TABLE OF CONTENTS

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CSM 2177 – Synchronous Machines with Comparison to Inductive Wound Rotor machines

Laboratory Report

Chapter 1

IntroductionThis report is based upon laboratory experiments using wound rotor induction motors and synchronous machines as discussed in the CSM 2177 module electrical energy conversion. This experiment was undertaken on the 12/03/2015. The purpose of this experiment was to apply the theoretical concepts discussed in CSM 2177 module and link it to industrial laboratory equipment in two differing experiments. The first experiment using synchronous machines to understand the operation and the procedure required to bring a motor into synchronisation with main electric frequency. The second experiment investigated the effect of increased resistance on a wound rotor induction machine.

The following report will take account of the apparatus used in the lab and include the results recorded from the experiment. This will include graphical analysis and calculations that will be further discussed to draw comparisons between the two machines.

Synchronous Motor TheoryA synchronous motor is an Alternating Current motor that when in a steady state operation the rotation of the shaft will synchronise with the frequency of the supply current.Synchronous motors contain multiphase AC electromagnets on the stator of the motor that create a magnetic field which rotates in time with the oscillations of the line current. [1]

Synchronous Motor Experimental MethodologyTo run a machine as a synchronous motor it must first be run as a generator. To achieve this state these three conditions must be followed:

- Generated EMF must equal that of the supply- The frequency from the generated EMF must be equal to the supply (In the UK around 50Hz)- The generated EMF must be in phase with the supply

Once the machine was synchronised the following experiments we carried out:

- Torque/speed test: An exercise relating output power and torque at a given current and efficiency

- Power factor control of a three phase synchronous motor.

Figure 1: circuit diagram of equipment set-up on the right is list of equipment used.

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Universal power supply (60-105) Three phase synchronous machine and

wound rotor (64-510) Dynamometer with 68-441 panel,

motor and 68-100 single and three phase measurements panel including synchronising lamps (8-120)

Universal frame (91-240) Shaft coupling and key (68-703) System frame (91-200) Pc with (68-912) Software Patch leads (68-800)

Wound Induction Motor TheoryA wound induction motor is where the rotor windings ae connected through slip rings to external resistances. Regulating the resistance allows control of the speed and torque characteristics of the motor. [2]

Wound Induction Motor Experimental MethodologyThe summary of the procedure can be found in CSM 2177 handout assignment. The key aspects of the experiment are listed below.

- The power factor and power is measured using two wattmeters. - The line voltage to the motor should be maintained at 250v - The motor is started using the rotor resistance unit, starting at the highest setting and

finishing with an open circuit.- At high loads readings are taken quickly as possible as high loads cause motor overload.

Figure 2: circuit diagram of equipment set up for second experiment using a wound three phase slip ring motor.

Chapter 2

Experiment: Synchronous Machine results & Graphical analysis

Part A:

TorqueNm

SpeedRPM

Line Current

I1

Power Factor

3PHInput

PowerWatts

OutputPowerWatts

Efficiency%

0 2905 0.216 0.455 79 0 0.000.1 2888 0.235 0.701 125 30.24307 24.190.2 2892 0.257 0.865 160 60.56991 37.860.3 2890 0.286 0.964 200 90.79203 45.400.4 2883 0.329 1 236 120.7628 51.17

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Table 1: Results from torque/sped test

Graph 1: relationship of output power/efficiency to torque

Part B:

Minimal Load Under 0.2 LoadDc PolarCurrent

Line Current

PowerFactor

Dc PolarCurrent

Line Current

PowerFactor

340 232 0.434 305 248 0.878300 197 0.505 273 228 0.929273 174 0.55 238 215 0.951239 146 0.608 208 206 0.99195 116 0.673 195 202 1164 97 0.73 172 206 0.991137 88 0.96 148 216 -0.933114 85 0.98996 86 168 96 -0.70550 107 -0.651

Table 2: Results of experiment part B.

To calculate the output power and efficiency that is shown in table one the following two equations were used:

Output power=2πNT60 Equation 1 – Output power (watts)

Efficency=Output powerInput power

×100

N = Speed (RPM)

T = Torque (Nm)

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0 0.05 0.1 0.15 0.2 0.25 0.3 0.350

102030405060708090

100Output Power / Efficency in relation to torque

Ouput power (W)

Torque (Nm)

Out

put P

ower

(W)/

Efficie

ncy

(%)

Graph 2: Line current vs polar current in synchronous motors with minimal load and 0.2Nm load comparison

Experiment: Synchronous Machine AnalysisGraph 1 highlights a directly proportional linear relationship between torque and output power. This is due to the fact the rotor is locked to the frequency of the AC supply. This proportional relationship is similar to power factor and torque, whereas torque increases the power factor increases. This relates as well to efficiency because as the power factor approaches one, the efficiency increases and is maximised. From table 1 it can be seen that as torque increases speed remains fairly constant decreasing by no more than 20 rpm which is taken as a negligible decreasing value of speed. This is because as the load increases the load angle increases resulting in the speed remaining constant.

