Power Supplies don't like generator

I have some of the cheaper UPS made by APC (model BK350). I have a 12kw generator and have these UPS to keep servers running until the generator starts. That works fine, but once the generator cuts on, the the BK350 will switch back and forth between battery and live power until it finally looses battery power and cuts off. The electricity coming from the generator is 110 volts. I'm sure it flucuates more than the line power, but I would expect the UPS to work with a generator. Does anyone know if these power supplies do not work with generators.


Reply to
Bruce Pate
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Bruce Pate schrieb:


what is the load of the 12 kW generator? Only several UPS with not more than 1 kW alltogether? I would suggest to add a resitive load of 2 to 4 kW. If the UPS units are the only load, the generator operates with no load at all or a very small electronic load less than 10 % of the maximum generator load, the voltage regulation of the generator produce a very unstable voltage under this load.


Reply to
Uwe Hercksen

Have you set the voltage and frequency on the generator? 12 kw is right on the edge for having its own voltage and frequency controls, depending on the manufacture. Plug in your VOM and check voltage at idle and frequency. Then get about 1/2 load on it and check again. Set the voltage and frequency with the load on it. Now check the voltage and frequency at idle. It might be in the 130's no biggie. Your not really using it at that level any way.

If all you using is the UPS's as a load your going to have some troubles until you get the load up to at least 25%.

Reply to

In addition to the previews replies.

Does APC UPS' have voltage/frequency variations tolerance adjustment ?

Typically 10 % Voltage & 5 % Frequency

Mind you APC UPS' are the OFF line type, which means power straight through to load with no invertor / convertor (AC ->DC the DC -> AC)

Frankly I'm not fan of APC, waste of money since 95% of utilities problems are not blackouts, rather dips and surges.


Reply to

Bruce, I have an ancient 3KW emergency generator and three UPS made by APC and all three function just fine on generator supplied power.

Not knowing the details of your situation, I'd suggest that you check your generator's output frequency. I do this by simply using an electric clock with a sweep second hand and compare how long it takes to complete a minite sweep against the same time measured by my watch. If the electric clock is running a bit fast or so when driven by the generator, I simply make minor corrections to the speed govenor on the generator until it's running within a second or two accuracy on a minite measurement. (It's sometimes amazing to me how stable these frequency/rpm settings are on small generators, even when they are run only infrequently.)

If your voltage and frequency settings are correct, the APC UPS unit should live with it just fine, as does my computer and TV when run on generator power. If your UPS doesn't, there is likely some other malfunction in your generator. (Check its brushes, general condition, and then monitor its output on an oscilloscope to give you additional clues that may lead to a solution to the problem.)

Pay particular attention to the noise present on the generator's output, since many quality UPS system will interpret excessive power source noise as presenting a problem and, as designed, switch to battery power. Source frequency is not usually an issue.

Harry C.

Reply to
Harry Conover

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