeed step tests. The speed step test was limited by time because of the formationof the test. Therefore, the drive was overloaded for only 3 seconds. The torquenever exceeded more than 80 Nm during the tests without resulting in a trip.

5.4 Drive Comparison

Table 5.1 summarises the best obtained results during the conducted tests for thethree drives. First column describes which test was conducted. Beneath the drivecolumns, torque and maximum current values are shown. In italic, the torque-over-current ratio or the shortest possible acceleration ramp time is shown.

56

5.4. DRIVE COMPARISON

Table 5.1. A comparison between the three drives during the best performancesfrom the tests.

Studying table 5.1 it is observable that Drive C could not provide as much torqueas the other two drives. The lowest possible acceleration time was 1.0 s for drive C.For drive A and B, the acceleration ramp time could be set to the lowest possiblevalue, 0.0 s. Drive A gave the overall highest torque value, 133 Nm. Drive B couldaccelerate the fastest from standstill to 50 Hz. It reached high torque values andcould manage to accelerate the fastest compared to the other drives.

57

Chapter 6

Analysis and Discussion

6.1 Test Bench Analysis and Discussion

6.1.1 Drive Size Selection

The nominal current of the motor at 27 A was tightly matched to the 30 A ratedcurrent of the drives, both 15 kW. A decision following drive manufacturer’s rec-ommendations for centrifugal pumps, keeping in mind that centrifugal pumps arenormally considered free of overloads [4]. Other drive manufacturers recommend anoverdimensioned drive with regards to the induction motor to be able to meet theoverload capability of the motor. It was therefore interesting to investigate how atightly matched drive would respond to a dynamically varying load. A load profileoccurring in waste water applications.

6.1.2 A Novel Evaluation Method

Because of the limitation of suddenly increasing the load in the test bench, newtest procedures were discussed in the group. The speed step test was predicted tobe a simple and accurate way to analyse the dynamic behaviour. A novel approachelaborated within the group. Furthermore, the other conducted tests would act asboth verifications and supplements to the speed step test. The combination of alltests present a new approach where the drive-motor performance is analysed fromdifferent aspects which widens the perspective of the obtained results.

6.1.3 The Test Results

The test runs with Drive A showed the impact of the parameter choices on the driveperformance. The most surprising result was that the highest overload was achievedwhen the drive was operating in V/f -control during the locked rotor test. DriveB, on the contrary, operated better in vector control, which was to be expected.The other major difference between the drives was that drive B could perform

59

CHAPTER 6. ANALYSIS AND DISCUSSION

equally well with or without an automatic motor parameter tuning. Keeping inmind that the drive was unable to shut off without tripping was a major drawback.An unacceptable behaviour. Drive A, on the other hand, showed better performanceif the drive settings were configured with consideration after an automatic motorparameter tuning. The test results from both drives did although show that it ismost reliable to operate in vector control.

As Drive C has a longer overload time compared to Drive A and B, the testsduration were significantly longer. The drive did not, however, reach as high over-load in percent as the other two drives. The most surprising test result of DriveC was that it performed better with the energy optimising function active. Thiswas not the case for Drive A and B. The energy optimising function should be usedonly when operating in non-varying load conditions, following drive manufacturer’srecommendations. The reason why the drive performance was improved could bethat the drive had a chance to follow the dynamically varying load with the longeroverload time.

6.2 Discussion

6.2.1 Drive Selection

There are a couple of heavy parameters to consider when selecting a drive for pumpapplications. What is pumped? Which pump is used? What motor is inside thatpump? How does the load curve look like? The questions are many, and the crucialkey matters, such as the transient load curve, remain uninvestigated. This reporthas put forward results of an empirical investigation on how to test drives andwhat to take into consideration when it comes to producing the maximum availabletorque for the least amount of current. As mentioned before, centrifugal pumpsare considered to not have varying load characteristics, but in waste water theycan encounter sudden load increases. Therefore, it is desirable to be able to allowthe motor to produce the maximum available torque without having the drive totrip. With that in mind, Drive C was the least suitable drive for centrifugal pumpapplications since it could not be overloaded as much as Drives A and B. Instead,it had a long term overload capacity, which is not a favourable option. Rather tohave the highest torque possible for just one second than a lower torque for a longertime. The reason is because if an object gets stuck in the impeller, it will needone strong push and not a slightly higher push over a longer time. The tests didalthough show that Drive C could be overloaded up to 60% more for a few seconds,but not for 60 seconds, as specified. Drive A and B, on the other hand, met theirspecifications.

As the drives were tightly matched to the motor, an interesting result is thatDrive A, which had a 100% overload capability could almost reach the startingtorque value of the motor in the locked rotor test. It reached 133 Nm while theDOL showed a capability of 137.5 Nm. Since there was no track in what exactposition the rotor was locked, it could be that the maximum starting torque was

60

6.2. DISCUSSION

reached in that locking position. Therefore, a drive that is rated as the motor couldwith a 100% overload reach the starting torque values.

