As one of the wide application of thyristor circuits, the inverter has a promising future and has been investigated vigorously. On the speed control of a squirrel cage induction motor by using a variable frequency thyristor
inverter, although few papers have been presented, there seems to be many problems to be solved imminently. In this paper, the stability of performance of a thyristor inverter on this theme has been confirmed and some particular points with relation to practical use also have been discussed

Fujitsuka, Takeshi

Himei, Toyoji

Inoue, Jyunichi

Okayama University Scientific Achievement Repository

MEMOIRS OF THE SCHOOL OF ENGINEERING, OKAYAMA UNIVERSITY, Vol. 4, No. I, SEPTEMBER 1969On the Speed Control of a Three Phase Squirrel CageInduction Motor Controlled by a Variable FrequencyThree Phase Thyristor InverterToyoji HIMEl, Takeshi FUJITSUKA and Jyunichi INOUEDepartment of Electrical Engineering(Received June 10, 1969)As one of the wide application of thyristor circuits, the inverter has apromising future and has been investigated vigorously. On the speed controlof a squirrel cage induction motor by using a variable frequency thyristorinverter, although few papers have been presented, there seems to be manyproblems to be solved imminently. In this paper, the stability of performanceof a thyristor inverter on this theme has been confirmed and some particularFoints with relation to practical use also have been discussed.D-CSOURCEdopted 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 Configuration2.1 Inverter CircuitThe thyristor three phase improved bridgeinverter which was adopted, is showen in Fig.1. This inverter has features that the peak ofload voltage, when an inductive load, is re-stained 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 FwLLFig. 1. Inverter circuit.Lsc.§ 1 IntroductionThe speed control of an induction motor byusing 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 motorsby thyristor inverters, few papers!,2) have beenpresented. However, there seems to be manyproblems that should be solved to be putinto 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 in-duction motor with a thyristorinverter. In this paper, experi-mental data and some conside-rations are presented. The a-6566 T. HIMEl, T. FUJITSUKA and J. INOUE (Vol. 4,2.2 Auxiliary CircuitThe block diagram of auxiliary circuits, isindicated in Fig. 2. The pulses, generated bythe UJT generator, are distributed by thethyristor ring 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 gene-rators insulate the pulses in order to meet therequiring from the inverter. Then, the outputpulses of the Royer generators about 7 kHzare converted by full-wave rectifying devices.2.3 Constant Speed Regulation CircuitThe regulation of constant speed, if errorsexist, is achieved by adjusting the inverterfrequency to the reference speed. In Fig. 3,the difference of voltage between that of refe-rence and of tachometer-generator 2Eo-Es=Eo+ (Eo- Es) applies to the emitter-base of atransistor Q. At equilibrium, the voltage iJEUJTgeneratorthyristorsix-stagering counterlogic circuit forreversion ofphase sequenceRoyer generator& rectifyingdevices thyristorf---~Uf.------.--)I,f.---~ Zr-----~Vr------~xI----~~W----{]-~ybackwardFig. 2. Block diagram of auxiliary circuit.vtachometer-generatorFig. 3. Constant speed control circuit. £1 = l8(V), £2= l8(V)Reference3-,/, ;I-C sourceAuxiliarycircuitTRectifying devicesJ.InverterlInductionmutor ~ D-C generator~TachometergeneratorFig. 4.:L Block diagram of constant speed control.1969) On Speed Control of Induction Motor by Thyristor Inverter 67Fig. 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, wtih no capacitor and motor with no load.(b) operation at 60 Hz, with capacitor and motor with no load.(c) operation at 60 Hz, with no capacitor and motor with load.(d) operation at 60 Hz, with capacitor and motor with load.(e) operation at 80 Hz, with no capacitor and motor with load.(0 operation at 80 Hz, with capacitor and motor with load.68 T. HIMEl, T. FUJITSUKA and J. INOUE (Vol. 4,is equal to zero. If the voltage of the tacho-meter-generator rises, LIE shifts to minus andthe frequency of UJT oscillation diminishes,and vice versa.2.4 Block DiagralUThe block diagram of the constant speedregulation of an induction motor is shown inFig. 4.§ 3. Driving Characteristics of InductionMotor3.1 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 of wave forms,the filter is needed. But, here, the only ca-pacitance was connected to improve the perfor-mance stability of the inverter.Without capacitors, as illustrated in Fig. 5,the wave forms are decisively affected uponvarying load conditions. With capacitors, ho-wever, the variation 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 regu-lation are given in Fig. 6. The speed variationratio with and without the feedback is ex-pressed as functions of load current. Where,the variation ratio is denoted byN.-NI; = T X 100 [percent]No: speed with no load (rpm)N : actual speed (rpm)The doc source voltage, however, was notregulated proportionally to the frequencies ofthe inverter. The curves in Fig. 6 shows thatthe deviation of motor speed with speed con-trol is very small. The difference of the vari-(%) (%)10 6.S';; 8 .S....';;pno feedb"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) feedbackload current a(a) a 0.5 1.0 1.0 (A)(%) load current14(c)/2 Ilo feedback P (%).S I 10~'O I II .St:i J{ ';; 8 no feedback ~o.