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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 back up power. But, most equipment like lights, motors, elevators, fire alarm, telephone equipment, copiers, and televisions will operate just fine at up to 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 like V-12 Caterpillar or Cummins. But they are often found on small units like the 4 cylinder 20 kW Onan ES 20, Kohler 20ROZ, or Generac SG20. On these 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.
Most generators use automotive engines just like in your car or pick-up truck. And just like your car when the engine encounters a load it will slow down. Sometimes a hill is steep enough no matter how hard you press on the gas your car slows down. It is the same with a generator. A load may be big enough (see my paper on starting circuits) that the engine cannot maintain its speed no matter how hard the governor "presses on the gas". That is why isochronous governors work better on big engines.
I have posted basic governor maintenance and adjustment
information from some of the common manufactures on this site. If you need
more help, email us at office@generatorservices.com., or call 1-800-888-4594.