6.2.2 The Topic

The vast topic with its many broad components could be approached in many ways.Each approach can be beneficial in its own way and contribute to the answer throughanother direction. Papers such as NEMA’s Application Guide for AC AdjustableSpeed Drive Systems [5] treats the topics, among others, of what to consider from theend users electrical perspective. Industry guides from drive manufacturers considerthe fact that they want to provide a reliable motor operation to their customers,focusing on protecting the drive and also making sure that the customers own motoris protected. Papers that present a drive system and motor simulations focuses onto show how the operating characteristics look like and are generalised. Moreover,the articles that take into account the load curve, that is the centrifugal pumpssubmerged under water speak about how to select the pump in the sense of flowcharacteristics and other pump characteristics, and not how to select a VFD thatcan manage transient load characteristics [1, 3]. Hence, work on how to select drivesfor pump applications are hard to come across. The initial idea of this project wasto make a generalised simulation that would correspond to the test bench resultsand that could be further developed by using different load characteristics in orderto investigate how to dimension a drive for such an unknown load. In the bestof cases, a model would be obtained where given parameters are entered and theresult would be a recommendation on how much to overdimension the drive for acertain load characteristic and motor size. The first obstacle that was faced was thefact that it was a long chain of topics that haven’t been thoroughly treated linkedto each other. Therefore, the decision to tackle the problem by empirically testingthe drive performance was chosen since little had been done in this field concerningdrive operated centrifugal pumps. In order to gain good knowledge on the drivecontrol methods, applied motor voltage should have been measured, as discussedpreviously. The simulation model could be tuned to behave more closely to reality.

The idea was that this project would be a part of further projects developingthis area. After investigating the possible motor-drive performance it would benaturally to start making thorough simulations. Next step is to investigate thetrue load characteristics of waste water and try to obtain a realistic response of thesimulation models to be able to answer the initial question.

6.2.3 Simulations

The first approach was to model the chosen induction motor in order to simulatedifferent models of variable frequency drives. The model was supposed to growfrom a specific case to a more generalised model of squirrel cage induction motorsconnected to variable frequency drives.

61

CHAPTER 6. ANALYSIS AND DISCUSSION

The motor model and the drive were supposed to correspond to the availableequipment. The aim was to be able to simulate transient loads and study the be-haviour of the system. The result would be a recommendation on how much thedrive should be overdimensioned given the motor loading case.

The choice of modelling tool was Simulink by MathWorks. The environmentoffers both circuit and signal modelling. The signal modelling of the system actedfaster than the system built up of circuits, therefore, the signal approach was chosen.

If the entire system should be built, with actual component characteristics, themodel should comprise of circuits. Each component can be modelled more realisti-cally by including for instance the stray inductances and capacitances between eachcomponent. There is always the possibility to develop a model to the smallest detailincreasing the complexity and the accuracy of the model.

Points of consideration when making a model for simulations are

• The output of a VFD can be hard to replicate since the modulation techniqueis a manufacturer’s secret.

• Measurements of the phase and line-to-line voltage waveforms can be a guideto how the waveform should be reconstructed.

• Studying the harmonic spectra can reveal the characteristics of the modulationmethod.

• The control algorithms can be adjusted by studying the applied motor voltageand the shaft speed.

• The induction motor model has to be dynamical.

• Higher accuracy can be obtained if the model is adjusted to suit the replicatedevent for each simulation case.

A lot of effort and consideration has to be made in order to obtain a model thatresembles the reality. Therefore, the conclusion made was that a great deal of workhad to be carried out that would lead to little valuable results.

The drive is a black box regarding software and a combination of both softwareand hardware, making modelling of the chosen drives difficult. A drive with cheapcomponents can still operate well if the programmed algorithms can compensate,and so on. With the mentioned arguments in mind, the approach of gatheringempirical data in the lab and post analysing was chosen.

62

Chapter 7

Conclusion and Future Work

7.1 Concluding Remarks

The test results from the locked rotor test and start test clearly indicated that itwould be possible to run a centrifugal pump that is in the same kW-size as theoperating variable frequency drive. If it is a bigger system on a remote location, thedrive should be overdimensioned. The reason is that the drive would be the bottleneck if the system would be overloaded. By introducing drive systems in pump ap-plications, already money is saved because of the possibility to decrease the speedinstead of flow-reducing valves. Furthermore, safe operation can be carried outsince the drive can soft start the motor avoiding high inrush currents and can alsoprovide remote monitoring. The motor will be prevented of going in overload andhence will not be excessively heated which can reduce the life time of the motor.The benefits, both regarding economy and sustainability, are numerous which canbe arguments in buying an overdimensioned drive in order to also obtain a reliablesystem.

Initially, when installing a drive to a motor, all limiting drive parameters shouldbe set to maximum. The name plate data of the motor should be entered and anautomatic tuning of the motor parameters should be performed. Lastly, in orderto maintain a non-varying, stable, reliable operation, vector speed control shouldbe selected. Vector control has shown that it can give high torque values and stillmaintain a low current.

The report implies that extracting a formula where the input parameters are themotor nameplate data and the load characteristics which will then yield to a finalanswer on which drive to select is difficult. First of all, the true load characteristicsare unknown and second of all it is not only the rated current, voltage, frequencyor the rated power that should be matched. Even if the drive would be perfectlymatched to the motor regarding the size of the two components, the drive can,depending on the design, run in overload for a certain time interval. The control

63

CHAPTER 7. CONCLUSION AND FUTURE WORK

method of the drive can allow it to surpass critical overloads by adjusting the outputto meet the torque demand. Additionally in pump applications, a clever control canfor instance adjust the shaft speed to overcome overloads and prevent clogging.

