Synchronous Speed
Synchronous speed is the exact speed at which a machine with a given number of poles must turn to produce the desired frequency. For 50 Hz it is 3000 rpm with 2 poles, 1500 rpm with 4 poles and 1000 rpm with 6 poles. On a synchronous alternator this value is not a target but a mandatory condition of operation.
The value follows from the relationship frequency times 120 divided by the number of poles. In 60 Hz systems the equivalents for the same pole counts are 3600, 1800 and 1200 rpm.
- In a synchronous machine the rotor turns at exactly this speed; a deviation is directly a frequency deviation.
- In an asynchronous (induction) machine the rotor turns slightly below synchronous speed; the difference is called slip and increases with load.
When setting up speed control these values give the target points. In a generator set, leaving the upper speed limit markedly above the synchronous speed leads the operator to raise the frequency without realising it. For this reason, in a GS740 or GS740-3P installation the upper limit should be taught according to the point at which the engine will hold synchronous speed at full load, and the setting must be verified under load.
Context in which this term is used: Why is constant speed important on a generator?