Brushless Exciter

This wonderful picture is of the clever part of a brushless exciter for a 17 MVA generator.

Exciter Animated Gif

The exciter rotor which is the rotating part is about 400mm across and is fixed to the shaft of the main generator which rotates at 1500 rev per minute. The exciter field windings, which do not rotate, are fed with dc from the excitation controller, a box of electronics usually located in a control panel, which in turn obtains its power from a permanent magnet generator also on the same shaft and not visible in the picture.

The exciter is sometimes referred to as an inside out generator. The dc field is fixed and the armature rotates, unlike a generator where the dc field rotates and the armature is fixed.

The exciter armature has 3 phase windings and the output is rectified by the 6 diodes visible, 3 with red leads and 3 with blue leads. These are connected to die cast heat sinks which also act as terminals for the exciter's output.

The output is dc and flows down heavy black conductors, just visible, 2 for positive and 2 for negative. These pass down the centre of the shaft directly into the field windings of the 17 MVA generator.

The small black modules visible are suppressors ancillary to the operation of the exciter.

There are 12 poles on the excitation stator and so the 3 phase output to the rectifier has a frequency of 150 Hz.  The 3 phase bridge rectifier then produces an output voltage with 900 Hz ripple.  The output current is heavily smoothed by the high inductance of the main generator rotor.