Mains Frequency and Engine Speed
In a generator set, engine speed and the frequency of the electricity produced are inseparably linked: on a 4-pole alternator, 1500 rpm means exactly 50 Hz. If speed drops by two percent, the frequency drops by two percent as well and falls to 49 Hz. The only way to hold the frequency is to hold the speed.
The practical consequences of this relationship are serious:
- Frequency-sensitive loads: synchronous motors, clock-driven devices and some electronic supply stages are affected by frequency deviation.
- Ease of measurement: if you have no tachometer, measuring the frequency at the alternator output gives the speed indirectly but reliably. If a 4-pole set reads 50.0 Hz, the engine is at 1500 rpm.
- Load effect: on an engine with a mechanical governor, the no-load frequency is higher than the full-load frequency because of droop; this difference is typically on the order of 1.5–3 Hz.
In terms of product selection the rule is clear: fixing the position of the throttle lever does not fix the frequency. Because the standard GS740 kit and the GS740-3P do not measure speed, the frequency shifts when the load changes. An installation that requires frequency stability needs either the engine's electronic governor or the optional speed-holding module of the GS740.
Context in which this term is used: Why is constant speed important on a generator?