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Brushless DC (BLDC) Motor Fundamentals

INTRODUCTIONBrushless Direct Current (BLDC) motors areone of the motor types rapidly gainingpopularity. BLDC motors are used inindustries such as Appliances, Automotive,Aerospace, Consumer, Medical, IndustrialAutomation Equipment and Instrumentation.

As the name implies, BLDC motors do notuse brushes for commutation; instead, theyare electronically com-mutated. BLDC motorshave many advantages over brushed DCmotors and induction motors. A few of these

are: Better speed versus torque characteristics High dynamic response High efficiency Long operating life Noiseless operation Higher speed ranges

In addition, the ratio of torque delivered to thesize of the motor is higher, making it useful inapplications where space and weight arecritical factors.

In this application note, we will discuss in

detail the con-struction, working principle,characteristics and typical applications of BLDC motors. Refer to Appendix B:Glossary for a glossary of terms commonlyused when describing BLDC motors.

CONSTRUCTION ANDOPERATING PRINCIPLEBLDC motors are a type of synchronousmotor. This means the magnetic fieldgenerated by the stator and the magnetic fieldgenerated by the rotor rotate at the samefrequency. BLDC motors do not experience

the slip that is normally seen in inductionmotors.

BLDC motors come in single-phase, 2-phaseand 3-phase configurations. Corresponding to itstype, the stator has the same number of windings. Out of these, 3-phase motors are themost popular and widely used. This applicationnote focuses on 3-phase motors.

Stator

The stator of a BLDC motor consists of stackedsteel laminations with windings placed in theslots that are axially cut along the inner periphery (as shown in Figure 3). Traditionally,the stator resembles that of an induction motor;however, the windings are distributed in adifferent manner. Most BLDC motors have threestator windings connected in star fashion. Eachof these windings are constructed withnumerous coils interconnected to form awinding. One or more coils are placed in theslots and they are interconnected to make awinding. Each of these windings are distributedover the stator periphery to form an evennumbers of poles.

There are two types of stator windingsvariants: trapezoidal and sinusoidal motors.This differentiation is made on the basis of the interconnection of coils in the stator windings to give the different types of backElectromotive Force (EMF). Refer to theWhat i s Back EMF? section for moreinformation.

As their names indicate, the trapezoidal motor gives a back EMF in trapezoidal fashion andthe sinusoidal motors back EMF issinusoidal, as shown in Figure 1 and Figure

2. In addition to the back EMF, the phasecurrent also has trapezoidal and sinusoidalvariations in the respective types of motor.This makes the torque output by a sinusoidalmotor smoother than that of a trapezoidalmotor. However, this comes with an extracost, as the sinusoidal motors take extrawinding interconnections because of the coilsdistribution on the stator periphery, therebyincreasing the copper intake by the stator windings.

Depending upon the control power supplycapability, the motor with the correct voltagerating of the stator can be chosen. Forty-eight

volts, or less voltage rated motors are used inautomotive, robotics, small arm movementsand so on. Motors with 100 volts, or higher ratings, are used in appliances, automationand in industrial applications.

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FIGURE 1: TRAPEZOIDAL BACK EMF

0 60 120 180 240 300 360 60

Phase A-B

Phase B-C

Phase C-A

FIGURE 2: SINUSOIDAL BACK EMF

0 60 120 180 240 300 360 60

Phase A-B

Phase B-C

Phase C-A

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FIGURE 3: STATOR OF A BLDC MOTOR

Stamping with Slots

Stator Windings

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Rotor

Therotor is

madeof per manentmagnetandcanvar yfromtwotoeightpolepair swithalternateNor th(N)andSouth(S)poles.

