A on line UPS is frequently cut off and on with generator supply why?

Answer 1

Often a generator output is not perfectly stable either in voltage or frequency. As the load changes, engine speed will change. This results in a change in frequency away from 60Hz, and often will cause the output voltage to sag or surge. Does the UPS cut in and out when the lights or fridge turn on or off?

The UPS likely wants to see 120V at 60Hz and will turn itself on if the generator voltage strays from that. There may be DIP switches on back of the UPS which will widen its operating window and make it less sensitive.

Answer 2

We probably need more information.

Do you have a source of commercial power, a sandby generator and a UPS? If so, I expect the UPS is there to carry you through the startup and stabilizing of the generator. Once the generator is up to speed and stabilized, the UPS should cut out, and remain out. If it starts and stops while the generator is running, it is likely a problem involving generator voltage. Perhaps it is surging or fluctuating in frequency or voltage. Maybe the UPS is sensitive about the quality of the sine wave, and sees differences. Perhaps the voltage drops below what the UPS wants to see.

Any additional information would be helpful.

Answer 3

Are asking why you are losing A phase on your UPS? If you are my guess is that your load banks are unbalanced (you have too much load on A phase). Your normal power supply may be able to handle this load imbalance but the UPS (being smaller) can not. Measure the amp loads on each phase of your UPS and switch breakers to the unloaded bus bars until they are as close to equal as possible.
Good Luck..