A Novel Approach to Design Stator Winding of Synchronous Generator

A Novel Approach to Design StatorWinding of Synchronous GeneratorSachin N. Gaikwad1,Poonam D. Daunde2, Rakesh Kumar Jha3

1Research Scholar, M Tech(Power system), IET, Alwar, [email protected] Student, M E (Power System), SNDCOE&RC,Yeola, [email protected]

3Asst. Prof., SNDCOE&RC,Yeola, [email protected]

AbstractStator winding of synchronous generator is designwith various types winding for manufacturing ofSynchronous alternator in an industry and the steelsheet laminations design is always kept constant.Lamination cannot be changed frequently because itneeds tool (die) development, for designing of newlaminations the time required and cost increasesenormously which is not acceptable to themanufacturers. At an instant very less information/data is available about the fact that a synchronousgenerator can also causes harmonics at the time ofgeneration of EMF, So it becomes paramount to dealwith this important equipment as far as harmonicsand related losses. Considering the harmonicspresent in the machine which, is mainly due tounequal distribution of field fluxes in the air gap.Many of researchers studies have been addressedthat Synchronous machines are also source ofharmonic currents on two counts: the frequencyconversion effect and the non-linear characteristicdue to magnetic saturation. Therefore weconcentrated our investigation work on thedistribution type of winding only. Considering theharmonic present in the machine, our concentrationwas on pitch factor.

Keywords: Synchronous Generator, Short pitchwinding, Harmonics, 2/3rd, 4/5th, 5/6th Pitch factor

I. INTRODUCTION

The Synchronous generator consistingstator and rotor. The stator of the ACgenerator is manufactured with placing

winding with some pitch factor. Thedistance between the two sides of anindividual coil of an AC armaturewinding is termed the coil pitch. Acoil whose sides are separated by onepole pitch (i.e., coil span is 180ºelectrical) is called a full-pitchcoil. With a full-pitch coil, the emfsinduced in the two coil sides are inphase with each other and theresultant emf is the arithmetic sum ofindividual emfs. However the waveformof the resultant emf can be improvedby making the coil pitch less than apole pitch. Such a coil is calledshort-pitch (or fractional or partialpitched) coil. The emf induced in ashort-pitch coil is less than that ofa full pitch coil. winding given by,Kp= cosα/2 which can be made bykeeping a 2/3rd, 4/5th or a 5/6th

winding pitch. Winding pitch of ACgenerators has influence on the shapeof waveform (harmonic contents) and onthe level of the fundamental voltage.The winding pitch will also have aninfluence on neutral circulatingcurrent in case of parallel operationand also will impact on the type ofneutral grounding method. Parallelinggenerators with different windingpitch will require careful

consideration if interconnecting starpoints of all running generatorsshould be necessary. The stator winding pitch of agenerator is a design parameter thatcan be used to optimise the generatorwaveform shape and minimise thegenerator cost, because shorter pitch(lower pitch ratios) use the generatorstator less effectively and requirethe use of more electrical steel andcopper for the same kW output thanhigher pitch machines.Quality of generated emf in the ACgenerator is how close to a true sinewave. The actual voltage waveform fromrotating machinery is never besinusoidal . Internal generator andexternal load characteristics causedistortions in the sinusoidal waveformcalled as harmonic. In brief,harmonics are energy levels existingin the system at multiples of thefundamental wave’s frequency. The mainsource of harmonic is non-linear loadconnected to AC generator.The MMF distribution (flux per pole)is sinusoidal which determines the EMFgenerated in an armature. As load isapplied on the alternator thegenerated sinusoidal waveform getdistorted For balanced three-phaseloads, distortion is caused by voltagedrop due to the harmonic currents inthe sub transient reactance of thegenerator. The sub transient reactanceof a generator is not a function ofcoil pitch. Therefore, the coil pitchdoes not affect waveform distortion.The impact of generator pitch on loadgenerated harmonic currents is highlydependent on the system configuration.Controlling circulating currents whenparalleling generators in a powersystem that shares a common neutralcan be difficult. In any parallelingoperation, it is extremely important

that voltages produced by thegenerating equipment are as closelymatched as possible. To properly matchvoltages, not only do the RMS valuesneed to be similar but theinstantaneous values, which aredetermined by the voltage waveshapes,should be similar as well. When thisis not possible, as in paralleling ofgenerators with different windingpitch configurations, circulatingcurrents may appear in the commonneutral which bonds the wyeconnections of the generating sources.These circulating currents can causeoverheating in the generator windings

