Why does an engine run slightly under rated speed and Frequency?

There are several reasons why an engine may run slightly under rated speed. the answer varies depending on the type of governing fitted. This is true for engines on generators that are not in parallel with other generators or the mains. For generators that are in parallel with the mains or other units, this article applies when the units are off-load.

Engines with a mechanical governor needs adjusting to run at a higher fixed speed.

Engines fitted with a manual governor can often be adjusted to run at a higher fixed speed. For an example of a mechanical governor speed change, see the How to change the speed on a Perkins 1103A, 1104A and 1106A mechanical governed engine. 

Remember for mechanical governors with droop, the off-load speed should be 3-5% higher than the rated speed to allow the rated speed to be reached after the droop has been applied.

Engines an electronic governor or ECU

The speed setting may need programming

For engines fitted with an electronic governor, it may be that the speed setting needs to be correctly programmed. An electronic governor will normally have a setting called "Rated Speed", "Nominal Speed" or similar as its target when governing. This would normally only need to be done at initial setup and is usually done by the engine manufacturer. It may not be possible to set the rated speed without the engine manufacture or their agent as they often don't release the software to the public.

The speed control inputs are requesting a lower speed.

For engines with electronic governors or an ECU, especially for generators that are running in a synchronizing system in parallel with other generators or the mains, they will have a speed input allowing the generator controller to adjust the speed. This is true if the engine governor is in droop or isochronous mode.

Typically this input is either a 0-5V or 0-10V input from the generator controller to the engines governor or ECU. You should use the engines and controller wiring diagrams to check the circuit and enquire the correct reference voltage is at the engines terminals.

On Welland generators synchronized units are usually fitted with a DSE 8610 MK2. Using this controller the engine speed control can be performed either down a CANBUS link for engines with electronic governors or  ECU's fitted, or via the governor outputs terminals 35 / 36.

For engines with 0-5V outputs, to run at rated speed, there should be an output of 2.5V DC between 35 / 36 reaching the engine terminals. For most engines,  if the voltage output is low or zero, the speed will be low. If the output is high, the speed would be high.

For engines with 0-10V outputs, to run at rated speed, there should be an output of 5V DC between 35 / 36 reaching the engine terminals. For most engines,  if the voltage output is low or zero, the speed will be low. If the output is high, the speed would be high.