IEEE Indicon 2005 Conference, Chennai, India, 11-13 Dec. 2005

Analysis, Simulation and Performnance Comparison

of AC Voltage Controller Fed Three Wire andFour Wire Connected Induction Motor Drives

K. Sundareswaran and Bos Mathew Joos

Abstract - The aim of this work is to evaluate and. compare theperformance characteristics. of a voltage controller driven starconnected induction, motor drives with three wire and four wsireconfigurations. Mathematical modefing and steady statesimulation of the two circuit topolog;ies are examined andperformance of the two schemes in terms of certain electricalcharacteristics such as input power drawn, input power factor,Total Harmonic Distortion (THD) of input current and motortermin al voltage etc is compared. It is shown that the three isirescheme is superior in performance when compared to four wireschemes. Experimental results are provided to supplement thecomputed results.Keywords - Induction motor, A.C. Voltage controller, modeling.

NOMENCLAT'URE

D d/d(cot)J Mcment of Inertia in Kg-M2ids, iqs 2- phlase stator curTents in Ampereidr, iqr Transformed two phase rotor currents in Amperers, Ls 2- phase stator resistance in ohms and

inductance in Henryrr,Lr 2-phase rotor resistance in ohms and

inductance in Henry referred to stator in ohmsTe Electromagnetic Torque in N-m.TL Load Torque in N-rnVds,Vqs 2- phase stator voltages in voltsLm Mutual inductance in Henry(or rotor speed in rad/secPo Number of poles

I. INTRODUCTION

T he stator voltage control of induction motors usingback-to-back connected thyristors for speed control isthe cheapest and most reliable scheme and is increasingly

employed in low to medium power applications, especially infan and pump drives. The stator voltage control of inductionmotor is achieved by employing three back-to-back con-nected thyristor pairs in each phase, in between the motor ter-

Electrical and Electronics Engineering Department,National Institute of Technology, Tiruchirappalli0-7803-9503-4/05/$20.00 C2005 IEEE

minals and power supply and the stator voltage is controlledsmoothly at line frequency- between zero and full value bysymmetrically controlling, SCR firing angle.The fundamental law relating motor speed control with this

scheme and motor input current is, established in [I]. It isshown that- peak input current is in;versely proportional tosquare root of-rotor resistance, emphasizing; that this methodof speed control is suitable for motors with high rotor resis-tance.. Each different scheme of power circuits suitable forphase control are given and it is shown that a circuit with apair ofSCRs in each phase of a star connected induction mo-tor is the best choice, when average torque per rms stator am-pere is used as the performance criterion. The closed loopspeed control of induction motor with voltage control and em-ploying digital phase locked loop scheme is reported in [2].Here, complete model of speed control of drive is designedand the schenme is evaluated experimentally. Compared toconventional method using analog devices, this method isfound to exhibit precise speed control. The design and imple-mentation ofSCR based starter suitable for 25 HP to 1000 HPinduction motors is explained in [3]. The use ofpower transis-tor for induction motor voltage control is given in [4]. How-ever, such a method is not viable for higher rating motorsowing to the non-availability of higher rating transistors.Despite the simplicity of SCR controlled induction motor

drive system, the analysis of such scheme is extremely diffi-cult due to non linear and complex nature of induction ma-chine equations. This problem was compounded in earlierdays due to large computation time required by the comput-ers. Different mathematical models were used to study thestatic response of the dlive with voltage control. The simula-tion results provide a useful information on the total systemdesign with various operating conditions. An earlier attemptin this direction is seen in [5]. However, the solution of the re-suliting equations is not carried out owing to their complexity.Nowotny and Fath [6] have used state-space approach andmatrix methods and brought out steady state solution subjectto switching constraints. Lipo [7] used this technique for theanalysis ofSCR controlled drive and the boundaiy conditionsfor the matirix differential equations are expressed as a func-tion of sytem parameters. Fourier series method is alsoadopted for the steady state analysis of such drives [8-1 1], but

133

134 IEE

computation is found to be tedious and results were not satis-factory as compared to the one obtained using state-space ap-proach. In [12],the motor is considered as a passiveresistance-inductance circuit. In the development of this cir-cuit, magnetizing current and stator resistance are includedand the amplitude and phase of the emf are determined asfunctions ofmotor torque and speed. However, the main drawback of the works in [5-14] is that only steady state solution isobtained with large number of system variables.While the analysis, implementation and related aspects are

well examined by the researchers in the past, the objective ofthe present work is to compare performance characteristics ofthyristorised voltage controller fed three phase star connectedinduction motor drives with and without neutral connection.The circuit without neutral connection is termed as three wirescheme in the present work and when the neutral of the motoris connected to the neutral of the three phase system, it is re-ferred as four wire scheme. The modeling ofthe two schemesis not of much different. But the performance characteristicsdiffer widely. It is shown in this work that a three wire schemeis more preferable than the four wire scheme due to its in-creased efficiency, reduced pollution of the line current etc.Calculated and test results are provided to validate thefindings.

