Hypothetical Transformer Question

TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
This was posed to me by and excellent apprentice a few years ago:

Let's say you go into a room and there is a transformer there. Conduits going to disconnects and continuing out of the room. You open the disconnects, look at the wiring, take voltage readings, and determine that the transformer is 480/277 3ph on one side and 208/120 3ph on the other, both with grounded neutrals, but not grounded in the transformer nor in the disconnects, but somewhere outside of the room.

With no more information, and without opening the circuit on either disconnect, and with just a normal meter, can it be determined if this is a step up or a step down transformer? In other words, is the power coming from the 480V or the 208V side?

I can think of a way with additional equipment, but not with just a regular meter, but that doesn't mean there isn't a way.
 
Two wattmeters one on each side
.higher reading primary
Chech voltage on that side know step up or stepdown
 
Transformer no load case
Check side no load current
That primary
Measure voltage that side know step up or step down
 
How about measuring the voltage across the unused taps or even change the taps on the transformer.
With the primary steady the secondary should vary.
 
Look at the disconnecting means on both sides of the transformer. The one that has the transformer on its load side is the primary. Measure the voltage; if it is 480V the xfmr is step down.
 
ggunn said:
Look at the disconnecting means on both sides of the transformer. The one that has the transformer on its load side is the primary. Measure the voltage; if it is 480V the xfmr is step down.
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Good thought, but what if they are wired so the transformer goes to the line side on both disconnects.
 
I have had a new thought, though. If the transformer has a load on it, there will be a voltage loss due to heat. So if the high voltage side is less than it would be with a theoretical, lossless transformer, then it is a step up transformer. And if the low voltage side is less than it would be with a theoretical, lossless transformer, then it is a step down transformer.

Other than that, if you could control an additional load on one side or the other at a disconnect, the amps would change on the conductors from where the power is coming from. Yes, you would need to use a clamp-on amp meter.
 
TwoBlocked said:
Good thought, but what if they are wired so the transformer goes to the line side on both disconnects.
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I could be wrong, but I don't think that should ever happen. I certainly wouldn't want to service the fuses in a disco where the fuses were hot when the switch was open.
 
If you have a clamp-on meter, for the 480V side you could multiply the L-N voltage on each phase times its line current, and sum those three to get the kVA on that side. Then do the same for the 208V side. The side with the higher kVA should be the primary because of transformer losses.
 
ggunn said:
I could be wrong, but I don't think that should ever happen. I certainly wouldn't want to service the fuses in a disco where the fuses were hot when the switch was open.
Click to expand...
Neither would I (and I have seen it...), but I didn't say they were fused disconnects. The OCPDs may be elsewhere.
 
synchro said:
If you have a clamp-on meter, for the 480V side you could multiply the L-N voltage on each phase times its line current, and sum those three to get the kVA on that side. Then do the same for the 208V side. The side with the higher kVA should be the primary because of transformer losses.
Click to expand...
Is measuring the amps necessary? See what I posted on #15.
 
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