7.1.1 Drive Selection Recomendation

This section presents a suggested work procedure in selecting a drive for a centrifugalpump. It is assumed that the choice of using a centrifugal pump is in line with theintended application. After following the steps, it should be possible to operate themotor accordingly.

1) Find out which fluid will be pumped, is it subjected to frequent hard objectsthat creates the risk of clogging the pump or is it clean water? If it is the latter,overdimensioning will not be necessary.

2) Match the nameplate data of the motor to the drive. Voltage, output frequency,rated current and rated power is to consider. If possible, select a drive that can gothe highest in overload, even if it is just for a second or two - rather than selecting asmaller overload over a longer time period if the pump is subjected to transient loads.

3) Enter the motor parameters that the drive asks for manually and perform anautomatic tuning of the rest of the parameters. It is important that the driverecognises the motor characteristics. If there are current or torque limits to beselected - select the highest possible values.

4) If the pump is submerged in a frequent clogging environment, find out whatthe starting torque is of the motor. If the pump is at stand-still and clogged, enoughcurrent must be supplied in order not to block the starting torque capability of themotor. The motor must be allowed to be started even if there is an obstacle onthe impeller. Additionally, find out what the maximum torque of the motor is andmake sure that the drive can supply enough current to reach that point when it isoperating in overload. If the pump is frequently subjected to transient overloads,make sure that the drive can supply enough current to allow the motor to producemaximum torque.

5) Lastly, make sure that environmental recommendations of the drive manufac-turer are met, such as ambient pressure and temperature, mounting and mainte-nance directives.

64

7.2. FUTURE WORK

7.2 Future WorkIn order to get a deeper understanding of the problem, investigation of the loadcharacteristics of centrifugal pumps in waste water needs to start. It is not only aproblem for electrical engineers since it also involves the flow of the pump, impellerand hydraulics. A study of what waste water actually contains and what impact ithas when it loads the electric motor needs to be investigated. The knowledge of thethe transient load curve would help in building a realistic simulation.

To further develop this project and add more depth to the results a final labtest where a large drive is connected to a small motor could be performed. Thedrive can then be set to different ratings fooling the drive to believe that the motorhas a different size. By varying the motor ratings set in the drive, such as ratedcurrent, the drive would trip on different levels and an overview on how much thedrive has to be overdimensioned in order to reach the maximum torque value wouldbe gained.

Future performed tests needs to measure the applied voltage to the motor. Thegain would be a deeper analysis of drive control methods. Additionally, shaft speedcan tell the total system response, stability and the behaviour of the motor. Dueto limitations in the test bench, these measurements were not performed.

Furthermore, in order to evaluate the speed step test method, frequency overtime could be plotted to study the shape of the curve, whether it is a linear re-lationship or if it follows another pattern. Comparing the frequency ramp whenstarting the drives from 0 to 50 Hz would also give another depth in a completeevaluation of the speed step test. These measurements would contribute to obtaina more accurate simulation model.

Further topics would be to try simulation and lab tests for different types ofmotors and other drive types.

65

Appendix A

Complementary Results

A.1 Drive A

A.1.1 Locked Rotor Test

Table A.1. Drive A: Results of the locked rotor test performed in V/f -control.

Parameter Settings ResultsAcc. Ramp Voltage fsw [kHz] Imax [A] Tmax [Nm] T/I- NoteTime [s] Boost [%] ratio

1 2.5 4 77 131 1.701 2.5 12 77 131 1.701 2.5 24 77 133 1.720 0 12 34 1 -0 0 12 63 1 -0 10 12 86 80 0.936 0 12 83 67 0.811 2.5 12 81 80 0.99 Energy Opt.

67

APPENDIX A. COMPLEMENTARY RESULTS

Table A.2. Drive A: Results of the locked rotor test performed in speed control.

Parameter Settings ResultsAcc. Ramp Voltage fsw [kHz] Imax [A] Tmax [Nm] T/I- NoteTime [s] Boost [%] ratio

1 2.5 4 58 96 1.661 2.5 24 59 98 1.660 0 12 58 94 1.620 0 12 61 101 1.660 10 12 61 97 1.596 0 12 63 94 1.491 2.5 12 58 94 1.62 Energy Opt.

A.1.2 Start Test

Table A.3. Drive A: Results of the start test, 0 to 50 Hz, performed in V/f -controlwith fsw = 12 kHz.

Acceleration Time [s] Voltage Boost [%] I [A] T [Nm] T/I-ratio Note4.0 0.0 74 68 0.923.5 0.0 80 72 0.903.4 0.0 82 73 0.903.3 0.0 83 76 0.913.2 0.0 86 76 0.89 Trip3.0 0.0 78 80 1.02 Trip4.0 5.0 91 18 0.19 Trip3.5 5.0 80 16 0.20 Trip3.3 5.0 86 17 0.20 Trip4.0 2.5 77 72 0.943.3 2.5 77 72 0.943.2 2.5 82 73 0.903.0 2.5 78 78 1.00 Trip

68

A.1. DRIVE A

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−60

−40

−20

0

20

40

60

Iu

t [s]

[A]

53

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

T

t [s]

[Nm

]

70

Figure A.1. Drive A: Phase current and torque, start test, V/f -control, accelerationramp time: 4.0 s, voltage boost: 0 %, fsw = 12 kHz with factory default parameters.

