Automatic Voltage Regulator. It's the box of tricks which maintains the terminal voltage of the generator at the demanded value by adjusting the strength of the magnetic field of the rotor. The rotor is an electromagnet and the field is controlled by the current through it.
The AVR measures the terminal voltage and through some form of singal conditioning and amplification it produces a current output. Some AVRs are based on a permanent magnet generator (PMG) to provide the power to the field, others use the main output of the generator for their power. Some have a rectifier on the shaft, some pass DC from the AVR through slip rings. Some use a signal of a few amps to control a pilot exciter and main exciter which in turn feed the generator field, others directly deliver perhaps 6000A directly to to the main generator field. There are a lot of permutations!
In the machine you're considering in your other thread it's a fair bet that it uses a PMG and a rotating rectifier.
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If we learn from our mistakes I'm getting a great education!
