Isochronous Speed Control
Isochronous control is the regulation mode in which droop is zero: the engine is held at the same speed from no load to full load. On a 4-pole alternator this means 1500 rpm and 50 Hz whatever the load. This mode requires a sensor that measures speed continuously and an electronic governor.
An isochronous governor does not merely reduce the speed error, it brings it to zero with integral action. When load is applied the speed drops for a moment, the governor increases fuel and the speed returns to the reference. Recovery typically appears as a transient deviation of a few seconds; in steady state no difference remains.
- It is the standard mode on stand-alone (island mode) generators, because the frequency is required not to drift with load.
- Running two isochronous machines in parallel without communication makes load sharing unstable; that is why droop or a load-sharing line is used in parallel systems.
In application terms the critical distinction is this: holding the position of the throttle lever fixed is not isochronous control. The standard GS740 kit fixes the throttle position, it does not measure engine speed; when the load changes, the speed falls according to the engine's own characteristic. On a generator where frequency accuracy is required, speed feedback is essential; otherwise open-loop throttle control alone is not enough.
Context in which this term is used: Electronic speed and throttle control on a diesel engine