WIXLIB

3/7/2011DC MotorsAlthough ac motors are used inmost of the cases, DC motorshave many applications andused for multi-purposeapplications.DC Motor PrincipleA machine that converts dc power intomechanical energy is known as dc motor. Itsoperation is based on the principle that when acurrent carrying conductor is placed in amagnetic field, the conductor experiences amechanical force. The direction of the force isgiven by Fleming’s left hand rule.1

3/7/2011Fleming Left Hand Rule2

3/7/2011Back or Counter EMFWhen the armature of a d.c.motor rotates under theinfluence of the drivingtorque, the armatureconductors move throughthe magnetic field and hencean e.m.f. is induced in them.The induced e.m.f. acts in opposite direction to theapplied voltage V(Lenz’s law) and is known as back orcounter e.m.f. Eb.Significance of Back E.M.FThe presence of back e.m.f. makes the d.c.motor a self-regulating machine i.e., it makesthe motor to draw as much armature current asis just sufficient to develop the torque requiredby the load.Back e.m.f. in a d.c. motor regulates the flow ofarmature current i.e., it automatically changesthe armature current to meet the loadrequirement.3

3/7/2011DC Motor Types Shunt Wound Series Wound Compound woundShunt Wound MotorIn shunt wound motor thefield winding is connected inparallel with armature.The current through the shuntfield winding is not the sameas the armature current.Shunt field windings are designed to produce thenecessary m.m.f. by means of a relatively large numberof turns of wire having high resistance. Therefore, shuntfield current is relatively small compared with thearmature current4

3/7/2011Series Wound Motor In series wound motor the fieldwinding is connected in serieswith the armature. Therefore,series field winding carries thearmature current. Since thecurrent passing through a seriesfield winding is the same as thearmature current, series fieldwindings must be designed with much fewer turns thanshunt field windings for the same mmf.Therefore, a series field winding has a relatively smallnumber of turns of thick wire and, therefore, will possessa low resistance.Compound Wound MotorCompound wound motor hastwo field windings; oneconnected in parallel with thearmature and the other in serieswith it. There are two types ofcompound motor connections1) Short-shunt connection2) Long shunt connectionWhen the shunt field winding is directly connectedacross the armature terminals it is called short-shuntconnection.5

3/7/2011Compound Wound MotorWhen the shunt windingis so connected that itshunts the seriescombination of armatureand series field it is calledlong-shunt connection.Parts of a DC MotorA simple motor has six parts Armature/ Rotor Commutator Brushes Axle Permanent Magnet DC Power supply6

3/7/2011Parts of a DC MotorCommutator and Brushes7

3/7/2011Commutation in DC MotorIn order to produce unidirectional force (ortorque) on the armature conductors of amotor, the conductors under any pole mustcarry the current in the same direction at alltimes.The function of commutator and brush gear in adc motor is to cause the reversal of current ina conductor as it moves from one side of abrush to the other.Voltage Equation of MotorsLet in a d.c. motorV applied voltageEb back e.m.f.Ra armature resistanceIa armature currentSince back e.m.f. Eb acts in oppositionto the applied voltage V, the netvoltage across the armature circuit is VEb. The armature current Ia is given byThis is known as voltage equation of the d.c. motor8

3/7/2011Condition For Maximum PowerThe mechanical power developed by the motor isSince, V and Ra are fixed, power developed by the motor dependsupon armature current. For maximum power,should bezero.Hence mechanical power developed by the motor is maximumwhen back e.m.f. is equal to half the applied voltage.Armature Torque of a DC MotorTorque is the turning moment of a force about anaxis and is measured by the product of force (F)and radius (r) at right angle to which the force acts9

3/7/2011In dc motor, each conductor is acted upon bycircumferential force, F at a distance r.10

3/7/2011Speed of a DC MotorSpeed of a DC MotorTherefore, in a dc motor speed is directly proportionalto back emf, Eb and inversely proportional to flux, φ.11

3/7/2011Speed RelationsProblemA 220 V dc shunt motor runs at 500 rpm whenthe armature current is 50 A. Calculate thespeed if the torque is doubled. Given thatRa 0.2 ΩTa2/Ta1 Ia1/Ia2; N2/N1 Eb2/Eb1Eb Va-IaRa12