Table A.4. Drive A: Results of the start test, 0 to 50 Hz, performed in speed controlwith fsw = 12 kHz.

Acceleration Time [s] I [A] T [Nm] T/I-ratio Note3.0 74 86 1.162.9 77 86 1.122.8 75 86 1.14 Trip2.7 74 86 1.16 Trip2.0 78 88 1.13 Trip1.1 75 89 1.18 Trip1.0 86 89 1.03 Trip3.0 75 86 1.13 Energy Opt, Trip2.9 80 1 - Energy Opt, Trip

A.1.3 Maximum Torque Test

Table A.5. Results of the maximum torque test performed in V/f -control.

fsw [kHz] Imax [A] Tmax [Nm] T/I-ratio Note4 81 103 1.2712 71 100 1.4124 91 101 1.11

69


Table A.6. Drive A: Results of the maximum torque test performed in speed control.

fsw [kHz] Imax [A] Tmax [Nm] T/I-ratio Note4 82 104 1.2712 86 103 1.2024 83 101 1.2212 88 102 1.16 Energy Opt.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5

−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A].

88

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.50

20

40

60

80

100

T

t [s]

[Nm

]

102

Figure A.2. Drive A: Phase current and torque, maximum torque test performedin speed control with fsw = 12 kHz and the energy optimising function active.

70

A.1. DRIVE A

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5

−80

−60

−40

−20

0

20

40

60

80

Iu

t[s]

[A]

71

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.50

20

40

60

80

100

T

t[s]

[Nm

]

100

Figure A.3. Drive A: Phase current and torque, maximum torque test performedin V/f -control with fsw = 12 kHz.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5

−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A].

86

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.50

20

40

60

80

100

T

t [s]

[Nm

]

103

Figure A.4. Drive A: Phase current and torque, load test, speed control, fsw = 12kHz.

71


A.1.4 Speed Step Test

Table A.7. Drive A: Results of the speed step test conducted in V/f -control.

Frequency Acceleration Rise fsw [kHz] I [A] T [Nm] T/I- NoteStep [Hz] Ramp Time [s] Time [s] ratio45-50 5.0 0.50 4 42 42 1.0045-50 2.0 0.20 4 54 54 1.0045-50 1.5 0.15 4 73 91 1.2545-50 1.0 0.10 4 54 60 1.11 Trip45-50 6.0 0.60 12 31 3745-50 1.5 0.15 12 61 9145-50 1.0 0.10 12 54 60 Trip45-50 1.0 0.10 24 73 64 0.88 Trip40-50 1.5 0.30 4 67 84 1.27 Trip40-50 2.0 0.40 12 76 9840-50 1.8 0.36 12 77 9840-50 1.7 0.34 12 77 9840-50 1.6 0.32 12 78 9940-50 1.5 0.30 12 80 9940-50 1.4 0.28 12 73 85 Trip40-50 2.0 0.40 12 77 9840-50 1.5 0.30 12 79 9940-50 1.4 0.28 12 77 99 Trip31-40 1.5 0.34 12 80 9831-40 1.4 0.32 12 81 97 Trip45-55 3.0 12 69 68 0.9945-55 2.0 12 78 85 1.0945-55 1.5 12 85 88 1.0445-55 1.5 12 80 87 1.0945-55 1.0 12 85 91 1.0745-55 0.8 12 69 1 0.01 Trip45-55 0.5 12 47 34 0.73 Trip

72

A.2. DRIVE B

Table A.8. Drive A: Results of the speed step test conducted in speed control.

Frequency Acceleration Rise fsw [kHz] I [A] T [Nm] T/I- NoteStep [Hz] Ramp Time [s] Time [s] ratio45-50 2.0 12 53 59 1.1045-50 2.0 12 49 61 1.2645-50 1.0 12 57 64 1.1345-50 0.5 12 60 63 1.0645-50 0.1 12 53 64 1.2040-50 0.5 12 75 90 1.1940-50 0.1 12 74 90 1.2235-50 0.5 12 75 90 1.1935-50 0.5 12 78 90 1.14 Enrg-Opt45-55 0.1 12 77 86 1.12

A.2 Drive B


Table A.9. Drive B: Results of the locked rotor test with an output frequency of 50Hz in V/f -control.

Acceleration Voltage fsw [kHz] I [A] T [Nm] T/I- NoteTime [s] Boost [%] ratio

1 0 4 83 96 1.151 0 24 83 98 1.184 0 4 88 99 1.134 0 24 85 98 1.160 0 12 86 108 1.260 10 12 88 31 0.356 0 12 88 97 1.1112 1 2.5 85 97 1.15 Energy Opt.

73


Table A.10. Drive B: Results of the locked rotor test performed in speed controlwith an output frequency of 50 Hz in speed control.

Acceleration Time [s] fsw [kHz] I [A] T [Nm] T/I-ratio Note1 4 85 102 1.201 12 86 101 1.171 24 82 100 1.230 12 82 94 1.156 12 82 102 1.241 12 83 102 1.23 Energy Opt.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−100

−80

−60

−40

−20

0

20

40

60

80

100

Iu

t [s]

[A]

82

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

120

T

t [s]

[Nm

]

102

Figure A.5. Drive B: Phase current and torque, locked rotor test, speed control,acceleration time: 6 s, fsw = 12 kHz.

