Question

Answer the following question regarding an a.c. generator.

When the current flowing in the external circuit is increased by decreasing the resistance, more power is required to drive the generator at the same speed. Explain.

Answer 1

In an a.c. generator, mechanical energy is converted into electrical energy. When the resistance of the external circuit is decreased, the current drawn from the generator increases. For the same speed of rotation, the induced e.m.f. remains almost the same, so the electrical power output (P = V I) increases. According to the principle of conservation of energy, this extra electrical energy must come from the mechanical energy supplied to rotate the armature. Moreover, by Lenz's law, the increased induced current creates a stronger magnetic effect that opposes the rotation of the coil, i.e. a larger retarding torque acts on the armature. To keep the generator rotating at the same speed, the driving torque must overcome this larger opposing torque. Therefore, more mechanical power has to be supplied to drive the generator.