Electrical audible buzz-like noise

[Anonymous user]

I just bought two new APC Back-UPS CS 650 and installed them on two different computer. Although the UPS are working and performing well, both of then have an electrical audible noise, something like a buzz. The noise is very audible and terribly annoying. I can hear it with the UPS on/off as long as it's connected to the outlet. The sound gets even louder if it's necessary to use the battery.

Does somebody know how to solve the problem?

[BillP]

What do you have plugged into them? A slight hum is normal while on battery operation.

I cant think of anything that would cause this unless there is something vibrating on them or if something else is causing the issue.

[Anonymous user]

I have just a computer and a monitor connected to each one. So it's about 250W per UPS. The noise sounds even when is not un battery mode, and it's annoying because it has an high frequency. When in battery operation, the noise is louder but not as annoying as the usual one.

[BillP]

How about when it sits with nothing plugged into it? Just turned on and plugged in?

[Anonymous user]

I did some additional tests and I can provide more info.

The buzz only appears if these conditions are met:

1. The UPS is plugged in the outlet and recieving power.
2. The battery has full (100%) charge.

It doesn't matter if I have something connected to the UPS or not. It doesn't matter if I have the UPS on or off, as long as I have it plugged.

If the UPS is plugged and charging the battery, then there is not buzz-like sound, but as soon as the charging is complete, the sound returns.

[Anonymous user · Accepted solution]

karloch wrote:
I did some additional tests and I can provide more info.

The buzz only appears if these conditions are met:

1. The UPS is plugged in the outlet and recieving power.
2. The battery has full (100%) charge.

It doesn't matter if I have something connected to the UPS or not. It doesn't matter if I have the UPS on or off, as long as I have it plugged.

If the UPS is plugged and charging the battery, then there is not buzz-like sound, but as soon as the charging is complete, the sound returns.

Do you have any lights or appliances that utilize a two level power/brightness control connected on the same branch circuit? These will introduce a disturbance on the line that will increase the noise of a transformer. This is only affected by two-level controls using a series diode.