the black start generator was recently serviced and was put back on line. but during start up, it is not running at the required frequency and voltage.

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Hi,

You didn't tell us what was done during the recent service to the black start generator. Most prime movers these days (the device (engine) that drives the generator) have electronic governors that look at a frequency feedback from some kind of speed pick-up on the prime mover shaft and then control fuel to make the actual speed (and hence the generator frequency) equal to the setpoint.

If someone changed the number of teeth on the wheel that the speed pick-up(s) monitor, then this could happen. If the electronic governor setpoints were changed (or perhaps the electronic governor was unintentionally "reset") during the outage this could cause the problem you are reporting.

Generator terminal voltage is also a function of speed, so if the speed is not at rated it's likely the generator terminal voltage will also not be at rated.

You haven't provided enough information for us to be of help. You didn't tell us what kind of prime mover (reciprocating engine; turbine; etc.) is driving the generator. You didn't tell us anything about the prime mover control system (manufacturer; model number). You didn't tell us anything about what was done during the outage--especially if it might have had an effect on the fuel system. There have been cases of the wrong fuel filter being installed restricting the flow of fuel to the fuel system (fuel injectors on a diesel engine) which caused the unit not to reach rated speed and voltage.

But, we just don't know enough about the outage and the equipment at your site because you didn't provide any actionable data for us to help you or to point you in the right direction.

Something is amiss--and the only thing we do know for sure is: The problem started after the service was done. So, it's likely something that was done during the service--unintentionally or not. This is a good first step, understanding when the problem started and what changed to cause the problem. But to be of any further help you would need to provide a lot more information.