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COMPREHENSIVE STUDY OF SINGLE WINDINGSQUIRRELCAGE INDUCTION MOTORS FOR TWO SPEEDSUSING PHASE-SHIFT MODIFICATION TECHNIQUEN. D. PRASAD a , H.V.K. SHETTY a & P.L PRADHAN aa Rotating Electrical Machines Division , R and D Centre, Jyoti Limited , Baroda, IndiaPublished online: 07 May 2007.

To cite this article: N. D. PRASAD , H.V.K. SHETTY & P.L PRADHAN (1982) COMPREHENSIVE STUDY OF SINGLE WINDINGSQUIRRELCAGE INDUCTION MOTORS FOR TWO SPEEDS USING PHASE-SHIFT MODIFICATION TECHNIQUE, Electric Machines &Power Systems, 7:5, 381-395, DOI: 10.1080/03616968208955460

To link to this article: http://dx.doi.org/10.1080/03616968208955460

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COMPREHENSIVE STUDY OF SINGLE WINDING SQUIRRELCAGE INDUCTIONMOTORS FOR TWO SPEEDS USING PHASE--5HIFT MODIFICATION TECHNIQUE

N. D. PRASAD, H.V.K. SHETTY, and P. L. PRADHAN

Rotating Electrical Machines DivisionRand D Centre, Jvoti LimitedBaroda, India

ABSTRACT

Modem industr,y demands a,c, drives heav~ for ail 0FElre.ttODS.However, certain processes require two or mre speeds. It is univer­salJ.y well known that a nOI'llBl induction motor has only one speed. Inrecent years the use of pole-changing techniques are used to achievedual speeds. In this paper, an attempt is nade to design a Winding fortwo speed opere.tiona in a nOI'lTel squirrelcage induction motor usingphase-llhift IIOdification (P.S .110.) technique. !'ajor emphasis is laid onthe experimentel results of this motor ....nufactured. Latest designtechniques are resorted to achieve better performances at two-speedoperations. Available techniques of pole-changing are briefly touchedupon. Advantages and economic aspects of using two-speed motors arehigh-lighted.

1. INTR(J)UCTIOO

The operation of the squirrelcage induction motor at two or morespeeds using a single winding, is Ilade possible in recent years, usingpole amplitude IOOdulation (PAM) principle or Phase-Shift l'",dificationTechnique (PSM). Pole amplitude IIOdulation is so called, because ofthe logical similarity between this technique with the technique ofamplitude modulation used in radio-eommunication. The fundamental diffe­rence between these two techniques, is that FAM involves modulation ofthe space distribution of a stator DIIlf wave, where as the later invol­ves time-lllOdulation. Though the classifical 1:2 speed ratio using aIlahlander wiming is a particular case of PAM principle, a generalisedand comprehensive theory of FAM is developed after Rawc,fiffe etal 1started syste!latiC research in late fifties. In principle, modulationis realised simp~ by reversing the current in a certain chosen numberof coils in the stator wiming. The essence of the PAM teChnique liesin identifying those coils which bring out modulation. Several variantsof FAM principle are available in the literature such as • (a) Phase­shift modification (PSM) 2 (b) Foly-phase ~trizatiQn3 (c) Slotvectorstar method 4 (d) Leastcommon-multiple (U:M) technique 5 etc.

Though, end results are the same, each method differs only in itsmethod of approach, The detailed description of these various methodsis f01md elsewhere. In this paper P.S.H. technique is used on a nonralinduction motor.

Electric Machines and Electromechanics, 7:381-395Copyright © 1982 by Hemisphere Publishing Corporation 0361·6967/82/050381·15$2.25

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382 N. D. PRASAD ET AL.

The variable-epeed operation of induction motors ie particular]ynecessaJY in process industries like textile - industries and Jlliclrl.ne­tools industries and also in pUlllf8dstorage plants. In pumped-storageplants water head varies depending on the seaaonal rain; and thedesired quantity of diecharge is realised by reduction gear or beltand pulley system. PSM motor can successf'ully replace the reductiongear in such a system. The advantages and application of PSM motor areexplained in more detail in Section-5.