II. LITERATURE REVIEW

Lyon and Waldo V was described that,the flux distribution in the air gap ofa synchronous machine consists of aseries of component distributions thatare simple harmonic wave trains, eitherstationary or moving at constantvelocities. Methods are suggested fordetermining the effects of slots andthe saturation of the magnetic circuiton the magnitudes of these componentdistributions. Expressions for thevoltage generated and the powerdeveloped thereby are given. The theoryis applied to the operation of a three-phase, synchronous machine underdifferent conditions of load, bothqualitatively and quantitatively.Chalmers, B.J, explained in his paperdescribes a type of double-layerwinding known as an interspersedwinding, which may be used in A.C.machines to reduce harmonic componentsof induced E.M.F. or load MMF. Theprinciples underlying the design ofinterspersed windings are described andcompared with the standard analysis ofconventional windings in terms ofharmonic winding factors.

Eleschova Zaneta , Belan Anton andMartin Mucha, Department of PowerEngineering, Slovak University ofTechnology, Ilkovicova , Bratislava,“Harmonic Distortion Produced bySynchronous Generator in Thermal –Power Plant ”, Slovak RepublicPublished Paper Sept.2006 .This paperdiscussed about voltage harmonicsproduced by synchronous machine. If themagnetic flux of the field system isdistributed perfectly sinusoidal aroundthe air gap, the flux is never exactlydistributed, particularly in salientpole machines non/sinusoidal fielddistribution can be expressed inharmonic series. The machine can beconsider to have 2p fundamental polestogether with 6p, 10p, … 2np harmonicpoles, all individually sinusoidal andall generating electromotive forces inan associated winding. The windinge.m.f. can be expressed in harmonicseries. The magnitude of the harmonice.m.f. is determined by the harmonicfluxes the effective electrical phasespread of the winding, the coil span,and the method of inter phaseconnection. For an integral slotwinding with m slots per pole per phaseand an electrical angle α betweenslots, the distribution factor for thenth harmonic can be calculated. Bysuitable choice of Kd and Ks manytroublesome e.m.f. harmonics can beminimized or even eliminated. Thetriplen harmonics in a three-phasemachine are generally eliminated byphase connection, and it is usual toselect the coil span to reduce 5th and7th harmonic. Slotting (the slots beingon the stator) produce variation ofpermeance. The fundamental rotor m.m.f.(magneto motive force) can berepresented as a travelling wave. Theslot ripple component of flux density.

Tsujikawa, T. Tokumasu, M. Kakiuchi, D.Hiramatsu, M, Fujita, T. Ueda, K.Ikeda, M.Ichimonji and T.Otaka,“Special winding to reduce space fluxharmonics caused by fractional-slot”,IEEE Transactions on ElectricalMachines and Systems, October 2012 .The authors said that space fluxharmonics caused by fractional-slotwinding and proposed a new windingmethod that reduces the low order spaceflux harmonic components (specialwinding method based on intersperse onecoil winding), The advantage of thisproposed winding method was confirmedusing a model winding. Here several numbers of research papersfrom IEEE and other reputed journalshave been studied and came to know thatdue to unequal distribution of fieldflux harmonics are introduced in outputvoltage waveform and also temperaturerises. Thus, the proposed problem isunique related to synchronous machinewhich the main equipment of our powersystem. So, it the challenge tominimize above problem by using twodifferent short pitches stator windingof synchronous alternator.Paolo mattavelli published paper relateto an implementation of synchronous-frame control for selected frequenciesin the output voltage. The regulationof the fundamental output voltage, aswell as that of some low-orderharmonics, is achieved using asynchronous-frame controller for eachselected frequency in addition to aconventional control

