Generator excitation

Generator excitation is the DC field current supplied to the rotor of a synchronous generator that creates the magnetic field, so terminal voltage and reactive power both depend on it. A brushless exciter with rotating diodes, regulated by an automatic voltage regulator (AVR), adjusts field current to hold terminal voltage as load changes. Raising excitation increases generated voltage and exported reactive power; in parallel operation the AVRs share reactive load through voltage droop. Loss of excitation collapses voltage and can motor the set, so under- and over-excitation protection trips the generator to protect the bus.