74

A.2. DRIVE B

A.2.2 Start Test

Table A.11. Drive B: Results of the start test, 0-50 Hz, in V/f -control.

V/f -control, voltage boost: 0 %, fsw = 12 kHzFrequency Step [Hz] Acceleration Time [s] Rise Time [s] I [A] T [Nm] T/I-ratio Note

0-50 10.0 10.0 29 33 1.110-50 5.0 5.0 56 60 1.070-50 4.0 4.0 66 75 1.140-50 3.0 3.0 80 98 1.220-50 2.5 2.5 83 102 1.220-50 2.3 2.3 89 102 1.14 Trip

V/f -control, voltage boost: 5 %, fsw = 12 kHz0-50 2.5 2.5 83 102 1.22

V/f -control, voltage boost: 0 %, fsw = 12 kHz, Energy Optimisation Function Active0-50 2.5 2.5 86 102 1.18

Table A.12. Drive B: Summary of results for the start test, 0-50 Hz in V/f -control,when factory reset.


0-50 3.0 3.0 72 92 1.280-50 2.9 2.9 78 95 1.210-50 2.8 2.8 77 98 1.270-50 2.7 2.7 74 98 1.330-50 2.6 2.6 85 105 1.24 Trip0-50 2.2 2.2 88 116 1.32 Trip0-50 2.0 2.0 38 28 0.75 Trip

75


−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A]

74

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

T

t [s]

[Nm

]

98

Figure A.6. Drive B: Phase current and torque, speed step test, 0-50 Hz, V/f -control, acceleration ramp time: 2.7 s, voltage boost: 0 %, fsw = 12 kHz, factoryreset.

Table A.13. Drive B: Results of the speed step test, 0-50 Hz, in speed control.

Speed control, voltage boost: 0 %, fsw = 12 kHzFrequency Step [Hz] Acceleration Time [s] Rise Time [s] I [A] T [Nm] T/I-ratio Note

0-50 2.5 2.5 83 97 1.170-50 2.4 2.4 80 96 1.210-50 2.3 2.3 80 97 1.210-50 2.2 2.2 82 97 1.190-50 2.1 2.1 80 98 1.22 Trip0-50 2.0 2.0 82 98 1.20 Trip

Speed control, fsw = 12 kHz, Energy Optimisation Function Active0-50 2.2 2.2 82 98 1.20

76

A.2. DRIVE B

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−100

−80

−60

−40

−20

0

20

40

60

80

100

Iu

t [s]

[A]

82

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

T

t [s]

[Nm

]

97

Figure A.7. Drive B: Phase current and torque, speed step test, 0-50 Hz, speedcontrol, acceleration ramp time: 2.2 s, fsw = 12 kHz.

Table A.14. Drive B: Summary of results for the speed step test, 0-50 Hz in speedcontrol, when factory reset

Speed control, voltage boost: 0 %, fsw = 12 kHzFrequency Step [Hz] Acceleration Time [s] Rise Time [s] I [A] T [Nm] T/I-ratio Note

0-50 2.7 2.7 77 103 1.350-50 2.5 2.5 N/A 112 N/A0-50 2.0 2.0 86 113 1.320-50 1.0 1.0 85 116 1.370-50 0.1 0.1 85 122 1.44

77



Table A.15. Drive B: Results for the load test with output frequency 50 Hz and avoltage boost 0% in V/f -control.

V/f -controlfsw [kHz] Energy Optimisation Function I [A] T [Nm] T/I-ratio Note

4 inactive 72 98 1.364 active 71 97 1.3824 inactive 85 98 1.16

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A]

72

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

T

t [s]

[Nm

]

98

Figure A.8. Drive B: Phase current and torque, load test, V/f -control, fsw = 4kHz.

78

A.2. DRIVE B

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A]

71

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

T

t [s]

[Nm

]

97

Figure A.9. Drive B: Phase current and torque, load test, V/f -control, fsw = 4kHz, Energy Optimisation Function active.

Table A.16. Drive B: Summary of results for the load test with output frequency50 Hz and a voltage boost 0%.

Speed Controlfsw [kHz] Energy Optimisation Function I [A] T [Nm] T/I-ratio Note

4 inactive 86 99 1.154 active 88 103 1.174 active 83 103 1.244 active 83 102 1.2224 inactive 86 104 1.2124 inactive 88 105 1.19

79


−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−100

−80

−60

−40

−20

0

20

40

60

80

100

Iu

t [s]

[A]

86

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

T

t [s]

[Nm

]

99

Figure A.10. Drive B: Phase current and torque, load test, speed control, fsw = 4kHz.

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−100

−80

−60

−40

−20

0

20

40

60

80

100

Iu

t [s]

[A]

86

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−20

0

20

40

60

80

100

120

T

t [s]

[Nm

]

104

Figure A.11. Drive B: Phase current and torque, load test, speed control, fsw = 24kHz.

80

A.2. DRIVE B

A.2.4 Speed Step test

Table A.17. Drive B: Summary of results for the speed step test in V/f -control.


