Working Principle of a DC Motor

The Working Principle or operation of DC Motor works on the principle that in a magnetic field when a current carrying conductor is placed, a mechanical force is produced on it. The direction of this force is determined by Fleming’s Left hand rule, and its magnitude is given by the relation shown below.


Consider only one coil of the armature placed in the magnetic field produced by a bipolar machine as shown below in the Figure A. When DC supply is connected to the coil OF A DC Motor, current flows through the coil which sets up its own field as shown in Figure B below.

WORKING-PRINCIPLE-OF-DC-MOTOR-FIG-1Now, by the interaction of the two fields (i.e., field produced by the main poles and the coil), a resultant field is set up as shown in the Figure C below.

WORKING-PRINCIPLE-OF-DC-MOTOR-FIG-2The tendency of this resultant field is to come to its original position. It tries to come in a straight line due to which force is exerted on the two coil sides and the torque is developed, which rotates the coil. Let us consider a figure shown below to understand the working principle of the Motor more clearly.

WORKING-PRINCIPLE-OF-DC-MOTOR-FIG-3The main field produces a field Fm, and its direction is shown in the above figure. When the current is supplied to the coil of the armature conductors, it produces its own field marked as Fr. This field Fr tries to come in line with the main field Fm and thus an electromagnetic torque is developed in clockwise direction.

In Practical or in DC Motor, a large number of conductors are placed on the armature. All the conductors, placed under the influence of one pole, say north pole carry the current in one direction, i.e., in the outward direction as shown in the figure below.


Whereas, the other conductors placed under the influence of other pole carry the current in the opposite direction. A resultant rotor field is produced. Its direction is marked by the arrowhead Fr. This rotor field Fr tries to align with the main field and the torque (Te) develops. Thus, the rotor start rotating.

To obtain a continuous torque in the DC Motor, the direction of flow of current in each conductor or coil side must be reversed when it crosses the magnetic neutral axis (MNA). This is achieved with the help of a commutator. The function of a commutator in DC motor is to reverse the direction of flow of current in each armature when it crosses the MNA to obtain continuous torque.

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