Basedontherequiredmagneticfielddensityintherotor,thepro

per

magneticmaterial ischosentomaketherotor.Ferr itemagnetsaretraditionallyused tomakeper manentmagnets.Asthetec

hnologyadvances,rareearthalloymagnetsare

gainingpopular ity.Theferritemagnetsarelessexp

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ensivebuttheyhavethedisad-vantag

e of lowfluxdensityfor agivenvolume.Incon-

trast,thealloymaterialhashighmagneticdensityper

volumeandenablestherotor tocompr essfurther for thesametorque.Also,the

sealloymagnetsimprovethesize-to-wei

ghtratioandgivehigher torquefor thesamesizemotor usingferritemagnets.

Neodymium

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(Nd),Samar iumCobalt(SmCo)andthe

alloy of Neodymium,Fer riteandBor on(NdFeB)

aresomeexamplesof rar eearthalloymagnets.Continu-ousresear

chisgoingontoimprovethefluxden

sitytocompr esstherotor further.

Figure4 sho

wscrosssections of diff erentarr angements of

magnetsin arotor.

FIGURE 4:

ROTORMAGNETCROSSSECTIONS

S

N

N

S

S

N

N

N

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NSS

NN

S

Circular core with magnets Circular core with rectangular Circular core with rectangular magnetson the periphery magnets embedded in the rotor inserted into the rotor core

Hall SensorsUnlike a brushed DC motor, the commutation of a Note: Hall Effect Theory: If an electric current

carrying conductor is kept in a magneticBLDC motor is controlled electronically. To rotate thefield, the magnetic field exerts a trans-BLDC motor, the stator windings should be energizedverse force on the moving charge carriersin a sequence. It is important to know the rotor positionwhich tends to push them to one side of in order to understand which winding will be energizedthe conductor. This is most evident in afollowing the energizing sequence. Rotor position isthin flat conductor. A buildup of charge atsensed using Hall effect sensors embedded into thethe sides of the conductors will balancestator.this magnetic influence, producing a

Most BLDC motors have three Hall sensors embedded measurable voltage between the twointo the stator on the non-driving end of the motor. sides of the conductor. The presence of Whenever the rotor magnetic poles pass near the Hall this measurable transverse voltage issensors, they give a high or low signal, indicating the N called the Hall effect after E. H. Hall whoor S pole is passing near the sensors. Based on the discovered it in 1879.combination of these three Hall sensor signals, theexact sequence of commutation can be determined.

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FIGURE 5: BLDC MOTOR TRANSVERSE SECTION

Stator Windings

Hall Sensors Rotor Magnet S

Rotor Magnet NAccessory Shaft

Driving End of the ShaftHall Sensor Magnets

Figure 5 shows a transverse section of a BLDC motor with a rotor that has alternate N and S permanent mag-nets. Hall sensors are embedded into the stationary partof the motor. Embedding the Hall sensors into the stator is a complex process because any misalignment in theseHall sensors, with respect to the rotor magnets, willgenerate an error in determination of the rotor posi-tion.To simplify the process of mounting the Hall sensors ontothe stator, some motors may have the Hall sensor magnets on the rotor, in addition to the main rotor magnets. These are a scaled down replica version of therotor. Therefore, whenever the rotor rotates, the Hallsensor magnets give the same effect as the main mag-nets. The Hall sensors are normally mounted on a PCboard and fixed to the enclosure cap on the non-drivingend. This enables users to adjust the complete assem-bly of Hall sensors, to align with the rotor magnets, in

order to achieve the best performance.Based on the physical position of the Hall sensors,there are two versions of output. The Hall sensorsmay be at 60 or 120 phase shift to each other.Based on this, the motor manufacturer defines thecommutation sequence, which should be followedwhen controlling the motor.

Note: The Hall sensors require a power supply. Thevoltage may range from 4 volts to 24 volts.Required current can range from 5 to 15mAmps. While designing the con-troller,please refer to the respective motor technical specification for exact voltage and

current ratings of the Hall sensors used.The Hall sensor output is normally an open-collector type. A pull-up resistor may berequired on the controller side.

See the Commutation Sequence section for anexample of Hall sensor signals and further details onthe sequence of commutation.

