Mem_Sch_Eng_OU_4_65

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MEMOIRS OF THE SCHOOL OF ENGINEERING, OKAYAMA UNIVERSITY, Vol. 4, No. I, SEPTEMBER 1969

On the Speed Control of a Three Phase Squirrel CageInduction Motor Controlled by a Variable Frequency

Three Phase Thyristor InverterToyoji HIMEl, Takeshi FUJITSUKA and Jyunichi INOUE

Department of Electrical Engineering

(Received June 10, 1969)As one of the wide application of thyristor ci rcuits , the inverter has a

promising future and has been investigated vigorously. On the speed controlof a squir rel cage induction motor by using a variable frequency thyristorinver ter, a lthough few papers have been presented , there seems to be manyproblems to be solved imminently. In this paper, the stability of performanceof a thyristor inver ter on this theme has been confirmed and some particularFoints with relat ion to practical use also have been discussed.

D-CSOURCE

dopted inverter is a type of thyristor threephase improved bridge inverter by Dr. Sato3).The reversal of motor revolution was made bychanging the phase sequence of the inverter.Also, the feedback of motor speed was addedfor the constant speed control. 2. Circuit Configuration

2.1 Inverte r Circu i tThe thyristor three phase improved bridge

inverter which was adopted, is showen in Fig.1. This inverter has features that the peak ofload voltage, when an inductive load, is restained within the source through the feedbackof reactive energy and the high efficiency iskept for the load, the power factor of whichvaries in wide range.

Fu Fv Fw

L

LFig. 1. Inverter circuit.

Ls

c.

1 IntroductionThe speed control of an induction motor by

using a variable frequency thyristor inverter,has distinctive merits such as compact size,easy maintenance and low cost over the methodof doC machinery. Also, compared with theconventional secondary resistor control of thewound-rotor induction motor, the speed cont.rol by a variable frequency thyristor inverterhas higher efficiency, lower cost and need nota wound-rotor induction motor.On the speed control of induction motors

by thyristor inverters, few papers!,2) have beenpresented. However, there seems to be manyproblems that should be solved to be pu tinto practical use such that rectangular waveforms of the inverter result inthe reduced efficiency; thatabrupt changes of load and ofpower factor affect the stabilityof operation; that the failureof the inverter performance canbe caused by outer noises; andthat noises generated by theinverter in turn will affect otherinstallations.

Authors attempted to makeclear the problems concomitantto driving a squirrel cage induction motor with a thyristorinverter. In this paper, experimental data and some considerations are presented. The a-

65

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66 T. HIMEl, T. FUJITSUKA and J. INOUE (Vol. 4,2.2 Auxiliary CircuitThe block diagram of auxiliary circuits, is

indicated in Fig. 2. The pulses, generated bythe UJT generator, are distributed by thethyristor r ing counter to six pulse arrays eachof which differs from 60 degrees and has apulse width of 60 degrees. The logic circuitreverses the phase sequence of pulses whichare fed to Royer generators. The Royer generators insulate the pulses in order to meet therequiring from the inverter. Then, the output

pulses of the Royer generators about 7 kHzare converted by full-wave rectifying devices.2.3 Constant Speed Regulation CircuitThe regulation of constant speed, if errors

exist, is achieved by adjusting the inverterfrequency to the reference speed. In Fig. 3,the difference of voltage between that of reference and of tachometer-generator 2Eo-E s=Eo+ (Eo- Es) applies to the emitter-base of atransistor Q. At equilibrium, the voltage iJE

UJTgeneratorthyristorsix-stager ing counter

logic circuit forreversion ofphase sequenceRoyer generator& rectifyingdevices thyristor

f - - - ~ U

f.------.--) I,

f . - - - ~ Z

r - - - - - ~ Vr - - - - - - ~ xI - - - - ~ ~ W

- - - - { ] - ~ y

backwardFig. 2. Block diagram of auxiliary circuit.

v

tachometergenerator

Fig. 3. Constant speed control circuit. 1 = l8(V), 2= l8(V)

Reference

3-,/, ;I-C source

Auxiliarycircuit

T

Rectifying devices

J.Inverter

lInductionmutor D-C generator

Tachometergenerator

Fig. 4.:L Block diagram of constant speed control.

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1969) On Speed Control o f Induction Motor by Thyristor Inverter 67

Fig. 5. Wave forms of load voltages and currents.Upper traces: load current (vertical scale: 100V/cm, I:Grizontal scale: 5 msec/cm)Lower traces: load current (vertical scale: 1 A/cm, horizontal scale: 5 msec/cm)Input voltage: 100 (V )(a) operation at 60 Hz , wt ih no capac it or an d motor with no load.(b) operation at 60 Hz , with capacitor and motor with no load.(c) operation at 60 Hz , with no capacitor an d motor with load.(d) operation at 60 Hz, wi th capaci to r an d motor with load.(e) operation at 80 Hz , with no capacitor an d motor with load.(0 operation at 80 Hz , with capacitor and motor with load.

