DC Motors || DC Motor Notes || Electrical Machines

 

         D.C. MOTORS

D.C. Motor Principle

The operation of a DC motor is based on the principle that when a current-carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. The direction of this force is given by Fleming’s left-hand rule and the magnitude is given by; F BIL Newtons. Where B is flux density, I is conductor current and L is the length of the pole face.

Working of D.C. Motor

Consider a part of a multipolar d.c. motor as shown in Fig. 1 below. When the terminals of the motor are connected to an external source of d.c. supply:
the field magnets are excited developing alternate N and S poles;
the armature conductors carry currents. All conductors under N-pole carry currents in one direction while all the conductors under S-pole carry currents in the opposite direction.

According to Fleming’s left hand rule, force on each conductor is tending to rotate the armature in anticlockwise direction. All these forces add together to produce a driving torque which sets the armature rotating. When the conductor moves from one side of a brush to the other, the current in that conductor is reversed and at the same time it comes under the influence of next pole which is of opposite polarity. Consequently, the direction of force on the conductor remains the same.

Types of D.C. Motors

Like generators, there are three types of d.c. motors characterized by the connections of field winding in relation to the armature:
Shunt-Wound Motor: In this motor the field winding is connected in parallel with the armature (Fig. 4). The current through the shunt field winding is not the same as the armature current. Shunt field windings are 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 current is relatively small compared with the armature current.

‌ Series-Wound Motor: In this motor the field winding is connected in series with the armature (Fig. 5). Therefore, series field winding carries the armature current. Since the current passing through a series field winding is the same as the armature current, series field windings must be designed with much fewer turns than shunt field windings for the same m.m.f. Therefore, a series field winding has a relatively small number of turns of thick wire and, therefore, will possess a low resistance.

‌Compound-Wound Motor: This motor has two field windings; one connected in parallel with the armature and the other in series with it. There are two types of compound motor connections (like generators). When the shunt field winding is directly connected across the armature terminals, it is called short-shunt connection (Fig. 6). When the shunt winding is so connected that it shunts the series combination of armature and series field, it is called long-shunt connection (Fig. 7).

The compound machines (generators or motors) are always designed so that the flux produced by shunt field winding is considerably larger than the flux produced by the series field winding. Therefore, shunt field in compound machines is the basic dominant factor in the production of the magnetic field in the machine.

SPEED RELATIONS

If a d.c. motor has initial values of speed, flux per pole and back e.m.f. as N1, 1 and Eb1 respectively and the corresponding final values are N2, 2 and Eb2, then,

(i)  For a shunt motor, flux practically remains constant . 

(ii) For a series motor prior to saturation.
where Ia1 = initial armature current
Ia2 = final armature current