Why AVR cards are getting faulty? Even load is not higher then the capacity of the genrator

AVR (automatic voltage regulator) is a device to maintain the output voltage at a set value under varying temp and load. The voltage sensing is done in reference to power generated in a coil to a stable reference. The reasons for fault in avr is mainly due to either rotor brushes or power getting generated in a stor itself. In either cases the mentioned component to be replaced