Engine Governors

How Engine Governors Operate on Commercial Emergency Generators

It is the engine speed that sets the generator’s frequency.

For example, an 1800 RPM engine connected to a four-pole generator produces 60 cycles.
That same engine slowed down to 1500 RPM produces 50 cycles.
The engine crankshaft and the generator rotor are mechanically locked together. Therefore, the engine speed and generator frequency cannot be separated.

All common electrical devices designed for use with AC current in the continental United States are designed to work on 60 cycles. If this frequency is increased or decreased more than 5%, damage may result.

Most generators are designed to operate from 5 % droop to no droop.
This range is acceptable to ordinary equipment and will produce no damage.
However, some things require tighter regulation.

UPS systems, clocks, and X ray equipment are a few examples.
You should always check with the manufacturer of any special equipment if you plan to run it on backup power.
But most equipment like lights, motors, elevators, fire alarm, telephone equipment, copiers, and televisions will operate just fine at up to a 5% droop.

Note: By “Droop” I mean droop in speed (frequency) from no load to full load.
If a generator runs at 60 cycles, 5% of that is 3 cycles.
Therefore, if I set the speed it runs with no load at 62 cycles, it is ok if it slows down to 59 cycles when fully loaded.
That just means the governor is set to 5% droop.

Some generators have governors that operate up to 0% droop.
These are called isochronous governors and they are usually electronic.
These are effective only on very large engines.
But they are often found on small units like a 4-cylinder 20 kW engine.
On small engines electronic governors are used because they are cheaper or more expedient for the manufacturer to use. They cannot be expected to maintain isochronous speed regulation.

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