Transformer

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Belknap24

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In a warehouse I was looking at a 45KVA step down transformer 480 - 120/208. The owner was complaining that his electric bills have been going up significantly the past 2-3 month with nothing changing as far as the load on the system. He called the power company and they said the transformer was bad which is what is causing the increase in the KW used. I amp probed it and they draw was very small (6 amps per leg). Could there be a problem with the transformer itself and how should I check it?

Thanks
 
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-Z based on rated pu
-measure R prim and sec each phase to gnd
-prim to sec, each phase to opposing 3
isolate phases and X0

operating
measure v and i for prim and sec and compare power out/in
avg or phase and summed

the xfmr specs should give you an idea if something is wrong
 
Belknap24 said:
The owner was complaining that his electric bills have been going up significantly the past 2-3 month with nothing changing as far as the load on the system.
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Not to be picky but if the power bill has gone up then the load has changed, whether intentional or not. When I worked in the utility world, one thing we found alot of was when customers called complaining about their bill going up the usual culprit was an element in a hot water heater had failed so one element was keeping the water hot. Any hot water heaters in the facility?
 
6 amps per leg is 5 kva!

Call the transformer manufacturer and see if that is normal for their transformer.

No, a transformer usually lives its long life without loss of efficiency.

5 KW on a 45 KVA transformer is more than 10% loss. That seems way high.

That 6 amps per phase was on the 480 side with NO LOAD, right?
 
160504-1501 EDT

Belknap24:

If you measured 6 A on each input line with the secondary unloaded, then I do not believe you have a transformer problem.

Some measurements from a small, 175 VA, transformer. Nominal input rating is 120 V. Calculated full load input current is 175/120 = 1.46 A.

No load measurements:
123 V input
0.19 A
4.0 input power
24.2 VA
0.16 PF measured, calculated 4/23.4 = 0.17

For your transformer measured unloaded VA per leg is about 277*6 = 1662. Calculated PF is 1662/15000 = 0.11 . Lower than my smaller transformer and to be expected. I used a VA ratio here instead of power but it gives us an estimate.

You should really use a power input measurement to the unloaded transformer. If your transformer efficiency was 98%, then you would expect an unloaded power loss per leg of about 15,000*0.01 = 150 W by assuming transformer core loss is 1/2 of full load transformer loss.

With a power factor of 0.1, then 150 W becomes 1500 VA. 1500 VA at 277 V equals 6.4 A.

Make sure all three legs read about 6 A with no load. Then your problem is in the load or how energy is being measured.

I have not proofread or checked my calculations.

..
 
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