III. CONSTRUCTION OF SYNCHRONOUSGENERATOR

Except for small ratings, thesynchronous generators have astationary armature and a rotatingfield. The field is housed in the rotor

for technical and economic reasons. Itmakes the rotor light, reduces the sizeand losses in the bearing, makes thecooling simpler, and the response fast.Also, it is not possible to extractlarge power produced in a rotatingarmature through slip-rings andbrushes. Therefore rotating fields areused from about a size of 500 Kw. Thegenerators coupled to steam turbines ina thermal power station run at a highspeed 3000 rpm in India. The armatureis housed in the stator of thealternators and the field in the rotor.The stator is made up of sector-shapedlaminations of high-grade Cold RolledGrain Oriented Steel. Normally fullpitched winding has been employed asstator winding. Which results in moreuse of copper hence copper loss, moreharmonic introduced in machine.The Stator frame is made of sheet metalwhich reduces the overall weight ofthe machine and is aestheticallybetter. The sheet metal enclosures arefixed on the steel bars welded on thestator core. The armature coils aremade from dual coated, class 200 copperwire. The stator core is made of highquality low content silicon steelstampings with C-4 coating for betterwelding core packs. These are oriented90 deg. after every one forth lengthfor better magnetic properties. Theslots are skewed to reduce the toothripples in voltage waveform.

IV. DESIGN APPROACH TO SYNCHRONOUSGENERATOR

Quality of electrical performance is ameasure of how close the electricaloutput of the generator is to a truesine wave. The actual voltage waveformfrom rotating machinery is never

perfect. Internal generator andexternal load characteristics causedistortions in the wave. These factorsimpair the consistency of the generatoroutput, and can result in voltageregulator sensing errors and incorrectinstrument reading. In brief; harmonicsare energy levels existing at multiplesof the fundamental wave’s frequency.Harmonics are currents or voltages withfrequencies that are integer multiplesof the fundamental power frequencybeing 50 or 60Hz (50Hz for Asian andEuropean power and 60Hz for Americanpower). For example, if the fundamentalpower frequency is 60 Hz, then the 2ndharmonic is 120 Hz , the 3rd is 180 Hz,etc. In modern test equipment todayharmonics can be measured up to the63rd harmonic. When harmonicfrequencies are prevalent, electricalpower panels and transformers becomemechanically resonant to the magneticfields generated by higher frequencyharmonics. When this happens, the powerpanel or transformer vibrates and emitsa buzzing sound for the differentharmonic frequencies. Harmonicfrequencies from the 3rd to the 25thare the most common range offrequencies measured in electricalpower systems. The main source ofharmonic is non-sinusoidal field form,which can be made sinusoidal and thetotal harmonics can be eliminated. Butit is not an easy job because air gapoffers maximum reluctance to the fluxpath due to which air gap cannot bemade to vary sinusoidally, if air gapis made to vary sinusoidally around themachine, then field form would besinusoidal and total harmonics would beeliminated. But field form cannot besinusoidal due to saturation in ironparts and hence we cannot totallyeliminate all harmonics but candecrease them. Field form is nothing

but distribution of flux in machine.This flux distribution determines thewave shape of generated voltage inarmature winding. In case ofalternators, the voltage and currentsinduced are having sinusoidalwaveforms. But practically we cannotget sinusoidal waveforms when suchalternators are loaded. Due to theloading conditions, the generatedwaveform deviates from ideal waveforms.Such a non-sinusoidal waveform iscalled complex wave. Additionally,harmonics are caused by and are the by-product of modern electronic equipmentsuch as personal or notebook computers,laser printers, fax machines, telephonesystems, stereos, radios, TVs,adjustable speed drives and variablefrequency drives, battery chargers,UPS, and any other equipment powered byswitched-mode power supply (SMPS)equipment. The above-mentionedelectronic SMPS equipment is alsoreferred to as non-linear loads. Thistype of non-linear loads or SMPSequipment generates the very harmonicsthey’re sensitive to and that originateright within your building or facility.SMPS equipment typically forms a largeportion of the electrical non-linearload in most electrical distributionsystems. There are basically two typesof non-linear loads: single-phase andthree-phase. Single-phase non-linearloads are prevalent in modern officebuildings while three-phase non-linearloads are widespread in factories andindustrial plants. In today’s environment, all computersystems use SMPS that convert utilityAC voltage to regulated low voltage DCfor internal electronics. These non-linear power supplies draw current inhigh amplitude short pulses. Thesecurrent pulses create significantdistortion in the electrical current