This paper is ailllS~ at presenting a two speed induction motor UsingPSM principle. 'ibis motor was designed for 11/7.5 kW rating for 4/6 poleoperation making use of the existing botW and stampings of availableframe. The winding ie care fully designed with· the latest techniqueavailable in the literature. Associated harmonic ana]ysie was carriedout using a computer program exclusive:ly written for this purpose andthe results are presented in section 3.40 The JllignetiC and electricloadings are kept minimum to avoid cogging, crawling, noise and to limitthe telllJl8rature. The perforIllSnce parameters under two speed operationswere checked based on the lstest design practice and are found to bewell comparable with the test results presented in section-40

2. PRmCIPIE OF POIE AMPLITUDE !1lJlUWION TECHNIQUE

The t'undamental basis of PAM or P.S.~. can be best explained by asimple example of a 8/10 pole Winding with 36 slots in the stator asshown in Fig. 1. The winding can be either connected for 8 pole or 10pole operation by simple reconnection of coils as shown.

The baaic theory of PAM is given below :

The IIIDf produced by a conventional 2 pole winding ma;y be writtenas M

1m A Sin (pe - wt) (1)

Suppose now this IIIIlf is modulated by a wave of (2p.:!: 2q) poles,that is by a wave haVing a number of poles equal to sum or differenceof the original and final pole numbers, the resulting mmf may be wri­tten aa

M2 .. A Sin (pQ - wt) Sin (p .:!: q) 9

.. ~ [Cos ( .t q9 + wt) - Cos { (2p:!!: q) Q - wtJ1 (2)

It can be seen rl-om equation (2) tha't two modulated waves arepossible. One is with the sum of pole numbers and the othEor is withthe difference of pole numbers. The first term in equation(2) repre­sents a rotating 2q - pole field. Hence, in order to obtain 2q-polefield from 2Epole winding, it is necessaJY to modulate the completewinding by a wave of(2p t. 2q) poles. In fact, the PSM technique ado­pted for 4/6 pole combination is basical]y falls in line with PAMtechnique. It will be clear from the sections to follow, that thesetwo techniquss are similar for the chosen pole numbers.

In order to study the perforlllSnce of the motor, available 4 polestator stampings haVing 4B stator slots were chosen. Rotor was multi­csge dieeast aluminium rotor haVing 54 slots. The Jllichine was designed

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1] 1415161716 19 20 21 n 23 24252627 26 29 3031 32]J 3435 366 POLE

~ -t' 8-A -e -A-A C A A 8'" - CONNECTION

A-C -e -8 -A-A -C 8 A A C C 8-A -c-c -8-8 -A C 88 A A C-8 -A-A -C-C -8 Ace 8 B - 10POlE

(e THE COIlS SO MARKED ARE REVERSED IN 10POlE CONNECTION,) CONNECT.

8POlEMMF WAVE

IIIw

II

_' IA

~e !!!!:! ARRANGEMENT OF STATOR WINDING IN A &!IOPOLE PAM MOTOR,

10POlf;'MMF

WAVE

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384 N. D. PRASAD ET AL.

for 11kW/7.5kW for 1,/6 pole operation. The perfoI'1ll\nce requirements wereai~d, as per the specifications laid down in the standards for textilering-frame motors.

The wiming was designed for 36 slots, leaving 12 slots symmetri­cally around statorbore periphery. The detailed study IlJide on this win­ding :La described in the following paragraP1s.

3.1 Winding Design

Win1ing design was carried for 4Iz6 pole combination with 36 slotsusing phaseshift modification toothod • The phase angle table is shownin Table-1.

The basic pole IlUIIlber is chosen as 4-pole and the IIIIJlf wave isdifference modulated by 2-pole wave, to get the required 6-pole IIIIIlfwave. The slot angles were written down for the basic 4 pole numberfirst and the sense of coils are same as the conventional one. Theslot angles for 6-poles were found, and the sense of coils were JUl.rkedpositive for those the phase angle lies in between 00 to 1800 and asnegative for those in between 1800 to 3600

• Now from tha phase angletable, it is easy to identify the coils undergoing current reversal inthe 6-pole case.

3.2 Sectional Diagram

The intercomection of coils is shown in the sectional diagram(Fig. 2.0). Each phase is JUl.de into two parts - (1) The coils forwhiCh the current direction remains same in both otigine.l and modu­lated case (2) The coils Which undergo current reversal in the modu­lated case.

3.3 Terminal Connection

The connection for 4-pole is two_paral1elstar (Fig. 2.1a). Butthere are two alternatives for 6-pole comection viz; (a) series­delta (Fig. 2.1b) (b) and series-star (Fie. 2.1c). Each connectionhas its own repercussions. It is true that the winding factor for themodulated pole number will be less than that for basic pole number.Either connection, Series-Delta or Series-Star in the modulated caseneed,to hring out only 6-terminals. The polechange aver can be manua­lly done or through a simple switch.

