Percent Droop
Percent droop is the ratio of the difference between no-load speed and full-load speed to the rated speed, given as a percentage. For example, on a machine with a rated speed of 1500 rpm, if 1560 rpm is measured at no load and 1500 rpm at full load, the droop is 4 percent. The larger the value, the more stable the load sharing and the weaker the frequency stability.
The calculation is simple: droop = (no-load speed − full-load speed) / rated speed. In generator applications the common setting range is between 3 percent and 5 percent; on a 50 Hz system this corresponds to a frequency shift of roughly 1.5–2.5 Hz. On adjustable electronic governors the band is usually kept at these values; since droop can reach 6 percent on engines with mechanical governors, a field difference of up to 3 Hz can be observed.
- Small droop: frequency stays more constant, but load sharing between parallel machines becomes touchy and prone to oscillation.
- Large droop: sharing is easy, but in return speed drifts noticeably as the load changes.
To measure this value in the field it is enough to remove the load and read the no-load speed, then read the speed at rated load. The measurement gives the characteristic of the engine's mechanical governor; the GS740 or GS740-3P does not change this value, because these kits act on throttle lever position, not on fuel regulation.
Context in which this term is used: Electronic speed and throttle control on a diesel engine