Graph 2 shows a shift to the right of the minimum point when a load is applied with respect to the minimum load. The minimum point on both lines is the moment when power factor is at a maximum of 1. Power factor is a value between 1 and -1. Power factor is the relationship between real power flowing to the load, to the apparent power in the circuit. A negative power factor occurs when the machine acts as a generator and power flows back to the source. A positive power factor means the machine injects reactive power. [3] At this point on the graph the synchronous machine is injecting reactive power.

Experiment: In this experiment the following equipment was used: Slip ring induction motor, slip/speed indicator, rotor resistance starter, eddy current brake, wattmeter’s (W1 & W2), 0-300V voltmeter and a 0-1A ammeter. Below is table 3 showing rotor resistance starter values.

Switch position 1 2 3 4 Short circuit

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0 50 100 150 200 250 300 350 4000

50

100

150

200

250

300

Line Current vs Polar Current(Synchronous Motors)

Minimal Load Polynomial (Minimal Load)Under 0.2 Load Polynomial (Under 0.2 Load)

DC Rotor Current (mA)

AC Li

ne C

urre

nt (m

A)

Resistance (Ω) 500 300 100 50 0Table 3: Rotor resistance starter values.

Open Circuit

TorqueNm

W1 Watts

W2Watts

SlipRPM

SpeedRPM

CurrentA

Power Factor

Output Power

Efficiency%

0 -20 85 11 1490 0.516 0.336557 0.00 0.000.1 -10 95 35 1460 0.518 0.423415 15.29 17.990.2 -5 100 54 1450 0.526 0.463002 30.37 31.970.3 5 107.5 74 1440 0.536 0.535259 45.24 40.210.4 11.25 115 97 1405 0.553 0.574867 58.85 46.620.5 16.25 125 119 1390 0.573 0.599945 72.78 51.530.6 27.5 135 140 1360 0.598 0.657539 85.45 52.59

Table 4: Results calculated for open circuit test.

300Ω

TorqueNm

W1 Watts

W2Watts

SlipRPM

SpeedRPM

CurrentA

Power Factor

OutputPower

Efficiency%

0 -20 90 95 1420 0.51 0.344865 0.00 0.00

0.1 -11.25 95 155 1355 0.513 0.414212 14.19 16.94

0.2 -2.25 100 230 1290 0.517 0.483223 27.02 27.64

0.3 1.25 110 300 1210 0.53 0.508547 38.01 34.17

0.4 8.75 117.5 357 1150 0.542 0.556763 48.17 38.16

0.5 15 125 450 1060 0.561 0.592137 55.50 39.64

0.6 25 135 500 980 0.584 0.643094 61.58 38.48

Table 5: Results calculated for 300Ω resistance.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.00

20.0040.0060.0080.00

100.00120.00

Effect of load (Torque) on output power with two levels of resistance in rotor circuit

0 OHM300 OHM

Torque (Nm)

Out

put P

ower

(w)

Graph 3: Output power against torque for short circuit and 300Ω

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.00

10.0020.0030.0040.0050.0060.00

Effect of load (Torque) on efficiency with two levels of resistance in rotor circuit

0 ohm300 ohm

Toque (Nm)

Efficie

ny (%

)

Graph 4: Efficiency over Torque

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

0.10.20.30.40.50.60.7

Effect of load (Torque) on current used with two levels of resistance in rotor circuit

0 ohm300 ohm

Torque (Nm)

Curr

ent (

A)

Graph 5: Input current against Torque

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

0.2

0.4

0.6

0.8

Effect of load (Torque) on power factor with two levels of resistance in rotor circuit

0 ohm300 ohm

Torque (Nm)

Pow

er F

acto

r

Graph 6: Power factor against Torque

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

500

1000

1500

2000

Effect of load (Torque) on speed with two levels of resistance in rotor circuit

0 ohm 300 ohm

Torque (Nm)

Spee

d (R

PM)

Graph 7: Speed against Torque

0 100 200 300 400 500 6000

0.10.20.30.40.50.60.70.8

Effect of rotor resistance on Torque-slip characteristic of the motor

0 ohm50 ohm100 ohm300 ohm500 ohm

Slip

Torq

ue (N

m)

Graph 8: Torque against slip for all tested resistances.

