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Old November 4th, 2017, 02:02 PM   #1
DrOrpheus
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Banging noise from transformer

Any experience with any sounds other than a hum from a transformer? Im not overly familiar with the setup, but from what I saw it was either a 100 or 200hp motor 230v. 480v fed into a soft start, through a step down transformer to 230v and then to motor. When motor attempts to start, we get a loud bang from the transformer and breaker trips. With load disconnected, still get a bang from transformer. I've heard the setup used to not have a softstart, but a regular starter, and the reason the soft start was put in was because of similar issue as today. Is the unit outputting full power rather than gradual? No fault lights or any other signs of problems coming from the soft starter. What could possibly make that kind of noise from within the transformer? Is that common/uncommon to have the transformer after the motor starter rather than before?
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Old November 4th, 2017, 02:31 PM   #2
Aabeck
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It may be related.

I used to work in an automotive stamping plant that had a line of automated spot welders, each was single phase on a single 3 phase conduit across the back. They were all interlocked so that only 1 welder on each phase could weld at once.

What happened along the longer sections of EMT conduit when a welder did weld was the wires inside magnetized quickly and banged together and into the conduit wall - with a loud bang each time.

Maybe something inside the transformer is loose and gets magnetized & pulled.
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Old November 4th, 2017, 07:00 PM   #3
DrOrpheus
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I sort of thought the same thing at first, it is located up against an outer wall of sheet metal.
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Old November 4th, 2017, 08:29 PM   #4
mendonsy
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I saw a somewhat similar case many years ago.
An electrician took the cover off of a 100 kva transformer to change the taps. He left the cover bolts and his wrench lying in the bottom of the transformer because he wanted to test the voltage before closing it up. When he turned the power to the transformer on the loose pieces all jumped up against the iron core for a few seconds then fell back to the bottom of the case.
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Old November 4th, 2017, 09:31 PM   #5
seth350
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I have not experienced this sound from a transformer, but like Aabeck stated, I have seen wires/cable jump in conduit.

Although, it too was in a welding app.

We have many 200+HP motors operating and get stopped and started just about everyday. No conduit banging to speak of, except the sound of the starter pulling in.

We also have 20,000A rectifiers that also have a soft start that is done over large resistors in parallel to main line. No banging or strange noises.

You may very well have something loose that is being magnetized for a moment. I would check that before traveling to the Twilight Zone of electrical physics.
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Old November 4th, 2017, 10:52 PM   #6
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Other details I didn't mention. The motor was not running,but was supposed to be. I checked voltage phase to phase on the output of the starter. Had 480v, I clamped each wire and got 1.5A. It was when we switched from auto to manual to start the motor is when we heard the initial bang. I havent been involved since.
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