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UPS on internal bypass, rectifier input still drawing 90A

UPS on internal bypass, rectifier input still drawing 90A

UPS on internal bypass, rectifier input still drawing 90A

Morning all,

I got called into work yesterday to attend a breakdown on a Siemens MasterGuard 125 kVA machine from 1999..

It had failed overnight and automatically switched itself to external bypass. The external bypass has a separate feed to the internal bypass.

During investigation I found that the current on the internal bypass matched the outgoing current from the UPS to the load (60 - 75A on RYB phases), however there were much larger currents (80 - 95A on each phase) going into the rectifier.

We'll never know why that was since there was no desire to fix as it was a machine from 1999, so we just put it into external bypass and have scheduled a replacement. I have been telling them to replace it for a long time, but whatever.

Anybody have any idea what those rectifier currents were doing? 90A is well within the capacity of the UPS however I'm not sure it's designed to sink all of that as heat through the components.

I confirmed that the current was no longer present after the UPS was isolated, so I don't think they were circulating which was a possibility that I considered.

RE: UPS on internal bypass, rectifier input still drawing 90A

If there's an isolating transformer fitted internally upstream of the rectifier then you're possibly looking at magnetising current for the transformer. If it is then it's almost purely reactive with no load from the rectifier on the secondary.

RE: UPS on internal bypass, rectifier input still drawing 90A

Hi ScottyUK,

I know this was a long time ago, but I never got an email about a reply so never checked.

Thanks for responding.

Guess I'll never know what it really was, but sounds like your answer could be plausible. Can't open the cabinet while energised for safety reasons, nobody ever shared drawings of internals.

RE: UPS on internal bypass, rectifier input still drawing 90A

Same reason an unloaded motor can have head scratching current draw showing on a clamp-on. Unloaded pf that's in the toilet.

Keith Cress
kcress - http://www.flaminsystems.com

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