To obtain the graphs in this section the subsequent calculations were needed:

Efficiency=Output powerInput power

×100

Output power=Torque×Motor speed (Radians per second )

Power factor=Cosɸ=cos ¿

Graph 3 shows how output power increase in a linear fashion as torque increase in a short circuit. But at 300Ω proportionality of the two differs and as torque increases the power output increases at a lesser rate. At a torque of 0.5Nm it can be seen that an approximate 20% dip in output power occurs with a 300Ω rotor resistance.

Graph 4 is the relationship of efficiency against torque. In this graph it can be seen that the difference between short circuit and 300Ω resistance occurs at low torque. The lines diverge at around 0.1Nm of torque. At 0.5Nm both lines being to maximise and plateau with short circuit rising

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slightly more than 300Ω resistance. At the maximum it can be said that short circuit has around 15% better efficiency.

Graph 5 of input current against torque shows it’s hard to differentiate between the two resistances. Current rarely varies with the change in resistance and it can be concluded that current remains constant for any given rotor resistance. The relationship is positive so as current increase so does torque.

Graph 6 shows that power factor increases as torque increases and the change in rotor resistance has little effect on the power factor. There are slight differences are 0.3 and 0.4Nm where 300Ω has lesser values of power factor but these are negligible. Therefore similar to graph 5 it can be deduced that power factor remains the same for all resistances.

Graph 7 is the relationship between speed and torque. It can be clearly seen that as torque increase speed decreases. For an increase in rotor resistance speed drops at a faster rate, where in a short circuit at 0.5Nm torque there is a speed of 1390 rpm, for 300Ω there is a speed of 1060 rpm.

Graph 8 shows the relationship for all rotor resistances of torque against slip. The graph shows that with an increase in resistance then less torque is required to give a greater value of slip. This means that increasing resistance decreases the motor speed (Graph 7 shows this as well) and increases the difference between the rotor speed and rotating field speed.

Chapter 3To compare each machine the following four must be taken into account: cost, efficiency, starting torque and speed. For a synchronous motor the cost is usually larger than a similar standard induction motor. It is more efficient than a similar standard induction motor. The motor will run at a constant speed and is not dependant on load it will also have the same frequency as the supply. The final point the motor is not self-starting; it must be first run as a generator using an external Dc supply. Once synchronised with the supply it can then be run as a motor.

For the induction motor it has a cheaper cost compared to a similar standard synchronous motor. It is not as efficient as its counterpart synchronous motor. It is able to start without the need to run it as a generator and its speed is less than that of a synchronous motor. Because the speed is dependent on load and as load increased speed decreases (see graph 7).

The induction motor works only under a lagging power factor, whereas by changing the excitation current a synchronous motor can run on a leading power factor. Induction motors of the two are considered more durable in harsher conditions due to the absence of brushers. This makes there construction simple and there maintenance time much lower.

Applications of synchronous motors include; Crushers, rotary kilns, power-houses to improve power factor and electricity generators at power stations. Applications of induction motors include; Lifts, cranes, compressors and pumps.

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ConclusionTo summarise, this report has successfully compared induction motors and synchronous motors by using the data calculated from lab results that have been presented in graphical and tabular form. The theory outlined in CSM module 2177 Electrical energy conversion has been tied into the findings of the experiments. Real world applications have been shown in the discussion.

References[1]: Fitzgerald, A. E.; Charles Kingsley, Jr.; Alexander Kusko (1971). "Chapter 6, Synchronous machines, steady state". Electric Machinery, 3rd Ed. USA: McGraw-Hill. pp. 283–330. Library of Congress Catalog No. 70-137126.

[2]: Harold J. Herbein Rotating Machinery, Rinehart Press, 1971 SBN 03-084675-7, pages 215-218

[3]: Almarshoud, A. F. (2004), et al, "Performance of Grid-Connected Induction Generator under Naturally Commutated AC Voltage Controller", Electric Power Components and Systems 32 (7),

Table of Figures

Figure Page

Figure 1 2

Figure 2 3

Table 1 3

Graph 1 4

Table 2 4

Graph 2 5

Table 3 5

Table 4 6

Table 5 6

Graph 3 6

Graph 4 7

Graph 5 7

Graph 6 7

Graph 7 8

Graph 8 8

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