New APC Back-UPS BX950MI-AZ

[John61]

I have a new APC Back-UPS BX950MI-AZ. 

I have noted that when the mains fails the unit beeps and  clicks loudly  when loaded with my PC, which is drawing 200W (measured with load meter). It maintains power to the PC.

If I connect a resistive load of 2 x 150w lamps and fail the mains, it maintains power to the lamps and beeps but does not click.

Is this normal behaviour for this unit?

[Teken]

The unit you have is a modified sine wave (stepped) vs true sine wave inverter UPS. As you noted the unit has no problems operating with purely resistive loads. 

Whereas reactive loads the unit may show odd operational behaviour. If you have a much older computer PSU. Redo the test and you might be surprised the behaviour is exactly the same vs not.

If the test results in the same behaviour on a very old computer PSU. It’s best to invest in a pure sine wave inverter UPS. If however the problem is NOT seen than it indicates the newer computer PSU is causing the UPS to react poorly. ☹️

Your choices are to accept that as is. Replace the unit with a higher quality (pure sine wave) inverter UPS. Or replace the computer PSU if possible. 

If the unit is new and still under APC Warranty you may also consider asking them to help troubleshoot if it’s the UPS, Load, or Both.

If it’s the UPS they will assist in the RMA process for a refurbished unit. You may find the replacement unit has the same problem! ☝️

But at this point you at least have identified and narrowed the root cause as to the PC. Where your choices are to replace the UPS with a pure sine wave inverter UPS! 🤦‍♂️

[John61]

Thank you for your answer / advice. I have another brand non-sine wave UPS and have just tried this one on the PC. It is  lower capacity unit so it could not support the load for very long, however it did not click repeatedly as the APC unit does. I repeated the test with the APC unit and found that the voltage is dropping to about 110V (nominal 230V) in time with the clicking of the unit.

The unit is less than 2 weeks old so I hope that it will be a warranty replacement.

[Teken]

Based on your reply it’s a combination of the AVR circuit kicking in to compensate for high / low input voltage. You may try to adjust the sensitivity on the unit to see if it behaves a little better from medium to low.

I have appended the user manual for you to make that change here: https://download.schneider-electric.com/files?p_enDocType=User+guide&p_File_Name=BU-UM-990-6291B-EN.... 

Please do let me know if that makes any positive difference. 👍

[John61]

I have tried changing the sensitivity setting from medium to both High and Low, it does not make a difference to the symptoms. I have opened a support ticket with APC (AU) so we will see what happens with that.

Thanks for your help.