Generator Performance

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New member
I am looking for documentation for generator performance standards. I am the Plant Operations Manager for a small 25 bed hospital. We had an old 75kw generator that pumped fuel out of 55 gal drums inside a bldg. OSHPD made us upgrade to an exterior unit with a skid mounted 24 hour tank. We put in a 125KW Olympian. When the generator company came out to load test the unit the load bank shut down from low power output from the generator. When 100% block load was applied the frequency dropped to about 38 Hz. The generator company came back and tweeked the governor and the load was applied without tripping the load bank. The problem I have is that at time of transfer it takes much longer than the old generator and that the frequency does not stay at 60 Hz. We transfer at less than 25% of the generators capacity and the frequency goes from 63-57 Hz. Is there any published standard of what an acceptable transient frequency is? Thanks for your input.


Senior Member
Re: Generator Performance

The performance has everything to do with the engineering specifications that you bought the unit with.
If you took the vendors "special", then you will be stuck with the standard manufacturers specs for that particular unit. The code does not specify permitted frequency deviation or voltage drop, unless essential equipment stops working.


Esteemed Member
Re: Generator Performance

Contact the product service department of the manufacturer. Your problem is partly due to the out of sync transfer.

Slowing down the transfer will sometimes correct the problem.

Make an accurate load schedule of the transferred equipment.

This is definitely a job for a company representative.

tom baker

First Chief Moderator
Staff member
Re: Generator Performance

Sounds like a voltage regulator issue, and related to the speed control. There are different types of both, can be electronic or mechnical. NFPA 110 is the standard for emergency and standby power systems, it does contain language on "generator system performance" in 3-5.9.6, but it says that the "frequency dip and rstoration to steady state shall not exceed the users needs". If you didn't specify any type of frequency limits the gen supplier may of sold you a unit thats was not suitable.

A generator is supplied to meet the load requirements. The only generator I've ever bid was to run a 100 hp submersible pump, we specifed a 150 kw unit with oversize alternator of 250 kw to limit the starting voltage drop to 20%. We ended up a with a 180 kw generator and it handles the 100 hp starting across the line just fine. But we had language in the specs that limited the voltage drop to 20% as measured by an O-scope...

brian john

Senior Member
Leesburg, VA
Re: Generator Performance

What type of governor did you spec.
what type of load
what about your fuel
block heater work
did you buy the standby unit
did you spec the product or did you accept an equal

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