STUDY OF ELECTROMAGNTUDY OF ELECTROMAGNETIC … · study of electromagnstudy of electromagnetic...

STUDY OF ELECTROMAGNSTUDY OF ELECTROMAGNSTUDY OF ELECTROMAGNSTUDY OF ELECTROMAGNETIC INTERFERENCE INETIC INTERFERENCE INETIC INTERFERENCE INETIC INTERFERENCE IN INVERTER DRIVEN ASYNINVERTER DRIVEN ASYNINVERTER DRIVEN ASYNINVERTER DRIVEN ASYNCHRONOUS MOTORCHRONOUS MOTORCHRONOUS MOTORCHRONOUS MOTORSSSS

Buletinul AGIR nr. 3/2012 ● iunie-august 1

STUDY OF ELECTROMAGNSTUDY OF ELECTROMAGNSTUDY OF ELECTROMAGNSTUDY OF ELECTROMAGNETIC INTERFERENCE INETIC INTERFERENCE INETIC INTERFERENCE INETIC INTERFERENCE IN INVERTER INVERTER INVERTER INVERTER DRIVEN ASYNCHRONOUS DRIVEN ASYNCHRONOUS DRIVEN ASYNCHRONOUS DRIVEN ASYNCHRONOUS MOTORMOTORMOTORMOTORSSSS

Eng. Ioan ŢILEA PhD-student1, Prof. Eng. Călin MUNTEANU PhD

1

1 Technical University of Cluj-Napoca, Electrical Engineering Faculty

REZUMAT. REZUMAT. REZUMAT. REZUMAT. Interferenţele electromagneticInterferenţele electromagneticInterferenţele electromagneticInterferenţele electromagnetice conduse din motoarele asincrone alimentate de invertoare PWM sunt o e conduse din motoarele asincrone alimentate de invertoare PWM sunt o e conduse din motoarele asincrone alimentate de invertoare PWM sunt o e conduse din motoarele asincrone alimentate de invertoare PWM sunt o problemă actuală în sistemele de acţionare cu turaţie variabilă. Interferenţele electromagnetice conduse din problemă actuală în sistemele de acţionare cu turaţie variabilă. Interferenţele electromagnetice conduse din problemă actuală în sistemele de acţionare cu turaţie variabilă. Interferenţele electromagnetice conduse din problemă actuală în sistemele de acţionare cu turaţie variabilă. Interferenţele electromagnetice conduse din aceste aceste aceste aceste sistemesistemesistemesisteme se datorează motorului electric şi a cablului de alimentare. Ase datorează motorului electric şi a cablului de alimentare. Ase datorează motorului electric şi a cablului de alimentare. Ase datorează motorului electric şi a cablului de alimentare. Analiza caracteristicii impedanţei motorului electric naliza caracteristicii impedanţei motorului electric naliza caracteristicii impedanţei motorului electric naliza caracteristicii impedanţei motorului electric şi a cablului de alimentare sunt fundamentale pentru interpretarea şi a cablului de alimentare sunt fundamentale pentru interpretarea şi a cablului de alimentare sunt fundamentale pentru interpretarea şi a cablului de alimentare sunt fundamentale pentru interpretarea emisiiloremisiiloremisiiloremisiilor de mod comun şi diferenţialde mod comun şi diferenţialde mod comun şi diferenţialde mod comun şi diferenţial din din din din sistemele sistemele sistemele sistemele de acţionare cu turaţie variabilăde acţionare cu turaţie variabilăde acţionare cu turaţie variabilăde acţionare cu turaţie variabilă.... Cuvinte cheie:Cuvinte cheie:Cuvinte cheie:Cuvinte cheie: supratensiuni la motor, invertor motor, interferenţe electromagnetice. ABSTRACT. ABSTRACT. ABSTRACT. ABSTRACT. Conducted electromagnetic interference (EMI) in induction motor fed from pulse width modulation (PWM) Conducted electromagnetic interference (EMI) in induction motor fed from pulse width modulation (PWM) Conducted electromagnetic interference (EMI) in induction motor fed from pulse width modulation (PWM) Conducted electromagnetic interference (EMI) in induction motor fed from pulse width modulation (PWM) inverters is one of the most difficult current technical problems in variable speed system. The conductedinverters is one of the most difficult current technical problems in variable speed system. The conductedinverters is one of the most difficult current technical problems in variable speed system. The conductedinverters is one of the most difficult current technical problems in variable speed system. The conducted EMI emissions EMI emissions EMI emissions EMI emissions in inverter motor system is related to the inverter load which is the induction motor and cable. The analysis of the in inverter motor system is related to the inverter load which is the induction motor and cable. The analysis of the in inverter motor system is related to the inverter load which is the induction motor and cable. The analysis of the in inverter motor system is related to the inverter load which is the induction motor and cable. The analysis of the impedance characteristic of the motor and cable is fundamental for understanding the common and differential mode impedance characteristic of the motor and cable is fundamental for understanding the common and differential mode impedance characteristic of the motor and cable is fundamental for understanding the common and differential mode impedance characteristic of the motor and cable is fundamental for understanding the common and differential mode of emissioof emissioof emissioof emissions. ns. ns. ns. Keywords:Keywords:Keywords:Keywords: motor overvoltage, inverter-fed motor, electromagnetic interference.