3.4 Harmonic Analysis of the Winding

It is needless to say that one should have balanced winding forboth the speeds. The coil distribution for the basic pole number issame as that of the conventional induction motor; therefore the IIIIJlfwave is harmonic free am the performance of the machine for that polenumber is identical to that of the conventional one. But the modulatedIIIIIIf wave may contain undesirable harmonics which might impair the per­formence. These effects can be Visualised on the speed torque chara­cteristics. Such harmonic effects was studied and efforts were JUl.deto limit their magnitude to the tolerable limits.

The amplitude and phase angle balance of the designlld winding canbe clearly seen in the vector diagrams shown in Fig. 3.0 and Fig. 400

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• • IIIE. • • I I I I • I .III!.

• 1111. • • 1;11.I a a • I I s I • :I• •

j I .a .1111 • I • IIIE .. • I § II if• •• · ..• II.. .. ,. .I.-.1111 •••• ;.11.1 I • , !! ! ! · ..-• • iii!'t

11.:jll! a.IIEE. a.II!I. • ! I I ! .• • f. 1•

1 II ... !l- •• ! II • • St!! • • f!!I.

::i.lisa • I iii":J ..•• •

Hll;- :I .I!! •• s • I!!I . :I • M!!!.

I ~:;1l

:. :I .1111 • :; • 1!SI. s .I!I~ •I.

•...• !aEE. .I!!! . -.• • :I :r • ! s I ! • •.. • ";..

•• •• J" •. !II!. • • J i ! , •

;i;: ..... ,. ........J. i!:- .. _-

I •• !Ii!is. i ~ ! ! •• • •• a a •• • • I • II• ....I" •- .tICJ.. .... . . . ... !a!,. •• , I I , • !":i •:iij•• I!!

:--:::~~i'-; -:=="S"~i: ~'='=~1:"i: .......'"S-:- ., .. : .. ., -:- ., ----• oil ••••• · ,. .. . .. • A ••• I •

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",.

>u1... I----3~--,..'"

~GENERA! 'W' !£CTOA J¥GRAN fOR

-f PQl..E: CDNHECllOH

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U

FG.l.lQ 4POL1: ~llEl STAR

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SINGLE WINDING SOUIRRELCAGE INDUCTION MOTORS

for both ~pole and 6-pole cases respective]y.

The m.ll'.f. waves of the total winding for 4 pole and 6 pole areshown in Fig. 5(b) ';'nd ·(c) respective]y. It is easy to recognise the4 pole m.m.f. wave. But the 6 pole m.1I'.f. wave is not self explanatory.The harmon:l.c ana]ysis giVen in the next section, offers an explanationto this. The presence of lerge 126.6 %- 2 Pole subhsrmonic in the m.ll'. f.wave, completely distorted the shape of the m.m.f. wave but the same isnot having al\Y perceptable effect on the operation of the mchine inpractice and Wholly acceptable.

Though the calculation and plotting of stator m.m.f. wave forms bythe usual conventional process - a graphical integration of the appliedampere conductors, in SOlll! sense a llli.sleading operation, is not comple­te]y a futile 8XJlercise. The lower the stator m.1I'.f. harmonics, thebetter the winding and a harmonicfree stator winding is ideal. The m.m.f.,thus deduced is the m.m.f. due to the stator mgnetising current only,in the absence of rotor current. The conventional stator \lIIIlf wave-form,therefora is 110 way, corresponds with the rasultant m.m.f. wave form,under actual or-erating conditions even for a standard induction motoron no-load and far less a for certain PAM change speed windings 4.

A computer program is developed spegially for the Harmonic analysisof PSM Winding based on Veinott's paper and applied for the designed36 slot J./6 pole winding. The basic equations are given in Appendix-A.

Harmonics can be broadly divided into three classes 4 nalll!ly: (a)subharmonics i.e. which have poles less than the operating pole number(b) medium order harmonics i.e. pole numbers which are not more thantwice the operating pole number and (a) higher order harmonics severaltimes more than the operating pole ID.IIIlber. Even the large amount of sub­harmonics will not have any significant effect on the performance ofthe machine, but more than 10 % of lII!dium and higher order harmonicsmay cause problems like crawling, cagging etc., and hence are not acce­ptable. Also the temperaturerise will be high. The results obtained forthe designed winding shown in Table-2 reveals that they are within thelimits.

4. TEST RESULTS OF THE EXJ'ERD1ENTAL MOrOR

Tests are conducted on the experimental machine as per the proce­dure prevailing in national standards. For the test purpose, pole ­Changing was adopted IIIlnually. Starting currents and voltage wave formsare recorded using multichannel recording device. During tasting, 0.5class accurecy meters were adopted. The results are tabulated in theTable) and 4.