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)1969) On Speed Control of Induction Motor by Thyristor Inverter 69feedbackno feedbackation ratio from a reference speed due to themagnetic saturation of the induction motorand the nonlinear characteristics of the de-tecting tachometer-generator.Under the same condition, the transientresponse 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 inverterdriving the induction motor, is shown in Fig. 8with frequency as parameter. The outputrpm150I:i.S 100~ 50'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 withwhere,E : input voltage of inverterI : input current of inverterW: output of inverterThe high efficiency is obtained with low fre-quencies even in the range of heavy load cur-rent.The power factor decreases, if wave formsof 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 current with fre-quency as parameter. The power factor ad-vances high with the increased frequency forrelatively small currents, but reverses for heavycurrents.('Yo)10040;- .-.- ,/ •r:f I,I III II II II II II I •.-.--e 40 HzI I, ,I I 0--0--0 5 QII If IJ I--60I II I1 I0 I 0-0-0120•2 3 CA)output currentFig. 9. Power factor.oo20E 60u~....ll):>8.. 40803.4 Transient BehaviorThe reversal of motor revolution is achievedby changing the phase sequence of inverteroutput, which provides the motor a pluggingand makes much loss result in the rotor. Thedissipation of loss can be expressed by32 Jw2 (Joule)(A)3 sec.22 :3time0--0--0 120Fig. 7. Response.output currentFig. 8. Efficiency.o80oo20('Yo)10060~u>::ll)'uSll)W"D 20'"..s70 T. HIMEl, T. FUJITSUKA and J. INOUE (Vo!. 4,which should be within the ratings of the mo-tor at continuous operation. Figs. 10 and 11indicate the transient performance of the mo-tor reversal. At 60 Hz, as shown in Fig. 11,the period to a standstill is about 0.17 secondsinputcurrentinputvoltageoutputcurrentoutputvoltagereactivefeedbackcurrentrevolutionperminutesvoltageacrosscapacitorvoltageacrossthyristor ._reversal signaland that to a reversal reference speed is about0.5 seconds. On the other hand, at 40 Hz,the period to a standstill is about 0.08 seconds,and about 0.25 seconds were necessary to thereversal reference speed from standstill. The1/IO[sec]Fig. 10. Transient response of the reversal of an induction motor, operation at 60 Hz.1969) On Speed Control of Induction Motor by Thyristor Inverter 71time of the completion of reversal at 60 Hz isaround two times to 40 Hz. It should be notedthat rush currents flow at the instant ofreversing. From the inspection of Figs. 10 andII, the peak value which must be within theratings of thyristors and diode devices, issix times over the normal value.input currentinput voltageoutput currentoutput vOltagerevolution perminutesvoltage acrosscapacitorvoltage acrossthyristorFig. 11. Transient response of the reversal of an induction motor, operation at 40 Hz.72 T. HIMEl, T. FUJITSUKA and ]. INOUE (Vol. 4,regulate doc input voltage proportional to thefrequency of the inverter.(2) Noise PreventionThe radio frequency interference4l resultsfrom the turn-on and turn-off switching per-formance of thyristors and is not particularphenomenon 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 theabrupt rise of current.(ii) add high frequency filter to input andoutput line.(iii) screen the installations with electro-static shielding.Disturbances to the inverter are dividedinto two kinds: that is, one to the aux-iliary circuit and the other to doc source.The false conduction of thyristors origi-nating from disturbances to the sourceline 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 anode-cathode of the thyristor with a R-C filter.Disturbances to auxiliary 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 parallel across the gata-ca-thode of the thyristor.§ 5. ConclusionThe stability of operation was confirmedwith relation to the speed control of asquirrel cage induction motor controlledby a variable frequency three phase thy-ristor inverter. 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. Moriwake for theirassistance in the experiments and the pre-paration of Figures and photographs.(b)Fig. 12. Operation of irregular revolution at 2100(rpm).Upper traces: 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 docsource voltage should be regulated proportion-ally 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 mag-netic saturation, it will be most convenientmethod to utilize thyristor rectifying circuitsor thyristor doc chopper control circuits to1969) On Speed Control of Induction Motor by Thyristor Inverter 73References1) N. SATO and I. JURI: Toshiba Review 18, (1963)No.7, 7542) E. ONO and M. AKAMATSU: Mitsubishi ElectricReview 38, (1964) No.6, 9313) N. SATO: J. I. E. E. of Japan 84, (1964) 7894) G. E.: SCR Manual, Third Edition (1964)

On the Speed Control of a Three Phase Squirrel Cage Induction Motor Controlled by a Variable Frequency Three Phase Thyristor Inverter

Abstract

Similar works

Full text

Available Versions

Okayama University Scientific Achievement Repository

Okayama University Digital Information Repository