Theory of Operation

Each commutation sequence has one of the windingsenergized to positive power (current enters into thewinding), the second winding is negative (current exitsthe winding) and the third is in a non-energized condi-tion. Torque is produced because of the interactionbetween the magnetic field generated by the stator coils and the permanent magnets. Ideally, the peaktorque occurs when these two fields are at 90 toeach other and falls off as the fields move together. Inorder to keep the motor running, the magnetic fieldproduced by the windings should shift position, as therotor moves to catch up with the stator field. What isknown as Six-Step Commutation defines thesequence of energizing the windings. See theCommutatio n Sequence section for detailed

information and an example on six-step commutation.

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TORQUE/SPEEDCHARAC

TERISTICS

Figure6 showsanexample of torque/speedcharacter-istics.Therearetwo

torquepar ameter sused todefineaBLDCmot

or,

peaktorque(TP )andratedtorque(TR).(Refer to AppendixA:TypicalMotor TechnicalSpecificationfor acompletelistof

par ameter s.)Dur -ingcontinuousoperations,the

motor canbeloadeduptotheratedtorque.Asdisc

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ussedearlier,in aBLDCmotor,thetorque

remainsconstantfor aspeedrangeuptothe

ratedspeed.Themotor canberunuptothemaximumspeed,whichcanbeupto150%of theratedspeed,butthetorquestar tsdropping.

Applicationsthathavefrequentstar tsandstopsandfrequentreversalsof rota

tionwithloadonthemotor,demandmor e

torquethantheratedtorque.Thisrequirementcomesfor abrief period,especiallywhen

the

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motor star tsfrom astandstillandduring

acceler a-tion.Dur ingthisperiod,extr atorque

isrequired toover-cometheiner tiaof theloadandtherotor itself.Themotor candeliver ahig

her torque,maximumuptopea

ktorque,aslongasitfollowsthe

speedtorquecur ve.Ref er tothe Selectinga

SuitableMotor Ratingfo r theApplicationsectiontounderstandhowtoselectthesepar ameter

sfor anapplication.

FIGURE 6:

TORQUE/SPEED

CHARACTERI

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STICS

PeakTorque

TP

Tor que

IntermittentTorqueZone

RatedTorque

TR

ContinuousTorqueZone

Rated SpeedMaximum

Speed

Speed

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COMPARING BLDC MO

TORS TO OTH

ER MOTOR T

Y

PESCompared tobrushedDCmotorsand

inductionmotors,BLDCmotorshavemanyadvantagesandfewdisadvantages.Bru

shlessmotorsrequirelessmainte-nance,sothey

have alonger lifecomparedwithbrushedDCmotors.BLDCmotorsproducemor eout-putpower per

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framesizethanbrushedDCmotorsand

inductionmotors.Becausetherotor ismadeof

per ma-nentmagnets,therotor iner tiaisless,comparedwithother typesof motors.Thisimprovesacceler ationanddeceler ationcharact

eris

tics,shorteningoperating

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cycles.Their linear speed/torquecharacteristicspro-ducepredictablespeed

regulation.Withbrushlessmotors,brushinspec

tioniseliminated,makingthemidealfor limitedaccessareasandapplicationswhereser vici

ng

isdifficult.BLDCmotorsoperatemuch

mor equietlythanbrushedDCmotors,reduci

ngElectromagneticInterfer ence(EMI).Low-voltagemodelsareidealfor batteryoperation,port

ableequipmentor medicalapplications.

Table

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1 summarizesthecomparisonbetweenaBLDCmotor andabrushe

dDCmotor .Table 2 compares aBLDCmotor toaninductionmotor.