Fig. I Induction Motor Drive with SCR Voltage Controller

II. SYSTEM MODELING

This induction motor is represented in the d-q axis framefixed to stator. The dynamic equations for the induction motorare given below:

ddtf Lsdiq, co,,4'di = I LsLdid, r L.

dtq, L,L,r

.,L,,,L4L

-r

4L,

L,r,L.L,L,

r,L.L,L,

C°rL.L,r,

Lr0,L,,L,

4L,L,

L,r,L,

-. L,0

I

Jd,ll

0

0 0 0

O.0L,01

L,00O

0

0

Lr-

+ Vr

10

I1)

EE Indicon 2005 Conference, Chennai, India, 11-13 Dec. 2005

where c = m and V&1 and Vqs are given byLrLs

F[-] 21 2 Vr2LvQS V3i3o° 2 - J vy

and

Vr l sin(ont)VYb LV. sin(ot - 2n /3)Vb sin(ot + 27r /3

.... (2)

.(3)

The electromagnetic torque is given by

Te = ,J PoL. (id,iqs-iidj) .(4)

The speed is govemed by

dc)r Te-Tjdt J

....(5)

111. SIMULATION RESULTS

Dedicated software is developed in matlab/simulink formodeling the drive system. Figure 2 shows the simulated mo-tor phase voltage and phase current waveforms for typical op-erating conditions. Under the same operating conditions,experiment is also conducted on the test motor with three wiresystem and four wire system and waveforms are obtained.The results are included in fig. 2.

IV. PERFORMANCE COMPARISON OF THREE ANDFOUR WIRE SYSTEMS

The performance of the drive with three and four wireschemes is compared based on certain electrical characteris-tics. These characteristics are obtained by simulation of thedrive using the model. The Computed results are verified bytaking measurements on the drive system. The details of thecharacteristics are explained below.

A. Input Power:

The power input to the drive system is calculated for the twoschemes is obtained using a Matlab model and is shown in

IEEE Indicon 2005 Conference, Chennai, India, 1-1-13 Dec. 2005

B. Inputpowerfactor:Input power factor of a drive with a power electronics inter-

face is given by the following equation:

Input PowerInput power factor - Input Voltamps

V11,cosp, - l1cosy,V,1

4J

(b)

a

(c)

(d)Fig.2. Measured and simulated voltage and current waveforms (a)Voltage waveform, (b) current waveform for a firing angle of 70

degrees with three wire system, (c) Voltage waveform,(d) current waveform for a firing angle of 100 degrees with four

wire system.

figure 3(a) for typical SCR firing angles. Measurements were

also taken on the drive system and the recorded values are

also shown in the figure. It can be seen that there is a good cor-

relation between the -measured and computed results. Furtherit is evident that the power drawn by the motor is larger withfour wire circuit than the three wire circuit for all operatingpoints ofthe drive systems. This is due to the presence ofneu-tral current with four wire scheme.

Where VI is the rms values ofthe input voltage and II is therms value of the fundamental component of supply current, Iis the rms value ofinput current and coscpj is the displacementfactor.As the per the above equation, the input power factor of the

drive is computed using the Matlab model for SCR firing an-

gle of 90 degrees and is plotted in figure3(b). The measuredresults are also shown in this figure. From the figure it is clearthat the input power factor of the drive system does not vary

much with the two schemes throughout operating domain ofthe induction motor. Thus whether the scheme is three wire or

four wire, the system power factor is not altered.

C. Total Harmonic Distortion (THD) ofinput current:

The harmonic content of the supply is now taken up as an

another electrical parameter for comparison. The THD of thesupply current is defined as follows:THD = (lh/il) where Ih is the rms value of the net harmonic

current.