and voltage wave shape. This isreferred to as a harmonic distortionand is measured in Total HarmonicDistortion (THD). The distortiontravels back into the power source andcan effect other equipment connected tothe same source. To give anunderstanding of this, consider a waterpiping system. Have you ever taken ashower when someone turns on the coldwater at the sink? You experience theeffect of a pressure drop to the coldwater, reducing the flow of cold water.The end result is you get burned! Nowimagine that someone at a sinkalternately turns on and off the coldand hot water. You would effectively behit with alternating cold and hotwater! Therefore, the performance andfunction of the shower is reduced byother systems. This illustration issimilar to an electrical distributionsystem with non-linear loads generatingharmonics. Any SMPS equipment willcreate continuous distortion of thepower source that stresses thefacility’s electrical distributionsystem and power equipment. Harmoniccancellation is performed with harmoniccanceling transformers also known asphase-shifting transformers. A harmoniccanceling transformer is a relativelynew power quality product formitigating harmonic problems inelectrical distribution systems. Thistype of transformer has patented built-in electromagnetics technology designedto remove high neutral current and themost harmful harmonics from the 3rdthrough 21st. The technique used inthese transformers is call "low zerophase sequencing and phase shifting".These transformers can be used to treatexisting harmonics in buildings orfacilities. This same application canbe designed into new construction toprevent future harmonics problems. In

case of alternators the voltagegenerated is mostly symmetrical as thefield system and coils are allsymmetrical. So the generated voltageor current will not have any evenharmonics in most of the cases. Thewaveform distortion can cause problemsin voltage regulation, generator andload overheating, and inaccurateinstrument readings. Both voltage andcurrent may have harmonic components.The current components produce heat andare therefore derating factors for thegenerator as well as system motors.

TABLE 1: EFFECT OF HARMONIC ON VARIOUS

ELECTRICAL MACHINES

V. TECHNIQUES FOR REDUCTION OF HARMONICS

Waveform Shaping: Field flux waveform can be madesinusoidal by decreasing the air gap atthe pole centre and large air gaptowards the pole end in salient polesynchronous machine. Skewing the polefaces if possible. In turbo-alternators the air gap is uniform. Sofield winding is distributed in theslots in such a manner as to make thefield flux waveform almost a sine wave.

Skewing :By skewing the armature slots, onlytooth harmonics or slot harmonics canbe eliminated.Fractional slot winding:The slot harmonic emfs can bedrastically reduced and even completelyeliminated from output voltage waveformby using fractional slot windings. Infact, present day synchronousgenerators invariably use fractionalslot windings on account of the factthat these windings give a much smallerdistribution factor for harmonics ascompared with that of fundamental.Larger length of air gap: If we use a large air gap length, thereluctance is increased and thereforethe magnitude of slot harmonics is

reduced.The fluxpulsationscan bereduced byhaving thenumber ofslots perpole arcas aninteger

plus 0.5.Alternator connections: Star or delta connections ofalternators suppress triplen harmonicsfrom appearing across the lines.Chording:Coil span less than the pole pitch theemf generated is proportional to Cos(n*α/2) Where, α is angle of chordingand n is order of harmonic. Theharmonic emf can therefore, beconsiderably reduced or entirelyeliminated by choosing a proper valueof α. To understand chording we shouldfirst go through the term coil span, Itis the distance on the periphery of thearmature between two

HarmonicOrder

1 2 3 4 5 6 7 8 9 10 11

Frequency(Hz)

50 100 150 200 250 300 350 400 450 500 550

Phasesequence

+ - 0 + - 0 + - 0 + -

Effect onEquipment

@ # $ @ # $ @ # $ @ #

@ Heating # heating plus motor problem $ heating plus noiseproblem

coil sides of a coil. It is usuallyexpressed in terms of number of slotsor degree electrical. So if coil spanis 180 degree electrical then the coilis called full pitched coil. As againstif coils are used in such a way thatcoil span is slightly less than a polepitch i.e. less than 180 degreeelectrical, the coils are called shortpitched coils.

Advantages Of Short Pitch Coils: The length required for the end

connections of coils is less i.eless copper is required, henceeconomical.

Short pitching eliminates highfrequency harmonics which distortthe sinusoidal nature of emf.Hence waveform of an induced emfis more sinusoidal due to shortpitching.

As high frequency harmonics geteliminated, eddy current andhysteresis losses which depend onfrequency also get minimized.This results in increase inefficiency.

Not only higher order harmonicsbut with specific degree ofchording the corresponding orderof harmonic can be eliminated.

Chording also helps to reduce thevibration level of the machine toa certain magnitude.