TABLE 1: COMPAFeature

Commutation EMaintenance Les

Life LSpeed/Torque FCharacteristics

Efficiency HiOutput Power/ HigFrame Size cha

thd

Rotor Inertia LowT

Speed Range Highb

Electric Noise LowGenerationCost of Building Hig

cControl Co

Control Requirements A corusp

TABLE 2: COMPAFeatures

Speed/Torque Flat

Characteristics ratedOutput Power/ HighFrame Size smRotor Inertia LowStarting Current Rate

Control Requirements A conrusp

Slip Nofr

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COMMUTATIONSEQUENC

EFig

ure7showsanexample of Hallsensor

signalswithrespecttobackEMFandthephase

curr ent. Figure 8showstheswitchingsequencethat

sho

uldbefollowedwithrespecttothe

Hallsensors.Thesequencenumberson Figure 7 corr

espondtothenumbersgiven inFigure 8.

Every60electricaldegrees of rotation,oneof theHallsen

sor schangesthestate.Giventhis,ittakes

six

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stepstocompleteanelectricalcycle.

Insynchr onous,withevery60electricaldegree

s,thephasecurr entswitch-ingshouldbeupdated.However,oneelectricalcyclemaynotcorr espondto acompletemechanicalrevolu-tionof the

roto

r.Thenumber of electricalcyclesto

berepeatedtocompleteamechanicalrota

tionisdeter-minedbytherotor polepair s.For eachrotor polepair s,oneelectricalcycle iscompleted.So,thenumber of electricalcycl

es/r

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otationsequalstherotor polepair s.

Figure9 showsablockdiagramof thecontroller used tocontrolaBLDCmotor.Q0toQ5are

thepower switchescontrolledbythePIC18FXX31micro-controller.Basedonthemotor voltageandcurr

entratings,theseswitchescanbeMO

SFETs,or IGBTs,or simplebipolar transistors.

Table3 and Table 4 showthesequenceinwhi

chthesepower switchesshouldbeswitchedbasedontheHallsensor inputs,A,BandC.Table 3 is

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for clockwiserota-tionof themotor and

Table 4 isfor counter clockwisemotor rotation.

This isanexample of Hallsensor sig-nalshavinga60degreephaseshiftwithrespect toeachother.Aswehavepreviouslydiscussedinthe HallSen

sor

s section,theHallsensor smaybeat

60or 120phaseshifttoeachother.When

derivingacontroller for aparticular motor,thesequencedefinedbythemotor manuf acturer shouldbefollowed.

Ref erringto Figure9, if thesignals

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mar kedbyPWMxareswitchedONor OF

Faccordingtothesequence,themotor willrun

attheratedspeed.This isassumingthattheDCbusvoltageisequaltothemotor ratedvolt-age,plusanylossesacr osstheswitches.Tovar y

the

speed,thesesignalsshouldbePulseWid

thModulated(PWM) atamuchhigher freq

uencythanthemotor fre-quency.Asaruleof thumb,thePWMfrequencyshouldbeatleast 10timesthatof themaximumfrequencyof themotor.

Wh

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enthedutycycle of PWM isvariedwithin

thesequences,theaveragevoltagesupplied tothe

sta-tor reduces,thusreducingthespeed.Another advan-tage of havingPWM isthat, if theDCbusvoltageismuchhigher thanthemotor ratedvolt

age

,themotor canbecontrolledbylimiting

theper centage of PWMdutycyclecorr espond

ingtothatof themotor ratedvolt-age.Thisaddsflexibility tothecontroller tohookupmotorswithdiff erentratedvoltagesandmatchthe

ave

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ragevoltageoutputbythecontroller,to

themotor ratedvoltage, bycontrollingthePWM

dutycycle.

Therearediff erentapproaches of controls. If thePWMsignalsarelimitedinthe

microcontr oller,theupper switchescanbeturn

edonfor theentiretimeduringthecorr espondingsequenceandthecorr espondinglower switchcan

be

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controlledbytherequireddutycycle

onPWM.

Thepotentiometer ,connectedtotheanalog-to-digitalconverter channel inFigure 9,

isfor settingaspeedreference.Basedonthisinputvoltage,thePWMdutycycleshouldbecal

culated.

