Induction Motor Operating as a Generator
When run faster than its synchronous speed, an induction motor runs as a generator called a Induction generator. It converts the mechanical energy it receives into electrical energy and this energy is released by the stator (Fig. 34.29). Fig. 34.28 shows an ordinary squirrel-cage motor which is driven by a petrol engine and is connected to a 3-phase line. As soon as motor speed exceeds its synchronous speed, it starts delivering active power P to the 3-phase line. However, for creating its own magnetic field, it absorbs reactive power Q from the line to which it is connected. As seen, Q flows in the opposite direction to P.
The active power is directly proportional to the slip above the synchronous speed. The reactive power required by the machine can also be supplied by a group of capacitors connected across its terminals (Fig. 34.30). This arrangement can be used to supply a 3-phase load without using an external source. The frequency generated is slightly less than that corresponding to the speed of rotation.
The terminal voltage increases with capacitance. If capacitance is insufficient, the generator voltage will not build up. Hence, capacitor bank must be large enough to supply the reactive power normally drawn by the motor.
Example 34.26. A 440-V, 4-pole, 1470 rpm. 30-kW, 3-phase induction motor is to be used as an asynchronous generator. The rated current of the motor is 40 A and full-load power factor is 85%.
Calculate
(a) capacitance required per phase if capacitors are connected in delta.
(b) speed of the driving engine for generating a frequency of 50 Hz.
Hence, the D-connected capacitor bank (Fig. 32.31) must provide 16/3 = 5.333 kVAR per phase.
