Troubleshooting a Blown Transformer

Help with a woodworking machinery problem (repeated failures in a transformer that steps down 480 volts to 120 for a control panel). November 15, 2011

We are installing a large equipment line with a control panel. 480v in with 7- 3 phase motors. The panel has a PLC touch screen. There is a transformer that steps down to 110v to run a cooling fan and the PLC computer within the panel. The problem is we have burned up two transformers in 30 minutes of run time. The panel manufacturer said to increase fuse size (located on transformer ) from 5a to 10a resulting in another burned up transformer, just no blown fuses. Does anyone have any suggestions?

Forum Responses
(Cabinetmaking Forum)
From contributor P:
The transformer is probably not large enough for the load applied. What is the VA of the transformer and what are the amperage requirements of the 110 load on this transformer? A larger fuse size is not going to help if the smaller fuse never opened. I would bet the transformer was undersized for the application.

From contributor S:
Is there any chance of getting a licensed commercial electrician to look this over?

From the original questioner:
It calls for 1/2A to 1-1/2A fuses. The electrical diagram shows 5A on two lines and 10A on one. The same licensed electrician who installed the panel we be called Monday. As of Friday we have only talked with the panel manufacturer (U.S. design/build).

From contributor M:
I would also check the transformers secondary voltage to make sure it is tapped properly. If I understand you correctly you are saying there is a 20 amp load on that transformer. That is a pretty big transformer for a control circuit. It sounds like there are some motors running on it as well.

From contributor O:
Putting in a larger fuse will never help solve a burnt out transformer. The fuse is designed to protect the transformer from being overused and if it is burning out with a small fuse there is no purpose in allowing even greater draw on the transformer. If the transformer is burning out then it is either undersized or incorrectly wired on the input side, or there is a short between the transformer and the fuse if the fuse is downstream of the transformer.

As recommended check the plates on the fan and logic module to see how much they draw. The transformer needs to be that size or preferably somewhat larger. Please do not take offense, but if the above is not crystal clear, then your experience is insufficient to troubleshoot this and you need an electrician or factory assistance.

From the original questioner:
It was the panel manufacturer that said to try larger fuses. The electrician is coming by on Wednesday to check everything.

From the original questioner:
Thanks to everyone, the electrician found the transformer to be undersized by a factor of ten.