Induction Generators

An induction generator is a comparatively simple and low-cost machine. The stator can be single- or three-phase and the rotor can be of squirrel-cage or wound design. Figures 25-1 and 25-2 are induction machines that may be used as a motor or generator.

Synchronous speed is a function of frequency and the number of poles that constitute the magnetic field. It is determined as follows:


f = Frequency in Hertz P = Number of poles

Fig. 25-1 Induction Machine. Source: United States Motors

An induction machine serves as a generator when it is driven above its synchronous speed. When it is operating below its synchronous speed, it serves as a motor. When operating as a motor (below synchronous speed), the induction machine absorbs current and produces torque (at a positive value). When operating as a generator, the machine does the opposite, absorbing torque and producing current.

Synchronous speed (RPM) =

120 f

Fig. 25-2 Induction Machine.

Source: Tuthill Corp. Murray Turbomachinery Division

Fig. 25-2 Induction Machine.

Source: Tuthill Corp. Murray Turbomachinery Division

The speed of an induction motor decreases below its synchronous speed as the load increases. The difference between synchronous speed and the motor's actual operating speed is referred to as slip, and is calculated as follows:

(Synchronous speed — Actual speed) Synchronous speed

To generate power, an induction machine is driven by a prime mover above its synchronous speed. Circulating currents are induced in the rotor bars by the reactive current of the grid or other power source. The magnetic field interactions between the rotor and stator are converted into electric power that flows into the grid. The faster the generator is driven, the more power generated.

Induction generators do not have exciters or voltage regulators. They require an external source of reactive power for excitation, typically provided by the electric grid. Excitation can also be provided from other external power sources, such as capacitors or local synchronous generators connected to the system bus.

Induction generators are much more simple to operate and control than synchronous units. Protective devices required for a typical induction generator installation include:

• Over/under voltage

• Over/under frequency

• Reverse power

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