Transformer Trips with "NO LOAD" ?

[stuartdmc]

A customer of ours wired up a 45kva 480V primary - 208V secondary 3phase step down transformer backwards, (that is the 208V in and 480V out) well, he said that without the 480V / 20A motor running or without any load on the transformer the primary 30amp OCPD trips out. His suppler told him that the transformer draws a load on it?s on without any load attached and that this could cause the circuit breaker to trip. I happened to have walked in on the conversation and was asked if that was true, and well I have never heard of a transformer drawing a load on it's on. I thought I would go to the experts on this one.
So my question is; does a dry type transformer ?on it?s on? have a load?

Stu.

 

[winnie]

All transformers have magnetizing current, which flows even with no load.

This magnetizing current is quite low, and will not trip a reasonably sized breaker unless something is very wrong with the installation, for example if the transformer were not only reversed, but the 208V side connected to 480V, in a mistaken attempt to get 1100V.

Magnetizing current can be especially high if the input voltage is high; eg 240V supplied to the 208V coils. One problem with reverse connected transformers is that there are no input taps available to correct supply voltage problems.

More likely is 'inrush' or 'startup surge'. When initially creating the core magnetic field, _very_ high current can flow into the transformer. Transformers are optimized to reduce this inrush on the primary winding. When reverse connected the inrush can be much higher than expected.

Finally, connecting the center tap of a wye connected primary can cause problems.

-Jon

 

[ron]

Considering that the regular FLA on the 208V primary side is 125A, the startup inrush could be 10-12 times FLA. It would surely trip a 30A primary CB.

 

[winnie]

Doh, I didn't even notice the 30A primary OCPD. With a 20A 480V load, the 208V load will be about 46A. (Of course, a '20A motor' probably doesn't really draw 20A, but that is another story.)

In any case, a 30A breaker on the 208V 'primary' of a 45KVA transformer is not even remotely reasonably sized.

-Jon

 

[ceb58]

Considering that the regular FLA on the 208V primary side is 125A, the startup inrush could be 10-12 times FLA. It would surely trip a 30A primary CB.

I did a install of the same situation Acme transformer told me that it may trip the breaker if when you close the breaker to the transformer you might do it between the sign waves and it will trip. But thinking about it did they connect any thing to xo? This sounds like a corner grounded delta situation. If it is nothing should be connected to xo.