DC MACHINES-DC Motor and its working

 D.C. MOTOR PRINCIPLE 
A machine that converts d.c. power into mechanical is known as a d.c. motor. Its operation 

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 magnitude is given by ; 

F =B I l newtons 


Basically, there is no constructional difference between a d.c. motor and a dc. generator. 

The same d.c. machine can be run as a generator or motor. 



WORKING OF D.C. MOTOR 
Consider a part of a multipolar dc. motor as shown in Fig. 4.1. When the terrninals ofthe
motor are connected to an extemal source of d.c. supply 


(1) the field magnets are excited developing alternate N and S poles ; 

(2)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. 


Suppose the conductors under N-pole carry currents into the plane ofthe and those under 
S-pole carry currents out of the plane of the paper as shown in Fig. 4. I. Since each

armature conductor is carrying current and is placed in the magnetic field, mechanical

force acts on it. Referring to Fig. 4.1 and applying Fleming's left hand rule, it is clear

that 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 conduct

or 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.