Closed-LoopContr ol

Thespeedcanbecontrolledin aclosed

loopbymeasuringtheactualspeedof themotor.Theerr or inthesetspeedandactualspeediscalculated.APropor -tionalplusIntegral

plu

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sDer ivative(P.I.D.)controller canbeuse

d toamplif ythespeederr or anddynamically

adjustthePWMdutycycle.

For low-cost,

low-resolutionspeedrequirements,theHallsignalscanbeused tomeasurethespeedfeed-back. A

timer fromthePIC18FXX31canbeuse

d tocountbetweentwoHalltransitions.With

thiscount,theactualspeedof themotor canbecalculated.

For high-resolutionspeedmeasurements,anopticalencoder canbefittedontothe

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motor,whichgivestwosignalswith

90degreesphasediff erence.Usingthese

signals,bothspeedanddirection of rotationcanbedetermined.Also,mostof theencodersgiv

e athir dindexsignal,which

isonepulseper revolution.Thiscanbe

usedfor positioningapplications.Opticalenc

od-ersareavailablewithdiff erentchoices of PulsePer Revolution(PPR),rangingfromhun

dredstothousands.

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FIGURE 7: HALL SENSOR SIGNAL, BACK EMF, OUTPUT TORQUE AND PHASE CURRENT

HallSensor Output

BackEMF

OutputTorque

PhaseCurrent

Sequence

Number

1 Electrical Cycle 1 Electrical Cycle

0 180 360 540 720

1A0

1B

01

C0

+A+

B- 0

+B+

C-0

+C+

0A-

+

0

+

0A

+

0B

+0C

(2) (3) (6) (2) (4) (5)(1) (4) (5) (1) (3) (6)

1 Mechanical Revolution

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FIGURE 8: WINDING ENERGIZING SEQUENCE WITH RESPECT TO THE HALL SENSOR

A A

C B C B(1) (2)

A A

C B C B(3) (4)

A A

C B C B(5) (6)

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FIGURE9: CONTROL BLOCK DIAGRAM

REF DC+

Q1 Q3 Q5PWM5A

RUN/ PWM4PWM1

STOP PWM3PWM3 PWM5PIC18FXX31 IGBT

PWM4FWD/REV PWM2 Driver PWM2

PWM1 PWM0PWM0

BCQ0 Q2 Q4

DC-

N SHall A

S NHall B

Hall C

TABLE 3:SEQUENCE FOR ROTATING THE MOTOR IN CLOCKWISE DIRECTION WHENVIEWED FROM NON-DRIVING END

Sequence Hall Sensor Input Active PWMsPhase Current

# A B C A B C

1 0 0 1 PWM1(Q1) PWM4(Q4) DC+ Off DC-2 0 0 0 PWM1(Q1) PWM2(Q2) DC+ DC- Off 3 1 0 0 PWM5(Q5) PWM2(Q2) Off DC- DC+

4 1 1 0 PWM5(Q5) PWM0(Q0) DC- Off DC+5 1 1 1 PWM3(Q3) PWM0(Q0) DC- DC+ Off 6 0 1 1 PWM3(Q3) PWM4(Q4) Off DC+ DC-

TABLE 4:SEQUENCE FOR ROTATING THE MOTOR IN COUNTER-CLOCKWISE DIRECTIONWHEN VIEWED FROM NON-DRIVINGEND

Sequence Hall Sensor Input Active PWMsPhase Current

# A B C A B C

1 0 1 1 PWM5(Q5) PWM2(Q2) Off DC- DC+

2 1 1 1 PWM1(Q1) PWM2(Q2) DC+ DC- Off 3 1 1 0 PWM1(Q1) PWM4(Q4) DC+ Off DC-4 1 0 0 PWM3(Q3) PWM4(Q4) Off DC+ DC-

5 0 0 0 PWM3(Q3) PWM0(Q0) DC- DC+ Off 6 0 0 1 PWM5(Q5) PWM0(Q0) DC- Off DC+

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WHAT

ISBACKEMF?WhenaBL

DCmotor rotates,eachwindinggeneratesa

voltageknownasbackElectromotiveFor ceor backEMF,whichopposesthemainvoltagesup

plie

d tothewindingsaccordingtoLenzsLaw.Thepolarity of thisbackEMF is

inoppositedirection of theenergizedvoltage.