5. ADVANTAGES & AJ'PLICATIrnS OF PSM MoreB

PSM motor can replece for any dual wound motor. From manufacturingpoint of view, housing a single winding is much simpler than two windingsin the same slot and also leads to lesser consumption of insulating mate­rials. Besides that, a great reduction in frame size, weight of copperand hence, a reduction in cost is obtained. The power factor and effi­ciency of a PSM motor are comparable to that of the corresponding singlespeed motor. Similarly, the starting and pullout torques are only margi­nally inferior to those of equivalent single speed motor. By careful

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--. IIID'l!lZ 1I0l!U1t\T1011 ArTS 1lODIJLt.T101l

•• Po....... -- P.-n ----.... .awe..... • .... 1IoC. -. lite.1 ..- ...-• 1.-«1

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'"'"o

TABlE _ 3:'1'ESTS ON THE EXPERlMENTAL MACHINE OF RATING 11leW/7.5kW

~) 1i2:-.Load Test and Heat_ run

Pole Connection Volts Input No-Y>ad Currents Iron loss Friction Te~rature rise( kW ) R Y B leW gt Windage

byme~~~~tt~ce~ kW

4 2 II y 415 1.50 12.l! 14.5 12.l! 0.72 0.5 13.5

6 y 415 1.356 8.l! 12.f 11.0 0.6 o.?- 24

B) Iocked-Rotor Test at reduced voltage

Pole At ~ed Volts At 41~ Volta let/In p-u Tst/Tfl PercentageVolts Current Amps) Input (leW) Current (Amps Input (leW)

4 38 15.0 0.512 163.8 61.18 7.2 290

6 111 14.55 1.62 54.4 22.64 3.0 140

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TABlE _ 4:PERFORMANCE TFST CN EXPERIMENTAL MACHDlE OF RATING 11kW/7.5kW

A) LQAD..'IEST

4 - Pole Operation (2-Pare.llel Star Connection)

Volts Input Qumnt (Am:oo) Ou~~ut Efficiency Percentage Power factor Speed at 50Hz RPMkW R y B

415 16.55 27.6 27.0 27.4 14017 85.61 0.84 1474

415 11.50 20.0 20.8 21.4 9.65 83.88 0.78 1483

415 7.44 16.5 16.4 16.4 5.85 78.62 0.64 1489

6 - Pole Operation (Star Connection)

415 1401 25.0 25.0 22.0 10.14 71.95 0.78 955

415 11.17 20.5 20.0 18.0 8.28 74.15 0.76 968

415 7.8 16.2 15.2 13.0 5.79 74.2 0.67 980

B) LOAD HEAT RUN TFST

Operation Number of Hours Run Current Amps Input kW Output kill Temperaturerise by resistance method

~

4 Pole

6 Pole

44

2400

17.5

1404

10.5612.3

7.82

32°C

78°C

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392 N. D. PRASAD ET AL.

design of windings, the noise and vibration can be brought to those ofstandard induction motor. Only six tenninals need to be brought out,asin conventional induction motors. As nentioned in the introduction, PSMmotors find application in driVing fans, pumps, Il8chine_tool drives,conveyors, escalators, hoists, centrifugals and in process industrieslike textile and in pumped-storage plants etc. Though PSM allows allYspeed ratio, close ratios (4/6, 6/8, 8/10 etc) pole combinations weremore successful and found demand in industry.

6. COOCLUSIONS

This paper has dealt with the implerrentation of the PSM principleto an industrial design. The test results shown in Table-3 and 4 areencouraging. Though the noload currents are slightly unbalanced undertwo speeds, the currents are well-balanced during load condition. Theefficiency is 85 %for 11kW output for 4 pole operation, where as thesame was 74 % for 7.5 kW output for 6 pole running,The full load powerfactors are 0.84 and 0.75 respectively for 4 pole and 6 pole operations(rable-4~. It is interesting to note that the temperature rise of win­ding under 4 pole operation was far less (320 C) where 8lI for 6 pole itwas 78oC. This shows that it is possible to obtain 22kW output directlj:for 4--pole operation without allY temperature problem, with the additi­onal benefit of increased efficiency and power factor (Table-4).

As it was pointed out earlier, this work was taken up only to studythe fundarrentals of PSM in principle and to see the implenentation inindustrial design, Moreover, it was concluded that 36 slots give wellbalanced load currents than 48 slots did. Certain slots were left outin the winding which resulted a poor winding factor particularly in 6­pole case. Hence, an improvement in winding factor will give higheroutput an:! better performance figures in 6-pole operation. This ispossible if the design is made in stampings having 36/28 slot combina­tions.

