Wrong output frequency 200Hz

[Anonymous user]

Hello Apc experts.

I had an old APC Smart UPS 750VA 500W 230V usb version (SUA750) Not used for sometime and yesterday I changed its batteries with new one. I turned it on and heared the tone then the onlinr (sine wave icon) along with  the left half leds which were dim flashed for about 10 seconds or less then stopped flashing and the battery led indicator kept on the rest were not. I then measured the output voltage and frequency using a Multimeter , it was 230 V and around 200Hz and highly fluctuating AC frequency. I plugged a table lamp just out of curiosity and It worked and the frq. Was the same ~200Hz. Now this is a totally wrong frequency and I guess in might damage my electrical component

I plugged a smal hair dryer and It didn't work, then I plugged a CFL lamp (which I tested and were working) with the hair dryer and the hair dryer worked slowely but the CFL lamp pruduced loud buzzing noise.

What is actually going on? How to fix this frequency issue?

[Anonymous user]

Any ideas, guys?

[Anonymous user]

No reply guys?

[voidstar_apc]

The SmartUPS doesn't have user-service-able parts inside, and isn't designed to put out 200 Hz AC. APC would recommend replacing it.

If your multimeter is correct, that seems like something seriously wrong: either it really is putting out 200 Hz AC or, more likely, the 60 Hz waveform is distorted and the multimeter is measuring the distortion. In the latter case you could try replacing capacitors. I suspect if you're skilled enough to fix the electronics then your time isn't worth it.

[Anonymous user]

Thank you for replying. I believe that my multimeter is correct, it is measuring the correct freq of the mains which is 50Hz here.

when I plug A cfl light it works but plugging a hair dryer wont.

can you specify which capacitors could be damaged? 

Also could there be any thing else other than the capacitors that could change the frequency ?

thank you

[voidstar_apc · Accepted solution]

A signal is composed of many frequencies -- existence of 200 Hz doesn't mean it's the fundamental. Multimeters are only designed to measure a pure signal. One way they measure frequency is by counting zero-crossings where the signal switches from positive to negative or vice versa. Distortion has higher frequency components that cause more zero crossings. We use oscilloscopes for a more detailed picture than a multimeter can provide.

The frequency of the output waveform isn't set by analog components but rather microprocessor timing, which is why although it's in the realm of possibility I'd be surprised if it were actually 200 Hz.

I'm afraid I can't give much advice on fixing it since the UPS isn't user-serviceable. It contains hazardous voltages stored in the capacitors and can kill you. There may be other people here who can chime in.