BackEMFdependsmainlyonthreefactors:

Angular velocityo

f

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ther otor

Magn

eticfieldgener atedbyr otor magnets

Thenumber of tur nsinthestator windi

ngs

EQUATION1:

Back EMF= (E)

N lr B

wher e:

N is the number of w

inding tur ns per phase, l

is

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the length o

f the r otor ,r

is the inter nal r adius of the r otor ,B is the r

otor magnet

ic f ield densi

ty and is the motor s angular velocity

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Oncethemotor isdesigned,therotor magneticfieldandthenumber of turn

s inthestator windingsremainconstant.Theonl

yfactor thatgovernsbackEMF istheangular velocity or speedof therotor andasthespeedincr

eas

es,backEMFalsoincr eases.Themot

or technicalspecificationgivesapar ameter

called,backEMFconstant(ref er to AppendixA:TypicalMoto r TechnicalSpecif ication

),thatcanbeused toestimatebackEMFfor

a

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givenspeed.

Thepotentialdiff erenceacr ossawindingcanbecalculatedbysubtractingthebackEMFvaluefromthesupplyvoltage

.ThemotorsaredesignedwithabackEMF

constantinsuch awaythatwhenthemotor isrun-

ning attheratedspeed,thepotentialdiff

erencebetweenthebackEMFandthesupply

voltagewillbesufficient for themotor todrawtheratedcurr entanddeliver theratedtorque.If themotor isdrivenbeyondtheratedspeed,backEM

F

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mayincr easesubstantially,thusdec

reasingthepotentialdiff erenceacr ossthewindin

g,reducingthecurr entdrawnwhichresultsin adroopingtorquecur ve.Thelastpointon

thespeedcur vewouldbewhenthesup

plyvoltageisequaltothesumof theback

EMFandthelossesinthemotor,wherethecurr entandtorqueareequaltozer o.

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SensorlessContr

olof BLDCMotor s

Untilnowwehave

seencommutationbasedontherotor position

givenbytheHallsensor.BLDCmotorscanbecommutatedbymonitoringthebackEMFsig-nalsinstead

of theHallsensor s.TherelationshipbetweentheHallsensor sandbackEMF,withres

pect tothephasevoltage, isshownin Figure7.

Aswehaveseen inearlier sections,everycommutationsequencehasoneof thewindingsenergized

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positive,thesecondnegativeandthethir d

leftopen.Asshownin Figure7,theHallsensor

signalchangesthestatewhenthevolt-agepolarity of backEMFcrossesfrom apositivetonegative or fromnegative topositive.Inidealcas

es,

thishappensonzer o-crossing of back

EMF,butpractically,ther ewillbeadelaydue

tothewindingchar-acteristics.Thisdelayshouldbecompensatedbythemicrocontr oller .Figure10showsablockdiagramfor sensorl

ess

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controlof aBLDCmotor.

Another aspecttobeconsidered isver ylowspeeds.BecausebackEMF is

proportionaltothespeedof rota-tion, ata

ver ylowspeed,thebackEMFwouldbeat aver ylowamplitudetodetectzer o-crossing.The

mot

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or hastobestar tedinopenloop,fro

mstandstillandwhensuff icientbackEMF is

built todetectthezer o-crosspoint,thecontrolshouldbeshif tedtothebackEMFsensing.The

minimumspeedatwhichbackEMFcanbe

sen

sediscalculatedfromthebackEM

Fconstantof themotor.

