Separate exciter. The most common way of supplying dc current to the rotor winding without brushes and sliprings is shown in Fig. 5.17.

Fig. 5.17 Separate brushless exciter

The output of the AVR drives a dc current If through the pole windings of the exciter, which are mounted in a stator frame. The poles produce a stationary field which induces a voltage in the exciter rotor winding as it turns.

The ac voltage produced by the rotor winding of the exciter is converted to dc by a bridge rectifier which is also mounted on the rotor shaft.

The dc output of the rectifier is connected to the main rotor windings by conductors laid in a slot along the rotor shaft. The inductance of the main generator rotor coils is usually sufficient to smooth out the ripple in the bridge rectifier output.

The power supply to the AVR is either provided by a separate excitation winding in the main generator stator, or by a small permanent-magnet generator mounted on the shaft of the main generator, often referred to as a ‘pilot exciter’.

The advantage of the pilot exciter is that the generator has a source of power available once the shaft is turning; the voltage supplied to the AVR is completely independent of generator load and there is no reliance on residual flux in the magnetic circuit of the main generator to start the self-excitation process.

The pilot exciter also enables the generator to supply current to a connected network even when a short circuit occurs, enabling the high current to be detected by protection relays which will then disconnect the faulty circuit.

If the AVR is supplied from an excitation winding in the main generator stator, the supply voltage is very small when the stator windings experience a short circuit, and the AVR is unable to drive an adequate rotor excitation current.

One manufacturer uses two excitation windings to provide a voltage from the AVR under short-circuit conditions, so that sufficient current is supplied into the fault to trip the protection system.

During a short circuit the air gap flux density in these machines shows a pronounced harmonic component. This component induces voltage in coils of one of the excitation windings, which are short pitched and therefore deliver a voltage to the AVR even under short-circuit conditions.

The second excitation winding is fundamental-pitched and provides the major drive for the AVR under normal operating conditions. It is claimed that the performance of this system is comparable to a machine using a permanent-magnet exciter.

Another method used to provide voltage to the AVR under short-circuit conditions is a series transformer driven by the generator output current.

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