45-50 2.0 0.20 53 62 1.1645-50 1.5 0.15 53 63 1.1745-50 1.3 0.13 62 65 1.2245-50 1.2 0.12 49 63 1.3045-50 1.2 0.12 55 63 1.1345-50 1.1 0.11 53 63 1.1845-50 1.0 0.10 55 63 1.1545-50 0.8 0.08 52 63 1.2245-50 0.5 0.05 53 64 1.2045-50 0.3 0.03 53 64 1.2045-50 0.1 0.01 53 64 1.2045-50 0.0 - 53 64 1.196545-55 3.0 0.5455 75 77 1.0245-55 2.0 0.3636 44 55 1.25 Trip45-55 0.5 0.0909 85 83 0.98 Trip40-50 3.0 0.60 83 96 1.0340-50 2.9 0.58 78 90 1.1540-50 2.8 0.56 85 90 1.06 Trip40-50 2.8 0.56 85 90 1.06 Trip40-50 2.5 0.50 80 93 1.17 Trip40-50 2.0 0.40 86 97 1.13 Trip

V/f -control, voltage boost: 0 %, fsw = 12 kHz, Energy Optimisation Function Active40-50 3.0 0.60 89 96 1.08 Trip40-50 2.8 0.56 88 98 1.10 Trip

81


−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−60

−40

−20

0

20

40

60

Iu

t [s]

[A]

53

−2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5−10

0

10

20

30

40

50

60

70

T

t [s]

[Nm

]

64

Figure A.12. Drive B: Phase current and torque, speed step test, 45-50 Hz, V/f -control, acceleration ramp time: 0.0 s, voltage boost: 0 %, fsw = 12 kHz.

Table A.18. Drive B: Summary of results for the speed step test in speed control.

Speed control, fsw = 12 kHzFrequency Step [Hz] Acceleration Time [s] Rise Time [s] I [A] T [Nm] T/I-ratio Note

45-50 2.0 0.20 47 63 1.3345-50 1.0 0.10 49 65 1.3245-50 1.0 0.10 57 64 1.1345-50 0.1 0.01 53 65 1.2245-50 0.1 0.01 50 64 1.2745-50 0.0 - 55 66 1.2045-50 0.0 - 57 64 1.1340-50 1.0 0.20 83 96 1.1640-50 0.5 0.10 80 96 1.2040-50 0.2 0.04 78 96 1.2340-50 0.0 - 85 96 1.1435-50 0.1 0.03 86 98 1.1335-50 0.0 - 88 98 1.1135-50 1.0 0.30 89 99 1.11 Energy Opt.45-55 0.5 0.0909 85 90 1.0745-55 0.1 0.0182 86 90 1.05

82

A.3. DRIVE C

A.3 Drive C


Table A.19. Drive C: Summary of results for the locked rotor test performed withautomatic parameter configuration in 60 AVM.

fsw [kHz] Acceleration Time [s] I [A] T [Nm] T/I-ratio Note2 2 44 64 1.442 2 47 62 1.312 1 51 65 1.282 1 44 65 1.478 2 46 58 1.27 (Trip)8 1 44 60 1.368 1 47 78 1.64 Energy Opt.8 1 51 60 1.19 Overmod, PWMrndm off8 1 49 60 1.23 PWM Random, Trip16 2 44 58 1.3116 1 47 60 1.27

−8 −6 −4 −2 0 2 4 6 8 10 12−50

−40

−30

−20

−10

0

10

20

30

40

50

Iu

t [s]

[A]

44

−8 −6 −4 −2 0 2 4 6 8 10 12−10

0

10

20

30

40

50

60

70

T

t [s]

[Nm

]

60

Figure A.13. Drive C: Phase current and torque, locked rotor test, 60 AVM,acceleration time: 1.00 s, fsw = 8 kHz.

83


Table A.20. Drive C: Summary of results for the locked rotor test performed withautomatically configured parameters in SFAVM.

fsw [kHz] Acceleration Time [s] I [A] T [Nm] T/I-ratio Note2 2 43 63 1.46 (Trip)2 1 51 65 1.288 1 46 61 1.34 (Trip)8 1 51 73 1.44 Energy Opt.8 1 51 61 1.20 Overmodulation8 2 47 57 1.20 PWM Random8 1 49 60 1.23 PWM Random, (Trip)10 1 52 60 1.1411

−8 −6 −4 −2 0 2 4 6 8 10 12−60

−40

−20

0

20

40

60

Iu

t [s]

[A]

51

−8 −6 −4 −2 0 2 4 6 8 10 12−10

0

10

20

30

40

50

60

70

80

T

t [s]

[Nm

]

73

Figure A.14. Drive C: Phase current and torque, locked rotor test, SFAVM, accel-eration time: 1 s, fsw = 8 kHz, energy optimisation.

84

A.3. DRIVE C

A.3.2 Start Test

Table A.21. Drive C: Summary of results for start test performed with manuallyconfigured parameters in 60 AVM.

fsw [kHz] Acceleration Time [s] I [A] T [Nm] T/I-ratio Note2 2 66 66 1.00 Trip2 5 76 61 0.80 Trip2 20 66 64 0.97 Trip

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A]

76

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−10

0

10

20

30

40

50

60

70

T

t [s]

[Nm

]

61

Figure A.15. Drive C: Phase current and torque, start test, manually configuredparameters, 60 AVM, acceleration time: 5 s, fsw = 2 kHz.

