Run-down speed (genset)

Run-down speed of a generator set is the lowest stable speed at which the engine holds frequency well enough to stay synchronized, the floor of its useful operating band before load sharing breaks down. Marine AC generators must hold their speed within a narrow window so output frequency stays near 50 or 60 Hz; below the run-down speed the governor can no longer maintain stable speed under fluctuating load, and a paralleled set drops out of step. The term also describes the controlled deceleration when a set is taken offline: the breaker opens, fuel is cut, and the rotor coasts down. Stable low-load running is set by the governor droop and the engine’s minimum firing limit.