For SCR firing angle of 90 degrees, THD of supply currentobtained through computation ands observation is depicted infigure 3(c). It is seen that the THD of supply current is gener-ally more with four wire scheme than the three wire circuit,except at a small speed of 1460 rpm around.

D. THD ofMotor terminal voltage

The variation of this index is plotted in figure 3(d). In thecase ofthis parameter four wire scheme offers lesser harmon-ics and more sinusoidal motor terminal voltage. This factor islittle bit high for three wire circuit.

V. CONCLUSION

The computed and measured performance characteristics ofvariable voltage star connected induction motor with threeand four wire schemes were examined in this chapter. Fromthe characteristics curves it was found that a three wirescheme is more preferable than a four wire scheme.

4DJ(a)

135

I

;.- I ! I I -1 ;4-4

--4-1

136 IEEE Indicon 2005 Conference Chennai,Lndia,l 1-13 Dec. 2005

(a)

(b)

(c)

(d)

SIMULATED EXPERIMENTALFig. 3. Simulated and Experimental performance characteristics

with 3 wire and 4 wire configurations. (a) Input power(b) Input power factor (c) Current THD (d) Voltage THD.

[4] AlexanderMozder and B.K. Bose, "Three phase AC power control us-ing power transistor$' {EEE Trans.on Industrial Applications,VoLIA-12 pp.499-505, Sep./Oct.,1976.

1[5] W.Sheperd, "On the analysis of the three phase induction motor withvoltage control by thyristor switching," IEEE Trans.on Industry andGeneral Applications, pp.304-311, May/June, 1968.

16] Nowtony and Fath,, "The analysis of induction machines controlled byseries connected semniconductor switches," IEEE Trans. OnPAS-87(2),pp597-605,1968.

[7] T.A. Lipo, "The analysis of induction motors with voltage control bysymmetrically triggered thyristors,"IEE Trans. on Power apparatus andSystems,pp.515-525, Mar/Apr. 1971.

[8] W.McMarray, "A comparative study of symmetrical three phase cir-cuits for phase controlled ac motor drives," IEE Trans. On Industrialapplications, Vol.IA-0O,pp.403-41 1, May/June 1974.

[9] M.Ramamurthy and M.F. Samek, "Steady-state analysis ofphase con-trolled induction motor drive with isolated neutral," IEEE Trans.onIECI, Vol.IECI-23, No.2, pp.178-184, May 1976.

[10] S.B. Dewan and A Straughen, Power semiconductor circuits, Wiley,London, 1975.

[11] N.H.Malik, S.M.Enamul Haque and W.Shepered, "Analysis and per-formance of three phase controlled thyristor ac voltage controllers,"IEEE Trans. on Industrial Electronics, Vol.lE-32, No.3, pp.192-199,Aug.1985.

[12] K.Padiyar and N. Prabhakaran, "Simplified analysis ofsteady state per-formance ofa voltage controlled induction motor drive," Electrical Ma-chines and Power systems, pp.149-162, 15,1988.

f 13] S.A. Hamed and B.J.Chalmers, "New method of analysis and perfor-mance prediction for thyristor-voltage controlled R-L loads," IEE pro-ceedings, Vol.137, Pt.B, No.3, pp.339-347, May 1990.

[14] S.A. Hamed and B.J.Chalmers "Analysis of variable-voltage thyristorcontrolled induction motors," IEE proceedings, Vol.134, Pt.B, No.3,pp.184-193, May 1987.

APPENDIX

REFERENCES

[I] D.A. Paice, "Induction motor speed control by stator voltage con-

trol," IEEE Trans. on Power Apparatus and systems,pp.585-590, Feb-ruary 1968.

[2] W.L.Kenley and B.K.Bose, "Triac speed control of three- phase induc-tion motor with phase locked loop operation", IEEE Trans. IndustrialApplications, Vol.IA-12,pp.492-498,Sept./Oct.,1976.

[3] J. Mungenast, "Design and application ofa solid state ac motor starter,"IEEE/lAS Annual meeting,pp.861-866,1974.

INDUCTION MOTOR DETAILS: Three phase, 400V,7.5A,5OHz, 5h.p,1400 rpm.Induction motor parameters:Stator resistance rS = 2.7 Q1Rotor resistance rr = 1.31lQMagnetising reactance Xm = 75 QlStator reactance Xs = 5.170Rotor reactance Xr = 5.17 Q

136 IEEE Indicon:2005 Conference,Chennai, India, 11-13 Dec. 2005