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68 T. HIMEl, T. FUJITSUKA an d J . INOUE (Vol. 4,is equal to zero. I f the voltage of the tachometer-generator rises, LIE shifts to minus andthe frequency of UJT oscillation diminishes,and vice versa.2.4 Block DiagralUThe block diagram of the cons tan t speedregulation of an induction motor is shown in

Fig. 4. 3. Driving Characterist ics of Induction

Motor3.1 Wave ForlUsThe wave forms of inverter output are rec

tangular and includes the fifth harmonicscomponent as the lowest harmonics. In orderto decrease the distortion rate o f wave forms,the filter is needed. But, here, the only capacitance was connected to improve the performance stability of the inverter.

Without capacitors, as illustrated in Fig. 5,the wave forms are decisively affected upon

varying load conditions. With capacitors, however, the var ia tion of wave forms is hardlyrecognizable between with and without load.Figs. 5 (e) and (f ) display that this holds trueon different frequencies.

3.2 Constant Speed RegulationThe characteristics of constant speed regulat ion are given in Fig. 6. The speed variationratio with and without the feedback is expressed as functions of load current . Where,the variation ratio is denoted by

N.-NI; = T X 100 [percent]No: speed with no load (rpm)N : actual speed (rpm)The doc source voltage, however, was not

regulated proportionally to the frequencies ofthe inverter. The curves in Fig. 6 shows thatthe deviation of motor speed with speed control is very small. The difference of the vari-

(%) (%)10 6.S';; 8 .S.... ';; no f e e d b " c ~ ..,o: / / .....S 6

....O ....llO feedback >: 4 /. / .S $ ) ', /.... ,D' . , /l 4 / ,0> ,D / .... /, l < }/"d ,0 ' > 2 /'" 2" " "d /0/g. _ 0 - fecdb""k " /0'- " /1;'a g. 0' /a 0.5 1.0 1.5 (A ) feedback

load current a(a) a 0.5 1.0 1.0 (A)

(%) load current14 (c)

/2 Il o feedback P (%).S I 10~ ' I II .St:i J{ ';; 8 no feedback ~ .S / .... / I';; 8 / t:i ,9 / .S /.>; / 6 , 0/ . /l 6 / /> .- .... ,.-0'"d /' 0 l 4 ," / > ,P '" 4 , 9 ' , ,g. , "d / 0'- " 2 0 - /0.. . . .. . .. . 0 " 0" - - - feedback2 feedback a- aa a 0.5 1.0 1.5 (A)a 0.5 1.0 1.5 (A) load currentload current (d)

(b)Fig. 6. Control characteristics of constant speed regulation.

(a) No: 1500 (rpm); doc input voltage: 70 (V) (c) No: 2100 (rpm); doc input voltage: 100 (V)(b) No: 1800 (rpm); doc input voltage: 70 (V) (d) No: 2800 (rpm); doc input voltage: 100 (V)

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1969) On Speed Control of Induction Motor by Thyristor Inverter 69

feedback

no feedback

ation ra tio from a reference speed due to themagnetic saturation of th e induction motorand the nonlinear characteristics of the de-tecting tachometer-generator.Under the same condition, the transient

response has been examined and is indicatedin Fig. 7. The maximum speed deviation issuppressed to 10 rpm with feedback, comparedwith 140 rpm deviation without a feedback.The response time is about two seconds.3.3 Efficiency and Power FactorThe measured efficiency of the inverter

driving the induction motor, is shown in Fig. 8with frequency as parameter. The output

rpm150I:i.S 10050'C O-t--i---r--r--,..--,.----,r---,-

1l rpmll) 50o - I - ~ = T = = r = . , . . - - r - - . . . - - - , - - -

voltages of the inverter were constrained to150 V. The efficiencies were calculated with

where,E : input voltage of inverterI : input current of inverterW: output of inverter

The high efficiency is obtained with low fre-quencies even in the range of heavy load current.The power factor decreases, if wave forms

of voltage differs from those of current. Themeasured curves of power factor of the inver-ter with the induction motor are shown inFig. 9 as functions of load cur rent with fre-quency as parameter. The power factor ad vances high wi th the increased frequency forrelatively small currents, but reverses for heavycurrents.

('Yo)100

40

;- .-.- ,/ r:f I,I III II II II II II I .-.--e 40 HzI I, ,I I 0 - -0 - -0 5 QI If IJ I - -60II I1 I0 I 0-0-0120

2 3 CA)output current

Fig. 9. Power factor.

o o

20

E 60u....ll):>8.. 40

80

3.4 Trans ient BehaviorThe reversal of motor revolution is achieved

by changing t he phase sequence of inverteroutput, which provides th e motor a pluggingand makes much loss resu lt in the rotor. Thedissipation of loss can be expressed by

32 Jw 2 (Joule)(A )

3 sec.