1. INTRODUCTION

More than 60% of the world’s energy is used to

drive electric motors. Due to growing requirements of

speed control, pulse width modulation inverters are

used in adjustable speed drives. Rapid developments in

semiconductor technology have increased the switching

frequency of power electronics dramatically. Increase in

the carrier frequency of pulse width modulation and the

faster switching rates of the power electronics can

induce serious problems in inverter fed induction motor

drive system.

One of the most difficult technical problems is the

generation of high frequency currents flowing in all

parts of the drive system due to capacitive couplings in

the motor drive system. As shown in Figure 1, a modern

motor drive system consists of a filter, a rectifier, a DC

link capacitor, an inverter and an AC motor [2]. The

EMI problems are very high in the cable, motor area

because of the high frequency of the inverters output

voltage.

Many small capacitive couplings exist in the motor

drive systems which may be neglected at low frequency

analysis but the conditions are completely different at

high frequencies were the influence of the parasitic

capacitance is noticeably higher.

When the inverter is connected to the motor through a

long cable because of the cable inductance, stray

capacitance distributed between the cable wires and high

rise times of the PWM voltage from the inverter,

overvoltages appear at the motor terminals [1], [9], [12].

Overvoltages at the motor terminal stress the motor

winding insulation reducing its life and causes partial

discharges that damage the insulation.

Common mode voltage creates shaft voltage through

electrostatic couplings between rotor and stator

windings and the rotor and a frame which can cause

bearing currents when the shaft voltage exceeds a

breakdown voltage level of the bearing grease [3], [5].

The overvoltage and conducted EMI problems in

variable speed system with high power motors and long

cables can be clarified by motor impedance analysis.

This paper analyzes conducted EMI generated by a

PWM inverter fed induction motor drive system

analyzing the overvoltages occurring at the motor

terminals when the motor is connected to a PWM

inverter through different cable lengths.

Using a finite element analyses (Maxwell 3D

software) for the extraction of the induction motor high

frequency equivalent circuit and a circuit analysis

(Simplorer) for the inverter fed induction motor drive

system.

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WORLD ENERGY SYSTEM CONFERENCE – WESC 2012


Fig.1. Motor drive system with capacitive couplings.

2. ASYNCHRONOUS MOTOR HIGH FREQUENCY MODEL

At low frequencies, the equivalent circuit of an

electric motor consists of inductances and resistances

without considering the motor capacitances. At high

frequencies, the electric motor can be modeled as

distributed capacitors, inductors and resistors. The stray

capacitance of electric motors is very important in

predicting EMI problems [6-7].

Asynchronous motor winding physical construction

is very complicated and detailed determination of its

capacitance is difficult.

Using Maxwell software a finite element analysis of

the induction motor is carried out to obtain the

inductance and capacitance of the motor. The numerical

3D model of the induction motor simulated in Maxwell

is presented in Figure 2. For an accurate determination

of the induction motor capacitance and inductance a 3D

model of the motor must be analyze for the reason that

it incorporates the end winding of the induction motor.

To acquire the equivalent inductance of the

induction motor a magnetostatic simulation must be

done and for the equivalent capacitance of the motor an

electrostatic simulation. Using the results from the

magnetostatic and electrostatic simulations an

asynchronous motor equivalent circuit can be obtained

consisting of inductances and capacitances.

