Apc pure sine wave UPS sending dc current through the transformer. - Page 1
Forum section: electronics / repair

author: johansen
author_rank: Super Contributor
timestamp: June 17, 2025, 05:24:09 pm
content: Anyone know a fix for this?I currently have an smt1000 from 2010, at idle its 6 amps of ac on the 13v primary, plus 3 amps of dc.I have, in the past, tried a very large capacitor in series, but what happens is the ac voltage on the output starts wandering ans the ups will error out.

author: bdunham7
author_rank: Super Contributor
timestamp: June 17, 2025, 05:54:31 pm
content: Quote from: johansen on June 17, 2025, 05:24:09 pmHow are you measuring that and what frequency is the AC?

author: johansen
author_rank: Super Contributor
timestamp: June 17, 2025, 07:36:54 pm
content: Quote from: bdunham7 on June 17, 2025, 05:54:31 pmvarious methods, i've put a dc shunt in series with the transformer back in 2013 when i had a 1500 watt rack mount ups. in this latest test i used a cheap dc and ac closed loop hall effect clamp meter. I also used my 22000ct uni-t multimeter on its 10 amp scale, it clearly shows 6 amps of magnetizing inductance and 3 amps of dc. (the dc current is not always constant, its somewhat random, and you can hear the saturation in the transformer vary and change)you can also see the distortion in the ac waveform on the output of the 120vac sine wave. there will be a flat spot on one peak but not the other, but this may be battery voltage dependent. basically the pwm reaches 100% on one half of the sine wave but not the other half.what makes these UPS a little interesting is the pwm frequency is not fixed. when you increase the leakage inductance of the transformer by adding a choke on the primary or secondary, the pwm frequency will drop. when you add extra turns around the CT that is in series with the 10uf output capacitor (or increase or decrease the capacitor) it will change the pwm frequency by fooling the unit into thinking the ripple current is higher or lower than it is.basically their algorithm has no dc offset closed loop and it probably just measures the average dc current from the battery.I've gotten a 1500 watt UPS down to 13 watts of no load power consumption from an initial 25 watts, but without fixing the dc current problem it wouldn't be possible to upgrade to a toroidal transformer because it would saturated far easier.I have another APC ups can't remember the year or if it works but i suspect it does the same thing.

author: bdunham7
author_rank: Super Contributor
timestamp: June 17, 2025, 07:49:08 pm
content: By "at idle" do you mean unplugged and on but with no load? For some reason that's not what I first thought of. I have an SMT1500 and it won't be soon, but the next time it's available I'll check it to see if it does the same. It sounds like some imbalance in the voltage measuring transformers or circuits. Can you check what the voltage across the transformer windings is under these conditions? If it is very low and thus the power is very low, I'd suspect that it is just normal or "good enough".

author: johansen
author_rank: Super Contributor
timestamp: June 19, 2025, 03:45:23 am
content: Quote from: bdunham7 on June 17, 2025, 07:49:08 pmThe problem may have fixed itself, and the transformer runs cooler now.I'll confirm later when I wind a 500 watt toroidal transformer for this ups and see if the losses decrease, or increase. they should significantly decrease because it seems the majority of the 24 watts of parasitic losses is in the transformer's core. They wound the core with a foil secondary and loaded it up completely full, so i can't wrap some more turns around it on the secondary side to decrease the flux density.I ran the primary low voltage leads 4 times through a metglass core, about 8x8mm cross section with a 1mm air gap, and it dropped the no load idle losses from 1.05 amps at the battery to 0.945as an inverter, the voltage on the primary side is 14.5 and the secondary is 120. when run on the mains, it adds the aux secondary in series and this drops the voltage on the primary down to 12.6, which is then PWM'd as a boost converter to charge the battery. -of course the moment the mains drops 10 volts low or high, it uses that aux winding to buck or boost and the transformer will now run at the full flux density.Anyhow the reason I want to investigate this is because if there is any dc offset at all, it becomes impossible to take one of these UPS' and scale it up to drive an H bridge and transformer 10 times larger.