Back emf effect Motor
The emf induced which opposes the emf running the motor is called the back emf of the motor.
When the coil of the motor rotates across the magnetic field by the applied potential difference V, an emf is induced in it. The induced emf is in such a direction to oppose the emf running the motion. The emf induced which opposes the emf running the motor is called the back emf of the motor. the magnitude of the back emf increases with the speed of the motor. since V and are opposite in polarity, the net emf in the circuit. If R is the resistance of the coil and the current drawn by the motor, then by Ohm’s law.
When the motor is just started back emf is zero and a large current passes through the coil. As the motor speeds up, the back emf increases, and the current becomes smaller and smaller. However, the current is sufficient to provide torque on the coil to drive the load and to overcome losses due to friction. If the the motor is overloaded, it slows down, the back emf decreases and allows the motor to draw more current. If the motor is overloaded beyond its limits, the high current may burn the motor.