DC Motors - 123seminarsonly.com€¦ · designed to produce the necessary m.m.f. by means of a relatively large number of turns of wire having high resistance. Therefore, shunt field

DC Motors

• Operation

• Construction

• Classification

• Applications

A DC Motor converts electrical energy intomechanical energy.

Special applications where dc motors are used include: in steel mills, mines and electric trains.

Its operation is based on the principle thatwhen a current carrying conductor is placed ina magnetic field, the conductor experiences amechanical force.

DC Motor Operation:

The direction of this force is given by Fleming's left hand rule and

magnitude is given by: F = BIl newtons . It acts in tangential direction

to the rotor, thus producing torque.

Working of D.C. Motor

Suppose the conductors under N-pole carry currents into the plane of thepaper and those under S-pole carry currents out of the plane or the paper.Since each armature conductor is carrying current and is placed in themagnetic field, mechanical force acts on it.

Applying Fleming's left hand rule, forceon each conductor tends to rotate thearmature in anticlockwise direction. Allthese forces add together to produce adriving torque which sets the armaturerotating. When the conductor movesfrom one side of a brush to the other, thecurrent in that conductor is reversed andat the same time it comes under theinfluence of next pole which is ofopposite polarity. Consequently, thedirection of force on the conductorremains the same.

part of a multipolar d.c. motor

How the Commutator Works

As the rotor turns, the commutator terminals alsoturn and continuously reverse polarity of thecurrent it gets from the stationary brushesattached to the battery.

Controlling Motor Direction• To change the direction of rotation:

– Simply switch the polarity of the battery leads going to the motor (that is, switch the + and –battery leads)

+-

-+

CW CCW

Direction of Rotation

Construction of DC Motor

All d.c. machines have five principal components : (i)field system (ii) armature core (iii) armature winding (iv) commutator (v)brushes

Back or Counter E.M.F.

When d.c. voltage V is applied across themotor terminals, the field magnets areexcited and armature conductors are suppliedwith current. Therefore, driving torque actson the armature which begins to rotate. Asthe armature rotates, back e.m.f Eb isinduced which opposes the applied voltage V.The applied voltage V has to force currentthrough the armature against the back e.m.f.The electric work done in overcoming andcausing the current to flow against Eb isconverted into mechanical energy developedin the armature.

shunt wound motor

Therefore, energy conversion in a d.c. motor is possible only due to the production of back e.m.f. Eb.

Power Equation

If Eq.(i) is multiplied by Ia ,

Types of D.C. Motors(a) D.C. Series motor(b) D.C. Shunt motor(c) D.C. Compound motor:

(i) Cumulative compound motor(ii) Differential compound motor

(a) Shunt-wound motor in which the fieldwinding is connected in parallel with thearmature. The current flows partlythrough the field and partly through thearmature. Shunt field windings aredesigned to produce the necessary m.m.f.by means of a relatively large number ofturns of wire having high resistance.Therefore, shunt field current is relativelysmall compared with the armaturecurrent.

Supply voltage, V = Eb + Ia Ra

or generated e.m.f., Eb = V− Ia Ra

Supply current, IL = Ia + Ish, from Kirchhoff’s current law.

(b) Series-wound motor in which the fieldwinding is connected in series with thearmature. Therefore, series field windingcarries the armature current. Since thecurrent passing through a series fieldwinding is the same as the armaturecurrent, series field windings must bedesigned with much fewer turns thanshunt field windings for the same m.m.f.Therefore, a series field winding has arelatively small number of turns of thickwire and therefore have low resistance.

Supply voltage, V = Eb + Ia(Ra + Rse)or generated e.m.f., Eb = V− Ia(Ra + Rse)Supply current, IL = Ia = Ise, from Kirchhoff’s current law.

(c)Compound-wound motor It is a combination of series and shunt motor. Thecompound motor has a shunt field winding in addition to series winding so that thenumber of magnetic flux lines of force produced by each of its poles is the resultantof flux produced by shunt and series coils. There are two types of compound woundmotor:(i) Cumulative compound, in which the series winding is so connected that the field

due to it assists that due to the shunt winding.(ii) Differential compound, in which the series winding is so connected that the field

due to it opposes that due to the shunt winding.

Applications of D. C. Motors Shunt Motors It is an approximately constant speed motor. It is therefore, used where the speed is required to remain almost constant from no-load condition to full load-condition.

Industrial Use : - Lathes, Small Drills, Shapers, Spinning and Weaving Machines etc.

Series Motors It is a variable speed motor. Its speed is low at high torque and vice-versa. However, at light or no-load, the motor tends to attain dangerously high speed. The motor has a high starting torque. It is therefore, used where(i) Large starting torque is required like in Elevators and Electric Traction.(ii)The load is subjected to heavy fluctuations and the speed is automatically required to reduce at high torques and vice-versa.(iii) Should not operate at no-load.Industrial Use : - Electric traction, elevators, cranes, hoist, conveyor belts, rolling mills etc.

Compound Motors

(i)Differential-compound motors are rarely used because of their poor torque characteristics.

(ii)Cumulative-compound motors are used where a fairly constant speed is required with irregular loads or suddenly applied heavy load .

Industrial Use : - Presses, Shears, Reciprocating Machines , Drilling etc.

Applications of D. C. Motors