The induction motor inductances and capacitances

obtained from the Matrix solution in Maxwell are

exported in Simplorer for circuit analyses. The

investigated motor is three phase induction motor with

an output power of 11 kW, rated voltage 400 V and a

speed of 900 rpm.

Fig.2. Numerical model of the induction motor.

3. OVERVOLTAGE ANALYSIS OF INVERTER FED INDUCTION MOTOR

For analyzing the overvoltages at the motor terminals

when the motor is connected to a PWM inverter the

simulation model presented in Figure 3, is used.

The model consists of an inverter, equivalent high

frequency circuit of the cable and the induction motor

capacitance and inductance imported from the finite

element analysis.

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Buletinul AGIR nr. 3/2012 ● iunie-august



Fig.3. Inverter fed induction motor simulated model

Overvoltage in inverter fed induction motor is due to

parasitic capacitance in the cable and the motor [4], [10],

[11]. Based on the results from a frequency sweep of the

motor it is possible to determine the resonance

frequency, which is the result of the winding parasitic

capacitance.

Current measurements from a frequency sweep of

the induction motor are presented in Figure 4. The

resonance frequency of the induction motor, when the

motor is connected to the inverter through a 1m long

cable (Fig.4.a), is 3 kHz, this is the frequency range

were there is a minimum load and this voltage

component is not attenuated by the motor. The

resonance frequency of the inverters load is

significantly affected by the motor cable (Fig.4.b) the

resonance frequency with a 10 m long cable is 1 kHz

and with a 50 m long cable is 0.5 kHz (Fig.4.c).

The parasitic capacitance of the motor cable moves

the resonance frequency towards the lower ranges. It is

a key reason for appearance of serious overvoltage

problems in inverter fed induction motor drive system

with long motor cable. Identifying the resonance

frequency of the inverter load (motor and cable) is

helpful for avoiding serious overvoltage problems in

inverter driven motor system.

a) Motor connected to the inverter through a 1m cable;

b) Motor connected to the inverter through a 10m cable;

c) Motor connected to the inverter through a 50m cable;

Fig.4. Current measurements from a frequency sweep when the

inverter is connected to the motor through different cable lengths.

When a PWM voltage is applied to the motor

through a long cable, the parasitic capacitance and

inductance of the cable generates overvoltages at the

motor terminals.

The moment that the PWM voltage is “off” because

of stored energy in the cable inductance the parasitic

capacitance of the cable is charging.


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When the PWM voltage is “on” the motor is subjected

to a theoretical surge of two times the amplitude of the

inverter output voltage. With each cycle “on” and “off”

of the PWM voltage the parasitic capacitance of the

cable is being charge and discharge.

a) Inverter output voltage;

b) Motor terminal voltage;

Fig.5. Inverter and motor terminal voltage waveforms.

In Figure 5, the inverters output voltage and motor

terminal voltage is presented, the overvoltage at the

motor terminal is clearly visible (Fig.5.b) due to the

parasitic capacitance of the motor cable, were the

inverters output voltage presents no voltage spikes

(Fig.5.a). The inverter output voltage has an amplitude

of 500 V and a fundamental frequency of 50 Hz with a

carrier frequency of 3 kHz, the induction motor is

connected to the inverter through a 1m cable (Fig.5).

In Figure 6 the voltage at the motor terminal is

presented when the motor is connected to the inverter

through different cable lengths and the carrier

frequency of the inverter output voltage is 1 kHz.

When the motor cable has a 1 m length the

overvoltage at the motor terminal (Fig.6, a) is clearly

visible, but the number of oscillations are fewer because

the resonance frequency of the cable and motor is

higher than the 1 kHz carrier frequency of the inverter

output voltage.

If the inverter is connected to the motor through a 10

m cable, the overvoltage at the motor terminal (Fig.6, b)

becomes a serious problem because the 1 kHz carrier

frequency of the inverter output voltage is much closer

to the resonant frequency of the inverters load (cable

and motor), thus the number of overvoltage oscillations

are much higher. When a 50 m cable is used to supply

the motor from the PWM inverter, the motor terminal

voltage is severely affected by the high parasitic

capacitance of the cable (Fig.6, c).

The parasitic capacitance of the motor cable moves

the resonance frequency towards lower frequencies,

amplitude and oscillation number of the overvoltage are

getting higher with cable length.

a) Motor connected to the inverter through a 1m cable;


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4. CONCLUSIONS

Determining the resonant frequency of the motor

and cable is very helpful in predicting serious EMI

problems in inverter driven motor system.

