Operation at Voltages Above or Below Rated Voltage

Author: E.E. Kimberly

The voltage at which a generator is designed to operate is always indicated on the name plate. At less than rated voltage, but at rated speed, the generator operates nearer the knee of the saturation curve and has a regulation greater (poorer) than normal. The armature reaction affects the weaker main field more than it would affect a field of normal strength and tends to cause still greater voltage regulation. Because the current is limited to the rated value to avoid excessive heating, the full rated power output cannot be obtained at voltages much below normal.

When a generator is operated at a voltage above the rated value, but at rated speed, the regulation is less (better) than normal. The higher voltage permits a power output somewhat greater than normal. Increased heating because of increased iron losses in the armature core usually limits the permissible armature current to a value less than the rated value, and hence the possible advantage is slight. The shunt field coils of most generators will not overheat when carrying the maximum current possible with self-excitation. Most generators are guaranteed to deliver their rated kilowatt output at any voltage between 90 and 110 per cent of the rated voltage.