Problem with 3 Phase Transformers

[nmyers]

We have a Mori Seiki MV50 and a Haas SL20 that were installed with an inline large transformer from our power service. We have had it installed this way for a while and everything has been working fine.

A couple days ago there was a bright flash of light and a boom at the transformer. It blew each of the fuses that are on each phase. We replaced the single transformer with two smaller transformers, one for each machine using the same leads from the power source. When power was turned on for the first time that transformer made a VERY LOUD noise and we quickly killed the switch to the power. A couple of the 150amp fuses had blown. We replaced the fuses and same thing. The fuses that were used were pulled from our building and are many years old.

Could this just be a case of 20 year old fuses that blew instantaneously and caused the transformers to only get 1 or 2 legs powered or could something else be going on. The noise is VERY LOUD like SCARY LOUD. Thanks for your help!

[Geof]

My inititial diagnosis is that you have a major short circuit downstream of the transformer. I have had this happen and the transformer does make a hell of a noise when it is powered up and the secondary has a dead short. Have you checked all the wiring for shorts.

[Al_The_Man]

I assume you have the secondaries of the two transformers separate, i.e. not in parallel?
If you do they have to be phased correctly.
Power up each transformer in turn with no load/connection on each secondary.
Is it only one that is showing a high load?
I would check the primary and secondary taps for correct position, a tap in the wrong place can cause a short circuit.
Al.