If a machine of this type is connected to a supply, it accelerates as a motor up to a speed near its synchronous speed. If the machine is driven faster than the synchronous speed by an engine or other prime mover, the machine torque reverses and electrical power is delivered by the machine (now acting as a generator) into the connected circuit.

A simple form of wind turbine generator uses an induction machine driven by the wind turbine. The induction machine is first connected to the three-phase supply, and acting as a motor it accelerates the turbine up to near the synchronous speed. At this point, the torque delivered by the wind turbine is sufficient to accelerate the unit further, the speed exceeds the synchronous speed and the induction machine becomes a generator.

It is also possible to operate an induction machine as a generator where there is no separate mains supply available. It is necessary in this case to self-excite the machine, and this is done by connecting capacitors across the stator winding as shown in Fig. 5.20(a).

The leading current circulating through the capacitor and the winding produces a travelling wave of mmf acting on the magnetic circuit of the machine. This travelling wave induces currents in the rotor cage which in turn produces the travelling flux wave necessary to induce the stator voltage.

For this purpose, some machines have an excitation winding in the stator which is separate from the main stator output winding. Figure 5.20(b) shows a single-phase version of the capacitor excitation circuit.

In small sizes, the induction generator can provide a low-cost alternative to the synchronous generator, but it has a relatively poor performance when supplying a low power factor load.

Although induction generators have useful characteristics for use in combination with wind turbines, the magnetizing current must be supplied by other generators running in parallel, or capacitors connected across the stator windings.

Another problem is that the efficiency of an induction generator drops if its speed differs significantly from the synchronous speed, due to high rotor copper loss in the rotor cage.

This can be overcome by using a slipring-fed wound rotor combined with a power electronic converter connected between the stator and rotor windings. Such schemes are often referred to as slip energy recovery using a doubly fed induction generator.

The slip s, of an induction machine is the per unit difference between the rotor speed and the synchronous speed given by:

s = (Ns − Nr)/Ns (5.8)

where Ns is the synchronous speed and Nr is the rotor speed. It can be shown that if Tr is the mechanical torque supplied by a turbine to the rotor of the induction generator, the generated electrical stator power transferred across the air gap is given by TrNs.

Since the input mechanical power to the generator is TrNr, the difference TrNr − TrNs must be the power lost in the rotor, produced mainly by copper loss in the cage. By substitution from eqn 5.8: power transfer to stator = TrNs = rotor loss/s.

With a simple squirrel cage rotor therefore the slip must be low to avoid high rotor loss with a resultant low efficiency.

If the cage is replaced by a three-phase winding, and sliprings are fitted, the same power balance can be achieved by removing the generated rotor power via the sliprings. This power can then be returned to the stator of the generator via a frequency converter. The rotor generated frequency is given by the stator frequency times slip ( f × s).

Fig. 5.20 Self-excitation of an induction generator (a) three phase (b) single phase

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