Module- II, Lecture-4

D.C.Motor.

Principle:Whenever a current coil is placed inamagnetic field the coil experiences a

Mechanical force, and isgiven by

F=BIlSinθnewtons. WhereBis thefluxdensityinTesla

Iis thecurrent through thecoil

l is the active length of the coil side

θ is the angle between the movement ofthe

coiland the direction of the flux

Thedirection of theforceacting can be decidedbyapplying Fleming‟s lefthand rule.

The construction ofa D.C.Motor is same as the construction of aD.C.generator.

Types of D.C.Motors:

DependingontheinterconnectionbetweenthearmatureandthefieldcircuitD.C.Motorsare classifiedas(i)ShuntMotor,(ii)SeriesMotorand(iii)Compoundmotorsjustlike D.C.Generators.

Back EMF:

Wheneveracurrentcoilisplacedunderamagneticfieldthecoilexperiencesamechanical forceduetowhichthecoilstartsrotating.Thisrotatingcoilagaincutsthemagneticlinesof forceresultinganEMFinducedinitwhosedirectionistoopposetheappliedEMF(asper Fleming‟s right hand rule), and hence the nameBACKEMFor CounterEmf.

SignificanceofBackEMF:BackEMFisamustinamotorwhichhelpstoregulatethe armature current and alsothe real cause for theproduction of torque.

Expression for thebackEmf is given byE=V-IaRa,

WhereEisthebackemf,Vistheappliedemf,IaisthearmaturecurrentandRaisthearmature circuit resistance. And alsoE=PZNФ/60A volts, from the machine parameters.

Production oftorque in a D.C. Motor.

Theproductionoftorqueinad.c.motorcanbewellexplainedwiththehelpofthefollowing figures.

Fig(a)representsthemagneticfielddistributionbetweenabipolarmagnetfromNorthpoleto South pole.

Fig(b)shows thefield setup arounda current carrying coil

Infig© the current carryingcoil is brought under the influenceof bipolarmagnetic field.

Theresultantfieldaroundthecoilduetotheinteractionofthemainfieldandthecoilfieldis seeninfig(d)whereinthefluxisstrengthenedintheleftpartoftheuppercoilsideand weakenedintherightpartoftheuppercoilsideandvice-versainthelowercoilside.The resultantfluxwhichstrengthenedatonepointexertsaforceontheconductorasperFleming‟s left hand ruleand therebythe coil side experiences a mechanical force.

Intheconstructionitisseenthatseveralcoilssidesareonthearmatureandthetangentialforce actingoneachofthesecoilsidesaddeachotherandresultinginaunidirectionalmovement which makes the armature to rotate at a uniform speed therebytorque is produced.