Operation of Speeds Above or Below Rated Speed

Author: E.E. Kimberly

When a shunt generator is operated at reduced speed, but with rated voltage, the regulation is better than normal because of the greater saturation required. If self-excited, a generator may not be capable of producing rated voltage at speeds less than 90 per cent of rated speed. At reduced speed the voltage generated per ampere of field excitation is less, and the effect on the maximum voltage is almost the same as an increase in shunt field resistance. Because of reduced ventilation at reduced speed, the temperature rise for a given armature current will be greater.

When a shunt generator is driven at higher than rated speed, but at rated voltage, the regulation is somewhat poorer than at rated speed because of operation lower down on the saturation curve. Also a change in the speed of the prime mover affects the terminal voltage more at higher speeds. It is not advisable to operate a generator at more than 15 per cent above rated speed because of dangerous centrifugal stresses, especially where the armature coils are held in their slots by band wires only.