Droop Governor
Droop is the regulation behaviour that deliberately allows engine speed to fall somewhat when load comes on. On engines with mechanical governors, the no-load speed is typically higher than the full-load speed by the equivalent of 1.5–3 Hz. On generators running in parallel, this deliberate droop is what makes the load share in proportion to power.
The governor increases fuel as the speed it measures falls. In a governor with droop this increase is not enough to null the error completely: each load level corresponds to a different, single stable speed. The system is therefore inherently stable and hard to drive into oscillation.
- Advantage: when two engines run in parallel on the same line, the machine whose speed falls more as load rises takes less load; the sharing balances itself.
- Disadvantage: on a generator running alone, the mains frequency drifts with the load.
On the AXI side this concept is a selection criterion: the GS740 and GS740-3P are not governors, the engine's mechanical governor stays in place. These kits electronically drive the position of the throttle lever; the drop under load is determined by the droop characteristic of the engine's own governor. If you want to hold speed constant under load, feedback that really measures speed is required; for the GS740 this is possible with the optional active-feedback speed stabilisation module.
Context in which this term is used: Electronic speed and throttle control on a diesel engine