UPS Input Amps

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cleveland

Member
Location
Midwest
Howdy. We have a new 400 kva UPS installed. This unit appears to be drawing 180 amps at no load... BUT... We have analog metering on the main switchboard. When the UPS is energized we see an inrush of about 150 amps on the meter. And then the meter goes right back down to where it was before the unit was turned on. A few people didn't believe this meter so I took an RMS FLuke out to the pad mounted transformer feeding the switchboad, got on every one of the 6-500 MCM on phase C and took readings, added them up, and they pretty much matched the swiitchboard meter. We have never quite understood the large amps at no load for the UPS and the manufacturer can't explain it completely either. The unit's power factor is not real good at no load was one explanation.... But it is pretty unique to see this occur. Could the no load current be filled with harmonics that somewhat cancel out at the main board????
 

cleveland

Member
Location
Midwest
Re: UPS Input Amps

The battery bank has been in place. There could be some charging taking place but I'm still not understanding why we do not see an increase in the main switch board meter when the UPS is energized. Other than an inrush there is no change. But a Fluke placed on the feeder to the UPS is showing 180 amps. There is no load on the UPS at this time.
 

ron

Senior Member
Re: UPS Input Amps

cleveland,
I would bet that the fluke you are using is a true RMS meter. The analog meter on the switchboard is an averaging type meter. The amperage you read is probably the high harmonic content of the small amperage the unit draws at no (or low) load.
UPS's are notorious harmonic contributors at low load. Is there an input filter on the unit. Is it an active type that takes itself out of the circuit at low load?
Also see below for similar type problems
http://www.powerware.com/About/technologies/index.asp
Overcoming UPS-Generator Interface Problems

[ May 07, 2003, 04:23 PM: Message edited by: ron ]
 

cleveland

Member
Location
Midwest
Re: UPS Input Amps

Ron,
You are correct in the Fluke being a true rms unit. So, we're measuring a small current with a very high harmonic when looking at the feeder to the unloaded UPS and get a false reading on the Fluke but we're looking at more current with less total harmonics when we measure the main feed into the building? Interesting because the panel meter right on the UPS is showing this somewhat bogus 180 amps too. I'm going to measure the current to the UPS with an older amprobe that is averaging. I should see less. I remember now that this is the brute force sort or way to verify the presence of harmonics, compare an averaging meter to a true rms meter....... Thanks.

[ May 07, 2003, 05:23 PM: Message edited by: cleveland ]
 

arneykaner

Member
Location
Illinois
Re: UPS Input Amps

The in-rush current at the moment of switching "ON" is the result of charging of a large capacitance in the primary loop of UPS, that is used by many manufacturers. Also, many manufacturers would not admit that they actually have a large capacitance installed, because it creates problems for an emergency generator, if one is feeding the UPS.
 

cleveland

Member
Location
Midwest
Re: UPS Input Amps

Well we measured with both a true rms and an averaging meter on the input to the UPS. On one conductor of a 2 conductor parallel feed we saw 83 amps with the averaging meter and 90 with the true rms. We then poked around a bit and talked with the technician for the UPS. The unit has an input filter with a lot of caps and some inductors. I'm now thinking that the unloaded UPS with the filter connected may be acting like a capacitor. That would sort of explain the lack of any big change to the averaging panelboard meter but seeing the current on the feed to the UPS. The current on the main switchboard is around 1000 amps and the building runs at a power factor of about .95 to .98
 
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