D. C. Motor

DC Motor

The Turning Effect (Motor Effect)

When a coil carrying current is placed in a magnetic field, the magnetic forces on the two sides of the coil act in opposite directions. These forces form a couple, which causes the coil to rotate. This is known as the motor effect and is the basic working principle of an electric motor.

Factors that Increase the Turning Effect

The strength of the turning effect can be increased by:

Increasing the number of turns on the coil: This multiplies the total force acting on the coil since each turn experiences a force.
Increasing the current: A stronger current increases the magnetic force acting on the coil, producing a greater torque.

Action of a Split-Ring Commutator

In a two-pole, single-coil motor, a split-ring commutator is used to reverse the direction of current in the coil every half turn. This ensures that the side of the coil moving up continues to move up, and the side moving down continues to move down — maintaining continuous rotation in one direction.

How it works: The commutator makes contact with carbon brushes. As the coil rotates, the split in the ring allows the current to switch direction at the correct moment, so the torque on the coil is always in the same direction.

Effect of Winding the Coil on a Soft-Iron Cylinder

A soft-iron cylinder placed inside the coil acts as an armature and enhances the magnetic field inside the motor. Soft iron is easily magnetised and demagnetised. It concentrates the magnetic field lines, resulting in a stronger interaction between the magnetic field and the current-carrying coil.