Transformer Question?

[dduffee260]

I had a situation where I had 240 3ph voltage and needed 480 3ph voltage. I thought I could take a transformer that I had and back feed it or take a step down transformer and make it a step up transformer. I took a 480-208/120 transformer and installed it with the 240 volt side on the low side of the transformer. I then took all the taps and put these on the lowest side or tap 7 since the voltage was 30 volts higher than the transfromer was rated. I then check the voltage with a Simpson 260, the voltage was right at 490 volts on the primary side of the transfomer. The transformer spraked a small amount on the grounding lug of the case. It hummed fairly loud, I checked the voltage again and things seemed like they may be ok. Then the ground sparked and burned into. I had 2/3 w/Ground SDT cable coming from a 100 amp fused disconnect to the transformer. The transformer was a 300 Kva. Then the grouynd sparked in the 100 amp disconnect so I turned it off quickly. The ground wire which was a #6 had burned completly into. No fuses had blown. What happened here? I am at a loss. I have to find a proper transformer to run about 30 horsepower at 480 volts. ANy help would be great.

 

[augie47]

Re: Transformer Question?

(a) double check your tap connections. all the same ?
(b)Did you connect to X0? don't!

 

[petersonra]

Re: Transformer Question?

Somehow you managed to get enough current in the ground wire to melt it without tripping the 240V OCPD. I suspect some kind of wiring error.

While sometimes what you did is OK, sometimes it is not. You need to check with the xfmr manufacturer to see whther it is OK. I suspect most xfmrs of that size are not rate for what you did.

OTOH, it probably would not have caused the problem you had. without more infomration it is hard to come to any conclusion about what did happen. I for one have a hard time believing that a #6 ground wire would burn up before tripping a 100A OCPD.

 

[catchtwentytwo]

Re: Transformer Question?

Is the neutral jumper (120/208 WYE side)still bonded to the can? Is the 480 three phase a delta or WYE system?

Please clarify: #2's feeding a 300 KVA XMFR for a 300 HP motor?

 

[dduffee260]

Re: Transformer Question?

Yes, the XO was still bonded to the can. I did not connect to XO only X1, X2 and X3 and had 240 volts on these phases. The 240 3 phase is delta coming to the transformer. The transformer itself is a 480 delta-208/120 wye step down transformer. The ground lug was tied to the case of the transformer also. It was energized for about 1 minute before it burned the ground completly into. My tap connections I felt were correct. X1,X2 & X3 having 240 delta on them. Someone told me because the adjustable taps were on the primary side of the transformer this is what may have caused the meltdown. The 300 kva was just something that has been sitting on a lot for a while, but I believe it worked before being taken in. All we are running is 30 hp but they wanted to try and power the equipment up last night temporarily. Thanks for the input.

 

[augie47]

Re: Transformer Question?

I believe the XO-ground bond got you. More learned folk will probaly reply, but from field experience, the transf, should be grounded (equipment ground), but no X-O bond link.

 

[iwire]

Re: Transformer Question?

I do agree the XO should be left floating in this case.

However that should not be the cause of a burn up.

There would have to be another issue in combination with the XO connection.