Withthismethodof commutation,theHallsensor scanbeeliminate

dandinsomemotors,themagnetsfor Hallsensor salsocanbeeliminated.Thissimplifiesthe

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motor constructionandreducesthecos

t aswell.This isadvantageousif themotor is

operatingindustyor oilyenvironments,whereoccasionalcle

aningisrequired inorder for theHallsen

sor s tosenseproperly.Thesamethingapplies

if themotor ismountedin alessaccessiblelocation.

FIGURE 10: BLO

REF

RUN/STOP

PIC18FFWD/REV

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SELECTING A SU

ITABLE MOTOR RATING F

OR THE APPL

ICATI

ONSelectingtherighttype of motor for

thegivenapplicationisver yimportant.Basedontheloadchar-acteristics,themotor must

beselectedwiththeproper rating.Thr eepar

am

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eter sgovernthemotor selectionfor the

givenapplication.Theyare:

Peakt

or quer equir edf or

theapplication

RMS

tor quer equir ed

Th

e

oper atingspeedr ange

PeakTorque(TP )

Requirement

Thepeak,or maximumtorq

uerequiredfor theapplica-tion,canbecal

culatedbysummingtheloadtorque(TL),torq

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ueduetoiner tia(TJ )andthetorquerequire

d toovercomethefriction(TF).

Thereareother

factorswhichwillcontributetotheoverallpeak

torquerequirements.For example,thewindagelosswhichiscontributedbytheresistanceofferedby

theair intheair gap.Thesefactorsare

com-plicated toaccountfor.Therefore,a20

%saf etymar ginisgivenasaruleof thumbwhencalculatingthetorque.

EQUATION2:

TP = (T L + T J+ T F) * 1.2

Thetorqueduetoiner tia(TJ )isthetorq

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uerequired toacceler atetheloadfrom

standstillor from alower speedto ahigher spe

ed.Thiscanbecalculatedbytak-ingtheproduct of loadiner tia,includingtherotor iner tiaand

loadacceler ation.

EQUATION3:

T J = J

w

her e:JL

+ M

is the sum of

the load and

r otor iner tia and is the

r

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equir ed acc

eler ation

Themechanic

alsystemcoupled tothemotor shaftdeterminestheloadtorqueandthefrictionaltorque.

RMSTorqueRequ

irement(TRMS )

TheRootMeanSquar e(RMS)torquecanberoughlytranslatedtotheaveragecontinuoustorquerequiredfor the

application.Thisdependsuponmanyfactors.

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Thepeaktorque(TP),loadtorque(TL)

,torqueduetoiner tia(TJ ),frictionaltorque

(TF)andacceler ation,deceler ationandruntimes.

ThefollowingequationgivestheRMStorque

requiredfor atypicalapplicationwhereTAis

the

acceler ationtime,TRistheruntimeandTDisthedeceler ationtime.

EQUATION4:

TRMS =

[{T P2

TA +

(T L +

TF)2

TR +

(T J

TL

TF)2

TD}/

(T A +

TR +

TD)]

SpeedRange

This isthemotor speedrequired todrive

the

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applicationandisdeterminedbythetyp

e of application.For exam-ple,anapplicationlike

ablower wherethespeedvari-ation isnotver yfrequentandthemaximumspeedof theblower

canbetheaveragemotor speedrequired.Wh

ere

asinthecase of apoint-to-pointpos

itioningsys-tem,likein ahigh-precision

conveyer beltmovementor roboticarmmovements,thiswouldrequireamotor with aratedope

ratingspeedhigher thantheaveragemove-

me

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ntspeed.Thehigher operatingspeedcan

beaccountedfor thecomponentsof thetrapezo

idalspeedcur ve,resultinginanaveragespeedequaltothemovementspeed.Thetrapezo

idalcur veisshownin Figure 11.

It isalwayssuggestedtoallow asaf etymar ginof 10%,asaruleof thumb,toaccount for miscellaneous

factorswhicharebeyondour calculations.