3/7/2011Losses and Efficiency of a DC MotorLosses in DC Motor:1) Copper Loss2) Iron Loss3) Mechanical LossThe efficiency of a d.c. motor is the ratio ofoutput power to the input powerDC Motor Characteristic(i) Torque and armature current characteristics(Ta/Ia): It is the curve between armaturetorque and armature current of a dc motor(ii) Speed and armature current characteristics(N/Ia): It is the curve between speed andarmature current.(iii)Speed and torque characteristics (N/Ta): It isthe curve between speed and armaturetorque.13

3/7/2011Characteristics of a shunt motorTa/IaN/IaN/TaCharacteristics of a Series Motor14

3/7/2011Characteristics of a compound motor15

3/7/2011Applications of DC MotorShunt Motor:The characteristics of a shunt motor reveal that it isan approximately constant speed motor.Industrial Use: Lathes, Drills, Boring Mills, Shapers ,Spinning, and weaving machinesSeries Motor: It is a variable speed motor i.e. speedis low at high torque and vice versa. This motor isused when large starting torque is required.Industrial Use: Electric Traction Cranes, Elevators,hair drier, Sewing machineProblemA 440 V shunt Motor has armature resistanceof 0.8 Ω and field resistance of 200 Ω.Determine the back emf when giving anoutput of 7.46 KW at 85% efficiency.Motor input power Motor input current 19.95 AShunt current V/Rsh 2.2 AArmature Current, Ia 17.75 ABack emf V-IaRa 425.8 V16

3/7/2011Comparison of three types of Motor1) The speed regulation in shunt motor is better than seriesmotor2) The starting torque of a series motor is more than othermotor3) Both shunt and compound motors have definite no loadspeed, however series motors have dangerously highspeed at no load.DC machineMotors and generators are dc machines.There are several tests to judge theperformance of a DC machine. The efficiencyof a dc machine depends upon the losses. Thesmaller the losses, the greater is the efficiencyof the machine.17

3/7/2011Armature ReactionWhat is meant by armature reaction?In dc machine, the main field is produced by thefield coils. In both the generating and motoringmodes, the armature carries current andmagnetic field is established, which is calledarmature flux. The effect of armature flux onmain field is called the armature reaction.What armature reaction does?It demagnetizes or weakens the magneticflux/field.It cross-magnetises or distorts it.Overcome:The demagnetizing effect can be overcome byadding extra ampere-turns on the main field.The cross magnetizing effect can be reduced byhaving common poles.18

3/7/2011Advantage of DC MotorAlthough a far greater percentage of electric motorsin service are a.c. motors,the d.c. motor is ofconsiderable industrial importance. The principaladvantage of a d.c. motor is that its speed can bechanged over a wide range by a variety of simplemethods. Such a fine speed control is generally notpossible with a.c. motors. In fact, fine speed controlis one of the reasons for the strong competitiveposition of d.c. motors in the modem industrialapplications.Speed Control of DC MotorThe speed of a dc motor is given byi) Flux Control: By varying the flux per pole.ii) Armature Control: By varying the resistance in thearmature circuit.iii)Voltage Control: By varying the applied voltage.19

3/7/2011Flux Control MethodIt is based on the fact that by varying the flux f, the motorspeed (N) can be changed and hence the name fluxcontrol method. In this method, a variable resistance isplaced in series with shunt field winding.Armature Control MethodThis method is based on the fact that by varying thevoltage available across the armature, the back e.m.fand hence the speed of the motor can be changed.This is done by inserting a variable resistance inseries with the armature.20

3/7/2011Voltage Control MethodIn this method, the voltage source supplyingthe field current is different from that whichsupplies the armature. This method avoids thedisadvantages of poor speed regulation andlow efficiency as in armature control method.However, it is quite expensive. Therefore, thismethod of speed control is employed for largesize motors where efficiency is of greatimportance.QuotationLearn to enjoy every minute of your life. Behappy now. Don't wait for something outsideof yourself to make you happy in the future.Think how really precious is the time you haveto spend, whether it's at work or with yourfamily. Every minute should be enjoyed andsavored. - Earl Nightingale21

The characteristics of a shunt motor reveal that it is an approximately constant speed motor. Industrial Use : Lathes, Drills, Boring Mills, Shapers , Spinning, and weaving machines Series Motor: It is a variable speed motor i.e. speed is low at high torque and vice versa. This motor