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 (Figure 2).
- (Figure 1) 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.
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| figure 1 |
- 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.
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| Figure 2 |
- 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 (Figure 3).
- 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.
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| Figure 3 |
- 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.