Table A.22. Drive C: Summary of results for start test performed with automaticparameter configuration in 60 AVM.

fsw [kHz] Acceleration Time [s] I [A] T [Nm] T/I-ratio Note2 5 47 49 1.032 2 58 58 0.992 1 62 63 1.028 1 79 59 0.75 Trip

85


−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A]

62

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−10

0

10

20

30

40

50

60

70

T

t [s]

[Nm

]

63

Figure A.16. Drive C: Phase current and torque, start test, automatically config-ured parameters, 60 AVM, acceleration time: 1 s, fsw = 2 kHz.

Table A.23. Drive C: Summary of results for start test performed with automaticconfigured parameters in 60 AVM and limits set to maximum.

fsw [kHz] Acceleration Time [s] I [A] T [Nm] T/I-ratio Note8 2.0 60 65 1.088 1.0 65 66 1.028 1.0 68 66 0.98 Overmodulation8 1.0 69 66 0.96 PWM Random8 1.5 69 68 0.98 Energy Opt.8 1.0 77 66 0.85 Energy Opt. Trip16 2.0 58 64 1.0916 1.2 63 66 1.0416 1.1 74 65 0.8816 1.0 72 66 0.9116 1.0 76 65 0.85 Trip

86

A.3. DRIVE C

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A]

65

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−10

0

10

20

30

40

50

60

70

T

t [s]

[Nm

]

66

Figure A.17. Drive C: Phase current and torque, start test, 60 AVM, accelerationtime: 1 s, fsw = 8 kHz.

Table A.24. Drive C: Summary of results for speed step test performed with auto-matically configured parameters in SFAVM.

fsw [kHz] Acceleration Time [s] I [A] T [Nm] T/I-ratio Note2 1.50 63 65 1.022 1.20 69 66 0.952 1.11 77 63 0.82 Trip2 1.10 77 63 0.82 Trip2 1.00 64 90 0.71 Trip8 1.15 68 65 0.968 1.00 63 77 0.82 Trip10 1.15 66 65 0.9810 1.00 66 66 0.9910 1.50 69 66 0.96 Energy Opt.10 1.15 77 63 0.82 Energy Opt. Trip10 1.00 87 66 0.76 Energy Opt. Trip10 1.00 71 65 0.92 Overmodulation10 1.00 63 65 1.03 PWM Random

87


−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A]

66

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−10

0

10

20

30

40

50

60

70

T

t [s]

[Nm

]

66

Figure A.18. Drive C: Phase current and torque, speed step test, SFAVM, acceler-ation time: 1.00 s, fsw = 10 kHz.

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A]

69

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5−10

0

10

20

30

40

50

60

70

T

t [s]

[Nm

]

66

Figure A.19. Drive C: Phase current and torque, speed step test, SFAVM, acceler-ation time: 1.50 s, fsw = 10 kHz, energy optimisation.

88

A.3. DRIVE C


Table A.25. Drive C: Summary of results for the load test performed with auto-matically configured parameters in 60 AVM.

fsw [kHz] I [A] T [Nm] T/I-ratio Note2 65 70 1.082 68 70 1.048 63 70 1.108 65 71 1.09 Energy opt.8 68 70 1.04 Overmodulation8 69 70 1.01 Overmodulation8 68 70 1.03 PWM Random16 68 70 1.04

Table A.26. Drive C: Summary of results for the load test performed with auto-matically configured parameters in SFAVM.

fsw [kHz] I [A] T [Nm] T/I-ratio Note2 63 70 1.102 66 70 1.058 63 70 1.108 62 72 1.17 Energy opt.8 60 70 1.17 Overmodulation8 62 70 1.14 PWM Random10 62 70 1.13

89


−3 −2 −1 0 1 2 3 4 5 6 7−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A]

66

−3 −2 −1 0 1 2 3 4 5 6 7−10

0

10

20

30

40

50

60

70

T

t [s]

[Nm

]

70

Figure A.20. Drive C: Phase current and torque, load test, SFAVM, fsw = 8 kHz.

−3 −2 −1 0 1 2 3 4 5 6 7−80

−60

−40

−20

0

20

40

60

80

Iu

t [s]

[A]

62

−3 −2 −1 0 1 2 3 4 5 6 7−10

0

10

20

30

40

50

60

70

80

T

t [s]

[Nm

]

72

Figure A.21. Drive C: Phase current and torque, load test, SFAVM, fsw = 8 kHz,energy optimisation.

