Answer to Question #189060 in Electric Circuits for Rohan Vadgama

Question

Explain how the back emf of a motor causes the development of mechanical power.

Answer 1

The back emf makes the DC motor self regulating machine i.e., the back emf develops the armature current according to the need of the motor. The armature current can be calculated using the formula:

$$I=\dfrac{V-E_b}{R_a}$$

Consider the motor is running at no-load condition. At no load the DC motor requires small torque for controlling the friction and windage loss. The motor withdraw less current> AS the back emf depends on the current their value also decreases. The magnitude of back emf is nearly equal to supply voltage.

If let sudden load is applied to the motor, the motor slows down. As the speed of motor decreases, the magnitude of their back emf also falls down. The small back emf will withdraw large current from the supply. The large armature current of the motor induces the large torque in the armature, which is need of the DC motor. Thus, the motor moves continuously at new speed.

In this way, back emf of a motor causes the development of mechanical power.