2 :3

time

0- -0 - -0 120

Fig. 7. Response.

output currentFig. 8. Efficiency.

o

80

o o

20

('Yo)100

60u>::ll)'uSll)

W"D 20'".s

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70 T. HIMEl, T. FUJITSUKA and J. INOUE (Vo!. 4,which should be within the ratings of the mo-to r at continuous operation. Figs. 10 and 11indicate the transient performance of the mo-to r reversal. At 60 Hz, as shown in Fig. 11,the period to a standstill is about 0.17 seconds

inputcurrentinput

voltage

outputcurrent

outputvoltage

reactivefeedbackcurrent

revolutionperminutes

voltageacrosscapacitor

voltageacrossthyristor ._

reversal signal

and that to a reversal reference speed is about0.5 seconds. On the other hand, at 40 Hz,the per iod to a standstill is about 0.08 seconds,and about 0.25 seconds were necessary to thereversal reference speed from standstill. The

1/IO[sec]Fig. 10. Transient response of the reversal of an induction motor, operation at 60 Hz.

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time of the completion of reversal at 60 Hz isaround two times to 40 Hz. I t should be notedthat rush currents flow at the instant ofreversing. From the inspection of Figs. 10 and

II , the peak value which must be within theratings of thyristors and diode devices, issix times over the normal value.

input current

input voltage

output current

output vOltage

revolution perminutes

voltage acrosscapacitor

voltage acrossthyristor

Fig. 11. Transient response of the reversal of an induction motor, operation at 40 Hz.

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72 T. HIMEl, T. FUJITSUKA an d ] . INOUE (Vol. 4,

regulate doc input voltage proportional to thefrequency of the inverter.(2) Noise PreventionThe radio frequency interference4l results

from the turn-on and turn-off switching per-formance of thyristors and is not par ticularphenomenon subsequent to the performance ofthyristor inverters. RFI is conveyed throughboth electric conduction and radiation. Toprevent the RFI, the following methods willbe very effective.( i) add small inductance to suppress the

abrupt rise of current.(ii) add high frequency filter to input and

output line.(iii) screen the installations with electro-static shielding.Disturbances to the inver ter are div ided

into two kinds: that is, one to the auxiliary circuit and the o ther to doc source.The false conduction of thyristors origi-nating from dis turbances to the sourcel ine occurs when the voltage exceeds overthe breakover value of thyristors or itsdv/dt surpasses allowance ratings. Thiskind of disturbances can be suppressedconsiderably by connecting the anodecathode of the thyristor with a R-C filter.Disturbances to auxil iary circuits inducethe noise voltage in gate wires and disturbthe performance of inverter or sometimesleads to a complete malfunction. To pre-vent the influence of noise voltages, theselection of an auxiliary circuit configura-tion must be stressed to the stability ofperformance. Noise voltages can be by-passed fairly well by connecting a resistor-capactor in para ll el across the gata-cathode of the thyristor. 5. ConclusionThe stability of operation was confirmed

with relat ion to the speed control of asquirrel cage induction motor controlledby a variable frequency three phase thyris tor inver ter. Some particular points ofthe thyristor inverter associated with apractical use were discussed.The authors are indebted to Mr. K.

Kawashima, Mr. T. Yoshida, Mr. M.Okada and Mr. K. Mor iwake for theirassistance in the experiments and the pre-paration of Figures and photographs.

(b)Fig . 12. Operat ion of irregular revolution at 2100(rpm).Upper t races : voltage (vertical scale: 100V/cm, ho-rizontal scale: 100 msec/em)Lower traces: voltage (vertical scale: 1 A/em, ho-rizontal scale: 100 msec/em)

(a) normal operation at doc input 85 (V)(b) irregular revolution at doc input 110 (V )

4. Discussion(1) Magnetic Saturation of Induction MotorFor the purpose of driving a motor, the doc

source voltage should be regulated proportionally to the performance frequency of an in-verter, unless the magnetic saturation of themotor causes load current to increase with thedecreased frequency. This current in turncauses the "irregular revolution" of the motor.Wave forms of voltages and currents were pre-sented in Fig. 12, compared with those ofnormal operation. In order to avoid the magnetic saturation, it will be most convenientmethod to uti lize thyristor rectifying circuitsor thyristor doc chopper control circuits to

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References1) N. SATO and I. JUR I : Toshiba Review 18, (1963)

No.7, 754

2) E. ONO and M. AKAMATSU: Mitsubishi ElectricReview 38, (1964) No.6 , 931

3) N. SATO: J. I. E. E. of Japan 84, (1964) 7894) G. E.: SCR Manual, Third Edition (1964)

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Mem_Sch_Eng_OU_4_65