I have a APC 1000VA BR Smart UPS.I have noted that the UPS Produces a Humming Sound when running on Battery Power.But my older a normal 500VA UPS(not from APC) seems to produce far lesser humming.The cannot say the humming is loud but you can hear a grrr+hmmm at mid frequency coming from the UPS.Is this normal for a stepped sine wave UPS.

I have a DSP Sine Wave Inverter at home(powering the house).I have gone near it kept my ear close to hear the sound coming from it(Very less sound)

Why is my UPS Producing such sound.Is this normal.


1 Answer 1


The humming and particularly the growling are indicative of demonic posession. Have you noticed dead cats outside and that birds don't fly over the house anymore?

OK, back to real science. Yes these things can hum. There are high currents sloshing around and magnetic fields coming and going at the power frequency and also at the switching frequency. Some noise from inductors and other parts is normal. This mostly comes from the force on the turns of wire resulting from the current and the magnetic field.

If the unit seems to be operating properly within expected efficiency (not getting excessively hot), then I wouldn't worry about it.

  • \$\begingroup\$ lolz.Okay.I get your point but why is the DSP Sine Wave Inverter not humming \$\endgroup\$
    – techno
    Nov 12, 2013 at 16:36
  • \$\begingroup\$ The whole-house DSP inverter probably has a much more tightly-constructed (and more expensive) transformer. Also, the sinewave synthesis doesn't generate high power-frequency harmonics that are more audible. \$\endgroup\$
    – Dave Tweed
    Nov 12, 2013 at 18:51

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