Contro-EMF

A motor converts electrical energy to mechanical energy. It consists of a current-carrying coil placed inside a uniform magnetic field.

When a current is passed through a coil, the magnetic field exerts a torque on the loop, rotating the coil inside the magnetic field. The magnetic flux through the rotating coil changes, inducing an emf.

According to Lenz's law, the induced emf opposes the current causing the rotation of the loop. Thus, the induced emf is called the back emf.

The total voltage available for supplying the current is the difference between the applied voltage and the induced back emf.

When the motor turns ON, the induced emf is zero, with maximum current through the loop. When the coil starts rotating, the induced emf reduces the current through the coil.

Suppose a current of 5 amperes passes through a coil with 20 ohm resistance at 100 volts. When the coil rotates at maximum speed, the back emf of 50 volts is induced, which reduces the current passing through the coil to 2.5 amperes.