Although some generators are still produced with brushes and sliprings, most now have a brushless excitation system. One of the main techniques for synchronous generators is capacitor excitation and this is described in the following sections.

The use of this technique is usually restricted to single phase generators with a rated output less than 10 kW. A separate excitation winding in the stator has a capacitor connected directly across its output as shown in Fig. 5.19.

 Fig. 5.19 Capacitor excitation

The rotor is usually of salient-pole construction as described previously, but in this case the rotor winding is shorted through a diode. On starting, the residual flux in the rotor body induces a small voltage in the stator excitation winding and a current flows through the capacitor.

This current produces two waves of magnetic flux around the air gap of the generator. One wave travels in the same direction as the rotor, to create the armature reaction.

The second wave travels in a direction opposite to the rotor, and induces a voltage in the rotor windings at twice the output frequency. The current circulated in the rotor windings by this induced voltage is rectified by the diode to produce a dc current.

This dc current increases the magnetic flux in the machine, which in turn drives more current through the stator excitation winding, which in turn produces more rotor current. This self-excitation process continues until the flux reaches a point at which the magnetic circuit is saturated, and a stable voltage results.

The process also produces an inherent AVR action, since any load current in the output stator winding induces more rotor current to offset the armature reaction effect.

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