Backup UPS switching issue under load.

[drummerdimitri]

So I have a Cyberpower CPS1000E backup power UPS that's been working fine so far however under a specific scenario, the UPS failed to switch to DC power when the AC power failed.

I normally have my pc system connected to this UPS but today, I also had additional loads that I don't usually have in my system and that's when I noticed that my unit was not working properly for some reason.

The problem is this: When under load (65% of rated capacity or over), the UPS fails to switch from AC to DC power but for some reason, when I power the UPS up without any AC power to begin with, it is able to surpass its rated capacity (107% test) without issues.

My guess is there's a faulty/underrated component in the switching circuitry that is not able to transfer the load from the AC to the DC power.

This unit has been repaired by the dealer in the past and maybe they switched out a faulty component with a lower spec one and that could be the issue here.

What should I be looking at here? Could it be a relay? It is still able to switch from AC to DC battery power under low loads (40% or so) so not sure if this is the issue here.

Any ideas where to start looking?

[drummerdimitri]

Bump

[perieanuo]

hi, if you say for small loads 40% the ups commutes correctly and with hi load it refuses, then the logic implies over-current protection activates on high loads
maybe battery is weak, maybe current supervisor is not working right
the ups supervise the battery current and the 220VCA output inverter current also afaik, any of them may trigger

[drummerdimitri]

hi, if you say for small loads 40% the ups commutes correctly and with hi load it refuses, then the logic implies over-current protection activates on high loads
maybe battery is weak, maybe current supervisor is not working right
the ups supervise the battery current and the 220VCA output inverter current also afaik, any of them may trigger

I don't think that is the case since I can load it up over 100% when I turn on the ups without AC power so that is acts like an inverter.

The problem with the load is when the UPS is connected to AC power and the power goes out and the UPS has to switch from AC to battery power and that's when the problem happens.

This surely means that there is a problem somewhere in the switching circuitry but I don't know what to look for. 

[perieanuo]

the ups isn't angry at you, some protection triggers or voltage loss detection circuit is not ok. if you're sure over-voltage/current protections are ok without identify them and put the scope or the meter at work, then go with voltage detection branch and test that
the ups logic may be different when you power without mains present (doing validation checks and starting the inverter) and when it has mains but it looses it, those checks may be done differently by the ups firmware. why? because starting inverter from batt is different from switching from mains input to starting-up inverter, put yourself in designer's head
in your place, i won't eliminate without putting the meter at work...
i don't know your ups, a relay with contact problems also can give symptoms like this (dunno if they commute with mechanical relay or triacs or what-else different)

[drummerdimitri]

So I removed all the relays from the PCB and tested each one with 12V on the coil and tested the contact resistance using a 4 wire resistance measurement and they all seems to be in good condition.

I have no idea what to test next as I'm not sure what could be the problem here.