Stator Winding Pitch:Stator winding pitch Reduces oreliminates certain harmonics, but notall harmonics. In general, when coil ispitched short or long by (or) ,

no harmonic of order n survives in thecoil emf.The triplen harmonics that maybe generated in a three phase machineare normally eliminated by starconnection of the phases.it istherefore usual to select the coil spanto reduce as much as possible the 5thand 7th harmonics a pitch of 83.3% ismost useful in this aspect, as it givesthe following factors.A. Distribution Factor Corresponding toOrder of Harmonics

TABLE 2: Distribution Factor

Corresponding To Order Of

HarmonicOrder ofharmonics

Fundamen

tal

3 5 7 9

Distributionfactor

0.966 0.7

07

0.2

59

0.2

59

0.7

07

The 5th& 7th harmonic factors are bothsmall while 3rd and 9th harmonic emfwill not appear in the line in casewith star and delta connection.Hence attime of winding design the attention ismainly directed for the attenuation of5th & 7th order harmonics by adopting a

suitable chording angle. Chording angleof 300 i.e. coil- pitch = 1500electrical is most useful,since itgives the above values.

B. Wave Distortion FactorThe distortion factor of a voltage waveis the ratio of effective value of theresidue after the elimination of thefundamental to the effective value ofthe original wave. The distortionfactor, Fdi of a wave is obtained bydividing the rms harmonic content, thatis the square root of sum of the squareof the rms value of all frequencycomponent except the fundamental, bythe rms value of the wave including thefundamental. Fdi= (En2) / Erms, Where, E = sumof the square of the rms value of allthe components of the voltage exceptthe fundamental ,Erms= rms value of thevoltage

VI. OPTIMIZATION TECHNIQUES OFHARMONICS WITH VARIOUS SHORT PITCHWINDINGWinding PitchA coil whose sides are separated by onepole pitch (i.e. coil span is 180degelectrical) is called a full- pitchcoil, the emf’s induced in the two coilsides are in phase with each other andthe resultat emf is the arithmetic sumof individual emfs. However thewaveform of resultant emf can beimproved by making coil pitch less thana pole pitch such a coil is called asshort pitch or fractional pitched coil.

A. Full pitch winding: Full pitch = pole centre (S) to

pole centre (N).For 48 slots stator Slot/pole= 48/4=12 slotsCoil span = 1-13

Fig. 1: Full Pitch Winding MachineDesign

B. 5/6th Short pitch winding: This winding pitch equals to5/6th of full pitch winding.For 48 slots stator= 5/6th of 12

slots=10 slotsCoil span = 1-11

Fig. 2: 5/6th Short Pitch WindingMachine Design

C. 2/3rd Short pitch winding: This winding pitch equals to2/3rd of full pitch winding.For 48 slots stator= 2/3rd of 12

slots= 08 slotsCoil span = 1-09

Fig. 3: 2/3rd Short Pitch WindingMachine Design

VII. CASE-STUDIES OF VARIOUS MANUFACTURER ON DESIGN OF ALTERNATOR

Mirus International Company;2/3-pitch generators produce littlethird harmonics current, they doproduce much higher fifth and seventhharmonics when compared with 4/5- and5/6-pitch generators. This increasesheating in motors, which can shortenlife. If inductive loads make up themajority of the load, 4/5- or 5/6-pitchgenerators can be used with correctsizing. These generators also result inphase-to-neutral faults much lower than2/3-pitch unit. There has been muchwritten and even more speculated aboutthe pros and cons of 2/3-pitchgenerators vs. 4/5- (and 5/6-) pitchmachines. Because the effects of thirdharmonics on electrical systems areinstallation-specific, few hard andfast rules apply. However, in general,the following points are consistentacross all generator and electricalsystems: 1. Third harmonics current is generatedalmost totally by connected load –computer systems, UPS, variable-speedand fluorescent lighting. Only anegligible amount is produced by thegenerator, no matter what its windingpitch. 2. Third harmonic currents in identicalparalleled gensets are no problem ifgensets are carrying equal load.However, it may be a problem if twogenerators of different pitches areparalleled. 3. While 2/3-pitch generators have verylittle third harmonic current comparedto other pitches, the fifth and seventhharmonics are nearly maximum at 2/3

pitch. Further, if a phase-to-neutralfault (the cause of 65% of all faults)occurs on a 2/3-pitch machine, therewill be higher fault currents, with thepotential of more system damage and theneed for higher interrupting capabilitycircuit breakers - adding cost to theinstallationpitch factors for synchronousgenerators of various pitch types.These pitch factors are multiplied bythe respective harmonic fluxes topredict the harmonic voltages [2].Since differently pitched machines havedifferent pitch factors for eachharmonic number, their harmonicvoltages and voltage waveshapes will bedifferent as well.Cummins Generator Technologies;Alternators are compatible if they havecompatible voltage waveform shapes. Toassure the optimal compatibilitybetween current and potential futuremachines, always specify the use of 2/3pitch alternators for line voltageapplications. When faced withparalleling dissimilar machines, someof which are not 2/3 pitch, the mostdesirable practice is to replace thedissimilar alternator(s) with acompatible alternator, so that all themachines are 2/3 pitch.

