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Working Principle of DC Motor

An Electric DC motor is a machine which converts electric energy into mechanical energy. The working of DC motor is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force. 

The direction of mechanical force is given by Fleming’s Left-hand Rule and its magnitude is given by F = BIl Newton.

There is no basic difference in the construction of a DC generator and a DC motor. In fact, the same d.c. machine can be used interchangeably as a generator or as a motor. Like generators DC motors are also classified in to shunt-wound, series-wound and compound-wound.

DC motors are seldom used in ordinary applications because all electric supply companies furnish alternating current. 

However, for special applications such as in steel mills, mines and electric trains, it is advantageous to convert alternating current into direct current in order to use dc motors. The reason is that speed/torque characteristics of d.c. motors are much more superior to that of a.c. motors. Therefore, it is not surprising to note that for industrial drives, d.c. motors are as popular as 3-phase induction motors.

Working of DC Motor

Consider a part of a multipolar d.c. motor as shown in Figure below. When the terminals of the motor are connected to an external source of d.c. supply: 
(i) the field magnets are excited developing alternate N and S poles; 
(ii) 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 of the paper and those under S-pole carry currents out of the plane of the paper as shown in Figure. Since each armature conductor is carrying current and is placed in the  magnetic field, mechanical force acts on it. On 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 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.

It should be noted that the function of a commutator in the motor is the same as in a generator. By reversing current in each conductor as it passes from one pole to another, it helps to develop a continuous and unidirectional torque.