In the present study, only ronual connections were resorted to,for polechanging operations. However, it is possible to incorporate anautomatic switChing arrangement with the timers in the circuit. Aspointed out earlier, the electric and magnetic loadings were kept mini­mum to limit the temperature rise. A careful study of heat transfer ofthis type of winding is necessary; since the losses are comparativelyhigher due to negative..aequence currents.

7. ACKNCMlEOOEMENTS

The authors are grateful to M/s Jyoti Id.mi.tad for the permissionto present this paper.D

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SINGLE WINDING SQUIRRELCAGE INDUCTION MOTORS

8. REFERENCES

1 G.H. RAWCLIFFE, R.F. BllRBIDSE and P'JNG, "Induction motor speedchanging by poJ.e amplitude modulation", Proe , IEE (LClIDON),Vol. 105, pp. 411-19, August 1958.

393

2

3

4

5

6

V•V. SASTRY, P.V. MO, G.S. MO and F.S. RAO, "A new philosophyfor speed changing induction motors", Electric machines andEl.ectro~chanics, Vol. 2, No.3, pp. 233-46, April-June, 1978.

W. FOOG, "Po)yphase ~trization, "A new DlBthod for the designof electrical windings", Prec, IEE (LONDON), Vol. 115, No. S,pp. 1123-1134, AugUst 1968.

G.H. RAWCLlJ'FE and W. FCNG, "Sum and Difference Winding Modulationwith Special Reference to 4/6 Pole PAM Winding" Proc , lEE, Vol.117,pp. 1782-92.

A.M. DUDIEY : "Connecting Induction Motors" - Book, McGraw-HillComparv, New York, 1936.

C.G. ViINorT, "S~ial Harmonic mmf in irregular windings andspecial connections of pol;ypha.se windings·, Trans. on PAS, IEEE,Vol. 83, pp. 1246-53, Dec. 19640

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394

APPENDII _ A

HARMONIC ANAIXSIS OF WINDING

N. D. PRASAD ET AL.

This section lays down the lIBthematicIU model used fCfI" harmonicanalysis of the winding designed by the authors. The main body ofthis mathematical model and lucid theory is found in reference (6).The subtle points are briefly described below :

Consider a coil having a throw between coil sides ofp radians.Pitch factor for the nth hannonic is given by

E =: sin Jl.#.pr.. Coo (J)

The space position of aI\Y coil C for nth order harmonic ill givenby : ::!/1lIC J where S is the number of slots.

Slet there be m phases, and be numbered consequently in order of

tilmlphase-sequence. let the phase number be denoted by K, Which maybe positive or negative depending on, whether that particuJ.e.r coil inquestion is connected to give positive or negative m.m.f. let zerotimehe taken as the reference, when the current in the nth phase is maxill1lllll.

The angular position in space of the fonrard revolving compo­nent of coil is given by :

G' .r ., nJ .\T-j nIfe = n Jt ," .'. ., ) -,,:.:.1=f ... 1.v , ....

. 11 '5' Il 11 ,~< D

Similarly, the position of the backward revolving wave is givenby

[

' -r- ")°1 1"ln11',,",,', , 1..l. •• ·: _~_

A ~ d~ '.' .,. ,. <" ::l~n I..... 11' 1.1 ~:.. :. -.s .

where both angles are electrical radians of the nth harmonic. If Q isthe number of coils carrying current, the forward field distributionfactor of nth harmonic is given by

l"1u

r

= ~'-.:-Oi-·I1-.-£,-,)..,...~-.~:.'-r-~-CO-8-8-r,-, )-~ ] (6)

BacknJard field distribution function is given by

j' .. .1 f~ /(:;: "in" ,::! .:( .... co c fa )?\. ~- - / ., VJm' .iIQ, bu<In 1J q,..

where all the sUllDl8tions are for C c 1, 2, ""S.

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SINGLE WINDING SQUIRRELCAGE INDUCTION MOTORS

1'00 resultant fOIWard revelving wave amplitude is given by

395

-, ~:l(8)

Similarly, the amplitude of the backward revolving wave is

ll1l· dn.._.~~ ~'-'..

The results of the present winding is given in Tabla-2.

Manuscript received in final form, April 19, 1982Send requests for reprints to H. V.K. Shettv, REM-R & 0 Centre,

P.O. Chemical Industries, Vadodara 390003, India

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