TABLE 3: PITCH FACTOR IMPACT ON HARMONIC

VOLTAGE MAGNITUDES IN SYNCHRONOUS

GENERATORS

VIII. CONCLUSION

One of the design considerations inselecting an appropriate pitchfactor is the harmonic content ofthe generated voltage wave form.Pitch factor can be used to reduceor eliminate specific harmonicfrequencies in the generated voltagewaveform as follows:A. Full Pitch: A full pitch willhave no damping effect on anyharmonic frequency.B. 2/3 Pitch: A 2/3 pitch willeliminate the third harmonic andsubsequent triplens i.e.: 9th, 15th,21st, 27th, … etc.C. 4/5 Pitch: A 4/5 pitch willeliminate the 5th harmonic.D. 6/7 Pitch: A 6/7 pitch willeliminate the 7th harmonic.E. 5/6 Pitch: A 5/6 pitch will:1. Minimize the 5th harmonic, butnot eliminate it as will a 4/5pitch.2. Minimize the 7th harmonic, butnot eliminate it as will a 6/7pitch.

It is important to note that it is notthe generator’s specific pitch valuethat causes the circulating current butrather the difference in voltage waveshape of the two differently pitchedgenerators. Therefore, the fact that a2/3 pitch generator has a very low

pitch factor for the 3rd harmonic doesnot mean that it will perform anybetter in paralleling operations. Infact, a 2/3rd pitch generator has verylow zero sequence reactance andtherefore, has less impedance to reducethe flow of circulating neutralcurrent. Circulating currents canresult with any generator pitch typewhen it is not matched with a similarlypitched unit or it is paralleled withthe Utility.

IX. REFERENCES

[1] Dr Jawad Al-Tayie, Chris Whitworth,Dr Andreas Biebighaeuser “AC Generatorswith 2/3rd and 5/6th winding pitch”,Issue WP105: Technical Information fromCummins Generator Technologies.[2] Pradipta K. Das And Amulya K. DasGupta, “Inductance Coefficients ofThree-Phase Inductor Alternators: Part-I Analytical Study”, IEEE Transactionson Power Apparatus and Systems, vol.PAS-88, no.-11, pp. 1725-1730, November1969.[3] Chalmers, B.J, “A.C. machinewindings with reduced harmoniccontent”, IEEE Transactions on ElectricMachines & Drives, vol.-111, no.-11,pp. 1859 – 1863, November 1964.[4] Xola B. Bomela and Maarten J.Kamper, IEEE Members, “ Effect of

Stator Chording and Rotor Skewing onPerformance of Reluctance SynchronousMachine” IEEE Transactions On IndustryApplications, vol.-38, no.-1, pp. 91-100, January/February 2002. [5] Paolo Mattavelli, IEEE Member,“Synchronous Frame Harmonic Control ForHigh Performance Ac Power Supplies”,IEEE Transactions On IndustryApplications, vol.-37, no.-3, pp. 864-872, May-June 2001. [6] Seok Myeong Jang , You, Dae Joon ,  Kyoung Jin Ko  and Sang Kyu Choi, “ Design   And Experimental Evaluation Of   Synchronous machine Without Iron Loss Using Double Sided HalbachMagnetized PM Rotor   In   High Power FESS”, IEEE Transactions On Magnetics,vol.-44, no.-11, pp. 4337-4340,November 2008.[7] Zhu,Xi Xiao And Yongdong Li, China,“ Permanent Magnet Synchronous MotorCurrent Ripple Reduction With HarmonicBack-Emf Compensations”, IEEETransactions on Electrical Machines andSystem, pp. 1094-1097, October 2010.[8]Rakesh Kumar Jha, Arvind S. Pande,Prof.Harpreet Singh “HarmonicsReduction in Synchronous Alternator byusing Short Pitch Winding”,International Journal of EmergingTechnology and Advanced EngineeringISSN 2250-2459, ISO 9001:2008 CertifiedJournal, Volume 3, Issue 9, September2013