Overvoltages in inverter fed induction motor system,

with long motor cable can be predicted using the

method presented in this paper.

Finite element analysis of the motor for extracting

the motors high frequency equivalent circuit is useful

when the motor is still a project, before entering

production.

ACKNOWLEDGMENT: This paper was supported by the

project "Improvement of the doctoral studies quality in

engineering science for development of the knowledge based

society-QDOC” contract no. POSDRU/107/1.5/S/78534,

project co-funded by the European Social Fund through the

Sectorial Operational Program Human Resources 2007-2013.

BIBLIOGRAPHY

[1] Luszcz J., Iwan K., Conducted EMI propagation in inverter

fed AC motor, Electrical Power Quality and Utilisation,vol. 2,

2006.

[2] Zare, F. EMI in modern AC motor drive systems, IEEE

International Symposium on EMC, 2007.

[3] WEG Equipamentos Eletricos S.A. Induction motors fed by

PWM frequency inverters. Technical Guide, 2006.

b) Motor connected to the inverter through a 10m cable;

c) Motor connected to the inverter through a 50m cable;

Fig.6. Voltage waveforms measured on the motor terminal when the inverter is connected to the motor through different cable

lengths.


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[4] Kosei T. , Kotaro W. Propagation of inverter surge and

voltage distribution in motor winding, Electrical Engineering

in Japan, vol. 161, 2007, pp. 771–777.

[5] GAMBICA/REMA Working Group, Motor shaft voltages

and bearing currents under PWM inverter operation, Technical

Guide, 2002.

[6] Arnedo L., Venkatesan K., High frequency modeling of

induction motor drives for EMI and overvoltage mitigation

studies, Electric Machines and Drives Conference, IEMDC'03,

IEEE International.

[7] Moreau M., Idir N., Modeling of Conducted EMI in Adjustable

Speed Drives, Electromagnetic Compatibility, IEEE

Transactions, 2009, pp. 665 – 672.

[8] Bartos S., Dolezel I., Theoretical and experimental

investigation of parasitic effects in induction motor drives

supplied from semiconductors inverters, Acta Electrotechnica et

Informatica ,vol. 8, 2008, pp. 3-9.

[9] Ogasawara S., Akagi H., Analysis and reduction of EMI

conducted by a PWM inverter-fed AC motor drive system having

long power cables, Okayama University Digital Information

Repository, 2000.

[10] Mutoh, N. Konuma T., A motor-terminal surge and induced

EMI noise controlling method effective in EVs with the high

density packaging structure, Vehicle Power and Propulsion

Conference, VPPC '08, IEEE, pp. 1-7.

[11] Włodek R. , Roehrich J., Analysis of overvoltage on motor

winding insulation fed by PWM pulses, High Voltage

Engineering and Application (ICHVE), International

Conference 2010, pp. 584 – 587.

[12] Wang L., Ngai-Man Ho C., High-Frequency Modeling of the

Long-Cable-Fed Induction Motor Drive System Using TLM

Approach for Predicting Overvoltage Transients, IEEE

Transaction on Power Electronics, vol. 25, no. 10, 2010.

About the authors Eng. Ioan ŢILEA PhD-student.

Technical University of Cluj-Napoca, Electrical Engineering Faculty, Department of Electrotechnics and

Measurements, Baritiu Street 26-28, 400027 Cluj-Napoca, Romania.

email: [email protected]

Graduated at the North University of Baia Mare, Engineering Faculty, study program – Electromechanic.

From 2011 he is a PhD student at the Technical University of Cluj-Napoca, Department of Electrotechnics and

Measurements. His research topic is electromagnetic phenomenons in electric motors powered in a harmonic

polluted regime.

Prof. Eng. Călin MUNTEANU PhD.

Technical University of Cluj-Napoca, Electrical Engineering Faculty, Department of Electrotechnics and

Measurements, Baritiu Street 26-28, 400027 Cluj-Napoca, Romania.

email: [email protected]

He received the MSc. degree in electrical engineering from Technical University of Cluj-Napoca, in 1989, and the

PhD degree in electrical engineering in 1999. He joined the Electrotechnics Department from Technical

University of Cluj-Napoca in 1991. Since 2003 he is Professor and the Head of the EMC Laboratory. His scientific

work is related to EMC, electromagnetic fields, numerical computation, optimal design techniques.


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