90

A.3. DRIVE C

A.3.4 Speed Step Test

Table A.27. Drive C: Summary of results for the speed step test, 40 to 50 Hz,performed with automatically configured parameters in 60 AVM.

fsw [kHz] Acceleration Time [s] I [A] T [Nm] T/I-ratio Note2 1.50 68 64 0.942 1.25 66 64 0.962 1.15 63 65 1.022 1.10 79 52 0.66 Adjusted2 1.00 63 53 0.83 Adjusted8 1.15 68 66 0.978 1.00 69 66 0.958 1.00 68 71 1.04 Energy Opt.8 1.00 66 66 1.00 Overmodulation8 1.00 63 66 1.05 PWM Random16 1.00 68 66 0.97

Table A.28. Drive C: Summary of results for the speed step test, 40 to 50 Hz,performed with automatically configured parameters in SFAVM.

fsw [kHz] Acceleration Time [s] I [A] T [Nm] T/I-ratio Note2 1.50 69 65 0.932 1.25 65 65 1.002 1.15 63 65 1.02 Adjusted2 1.10 76 53 0.70 Adjusted2 1.00 80 51 0.64 Adjusted8 1.25 62 65 1.058 1.00 71 70 0.988 1.00 62 66 1.07 Energy Opt.8 1.00 63 67 1.06 Overmodulation8 1.00 65 66 1.02 PWM Random10 1.00 69 65 0.93

91

References

[1] "Adjustable Speed Drives as Applied to Centrifugal Pumps", Publication D-7737March 2000 Rockwell Automation International Corporation, 2000

[2] "Flygt N-Pump Series", Xylem Inc., 2010

[3] Larralde, E. Ocampo, R. "Centrifugal Pump Selection Process" World Pumps,no. 2, pp. 24-28, 2010

[4] Griggs, M. Hartzo, G. "Overload for ASD Applications - How Much IsRequired?" IEEE Transactions on Industry Applications, vol. 42, no. 4,July/August 2006

[5] Bezesky, D. Kreitzer, S. "NEMA Application Guide for AC Adjustable SpeedDrive Systems" Record of Conference Papers, Sept. 2001, pp. 73-82, 2001

[6] Mohan, N. Undeland, T. Robbins, W. "Power Electronics - Converters, Appli-cations, and Design" (3rd Edition). John Wiley & Sons. 2003

[7] Harnefors, L. "Control of Power Electronic Converters and Variable-SpeedDrives"

[8] Holmes, D. Lipo, T. "Pulse Width Modulation for Power Converters:Principlesand Practice" Wiley-IEEE Press 2003

[9] Kusko, A. and Peeran, Syed M. "Application of 12-pulse Converters to ReduceElectrical Interference and Audible Noise from DC Motor Drives", Industry Ap-plications, IEEE Transactions on , vol. 29, no. 1, pp. 153-160, 1993

[10] Norrga, S. "EJ2311 Modulation of Power Electronic Converters, Lecture Slides"KTH, 2013

[11] Franquelo, L.G. Rodriguez, J. Leon, J.I. Kouro, S. Portillo, R. Prats, M.A.M."The Age of Multilevel Converters Arrives" Industrial Electronics Magazine,IEEE, vol. 2, no. 2, pp. 28-39, June 2008

[12] Kazmierkowski, M.P. Franquelo, L.G. Rodriguez, J. Perez, M.A. Leon, J.I."High-Performance Motor Drives" Industrial Electronics Magazine, IEEE , vol.5, no. 3, pp. 6-26, Sept. 2011

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REFERENCES

[13] Ogasawara, S. Akagi, H. Nabae "A Novel PWM Scheme of Voltage SourceInverters Based on Space Vector Theory" Archiv für Elektrotechnik, Springer-Verlag, 1990

[14] "AC Drives Using PWM Techniques" Publication DRIVES-WP002A-EN-P -Rockwell Automation International Corporation, USA, June 2000

[15] Buja, G.S. Kazmierkowski, M.P. "Direct Torque Control of PWM Inverter-FedAC Motors - a Survey" Industrial Electronics, IEEE Transactions on, vol. 51,no. 4, pp. 744-757, August 2004

[16] Krause, P. Wasynczuk, O. Sudhoff, S. Pekarek, S. "Analysis of Electric Ma-chinery and Drive Systems" (3rd Edition). John Wiley & Sons. 2013

[17] Novotny, D.W. "Vector Control and Dynamics of AC Drives" Oxford UniversityPress 1996

[18] F. Blaschke "The Principle of Field-Orientation as Applied to the TransvectorClosed-Loop Control System for Rotating-Field Machines" Siemens Rev., vol.34, pp. 217-220, 1972

[19] K. Hasse "Drehzahlgelverfahren für Schnelle Umkehrantriebe mit Strom-richtergespeisten Asynchron-Kurzschlusslaufer-Motoren" Reglungstechnik, vol.20, pp. 60-66, 1972

[20] Cadirci, I. Ermis, M. Nalcacl, E. Ertan, B. Rahman, M. "A Solid State DirectOn-Line Starter for Medium Voltage Induction Motors with Minimized Currentand Torque Pulsations" Energy Conversion, IEEE Transactions on, vol. 14, no.3, pp. 402-412, September 1999

[21] Freiherr von Karnten, R.E. "Starting of Drives in the Cement Industry Re-quirements, Methods and Solutions," Cement Industry Technical Conference,2003. Conference Record. IEEE-IAS/PCA 2003, pp. 75,94, 4-9 May 2003

[22] Trzynadlowski, A. M. "The Field Orientation Principle in Control of InductionMotors", Springer Science+Business Media 1994

[23] Huai Wang, Blaabjerg, F. "Reliability of Capacitors for DC-Link Applications -an Overview" Energy Conversion Congress and Exposition (ECCE), 2013 IEEE,pp. 1866-1873, 15-19 Sept. 2013

94

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