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FIGURE 11: TRAPEZOIDAL SPEED CURVE

MaximumSpeed

Average Motor Speed

Speed

TA TR TD

Time

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TYPICAL

BLDCMOTORAPPLICATIONSBLDCmotorsfindapplications ineverysegmentof themar ket.Autom

otive,appliance,industr ialcontrols,automatio

n,

aviationandsoon,haveapplicationsfor BLDCmotors.Outof these,wecan

categorizethetype of BLDCmotor controlintothreemajor types:

Constantlo

ad

Var yingloads

Posi

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tioningapplications

ApplicationsWithConstantLoads

Thesearethetypesof applicationswhereavariablespeedismor eimportantthankeepingtheaccuracyof the

spe

edat asetspeed.Inaddition,theacceler

ationanddeceler ationratesarenotdynamicall

ychanging.Inthesetypesof applications,theload isdirectlycoupled tothemotor shaft.For exa

mple,fans,pumpsandblower scomeund

er

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thesetypesof applications.Theseapp

licationsdemandlow-costcontrollers,mo

stlyoperatinginopen-loop.

ApplicationsWithVaryingLoads

Theseare

thetypesof applicationswheretheloadonthe

motor variesover aspeedrange.These

applica-tionsmaydemandahigh-spe

edcontrolaccuracyandgooddynamicresponses. Inhomeappliances,wash-ers,dryersandco

mpr essorsaregoodexamples.Inautomotive,

fuel

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pumpcontrol,electronicsteeringcon-

trol,enginecontrolandelectricvehiclecontrolare

goodexamplesof these.Inaer ospace,ther eareanumber of applications,likecentrifuges

,pumps,roboticarmcontrols,gyr oscopecon

trol

sandsoon.Theseapplicationsmay

usespeedfeedbackdevicesandmayrunin

semi-closedloop or intotalclosedloop.Theseapplicationsuseadvancedcontrolalgorithms,

thuscomplicatingthecontroller.Also,thisincr

eas

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estheprice of thecompletesystem.

PositioningApplicat

ions

Mostof theindustr ialandautomationtypesof applica-tioncomeunder thiscatego

ry.Theapplications inthiscategoryhavesomekind of power transmission,whichcouldbemechanic

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algearsor timer belts,or asimplebelt

drivensystem. Intheseapplications,thedynami

cresponseof speedandtorqueareimportant.Also,theseapplicationsmayhavefrequent

reversalof rota-tiondirection.Atypicalcycl

e

willhaveanacceler atingphase,acon

stantspeedphaseandadeceler ationandpos

itioningphase,asshownin Figure 11.Theloadonthemotor mayvar yduringallof these

phases,causingthecontroller tobecompl

ex.

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Thesesystemsmostlyoperateinclosed

loop.Therecouldbethreecontrolloopsfun

ctioningsimultaneously:Tor queControlLoop,SpeedControlLoopandPositionControl

Loop.Opticalencoder or synchr onousresolvers

are

usedfor measuringtheactualspeedof

themotor.Insomecases,thesamesensor s

areused togetrelativeposi-tioninformation.Otherwise,separatepositionsensor smaybeuse

d togetabsolutepositions.Computer Numer ic

Co

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ntrolled(CNC)machinesareagood

example of this.Processcontrols,machiner

ycontrolsandconveyer controlshaveplentyof applications inthiscategory.

SUMMAR

YInconclusion,BLDCmotorshav

eadvantagesover brushedDCmot

orsandinductionmotors.Theyhavebetter spe

edver sustorquecharacteristics,highdynamicresponse,highefficiency,longoperatinglife,noiselessoperation,higher speedranges,rugged

con

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-structionandsoon.Also,torquedeli

ver edtothemotor sizeishigher,makingituse

fulinapplicationswherespaceandweightarecriticalfactors.Withtheseadv

an-tages,BLDCmotorsfindwidespr ead

applicationsinautomotive,appliance,aer ospace

,consumer ,medical,instrumentationandautomationindustr ies.

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