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Old November 4th, 2017, 03:02 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, 03:31 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, 08:00 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, 09:29 PM   #4
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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, 10:31 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, 11:52 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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Old March 26th, 2018, 11:03 PM   #7
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Aaaaaaaand it's back. So back in November when this initially happened,I contacted ABB for help. The dude sent me a sheet on how to test the thyristors inside the unit. So I did what it said, I disconnected all the wires attached to the outside, took the cover off, disconnected all the wires inside, did some ohm checks, all seemed to check out ok. Put the thing back together, and it worked perfectly like it should. Ran fine for 5 months, now it's acting up again.

Same issue occured, fan tripped and it's now down. Same set up however, it was time to swap the fan motor so the old one can be serviced/rewound. Got a new motor in it, transformer has been fully tested by outside shop with a clean bill of health, not shorts,etc. Also got a brand new identical ABB softstarter in it. I copied the settings from old unit and entered it into the new one. First test, smooth start up, all is well, as were subsequent manual starts. Put it in for production run and it craps out.

Things I've observed. With motor disconnected and only the transformer on the load side, it makes some gnarly noises, the voltage output on softstart is erratic. Not a smooth 20 sec progression like it's set. Transformer (no load attached) was pulling erratic current, spiking as high as 60A and back to 0 and everything in between. With motor attached, (100hp 230V) I watched the amps spike to 420A as soon as it started and worked it's way down, then slowly up to tripping point of breaker attached inline with motor. I was convinced it was something to do with the xformer, even though all the experts told me otherwise.We pulled wires off of the transformer, hooked up a portable 480V fan to the softstarter, fired it up, ramped up nice and smooth, amps were like 1.5A, voltage ramped up like it was supposed to. I was certain at that point it was the transformer. Took out the portable fan, came back the next day to check some more stuff. Nothing hooked to softstarter, no control calling for action, the output is sitting at 480V. Display screen shows 0% output, removing the fuse for 120VAC control power to the softstarter so it powers down, and it's still outputting 480. Pulling disconnect, and powering back on, still 480V. Pretty sure we have a shorted thyristor now. No longer do I think it's the transformer down stream. Now I need to look upstream, any suggestions? I've checked control voltage to unit which was odd that my meter hesitates when checking high voltage and does a beep first. It did it on the 120V, showed something like 500 volts then immediately down to 88, then 122. PLC commands the softstart as well as a HOA switch on panel. There's a purple and an orange wire on the softstart I assume one provides steady voltage and the orange connects to the start and stop terminals to control. Traced the purple and orange to an ice cube relay that I assume comes from PLC to make the connection. During earlier tests, removing that relay didn't break the 480 output either. I read around a little and it sounds like a no voltage scenario wouldn't necessarily stop the SCRs from conducting, it would actually take a voltage signal to make it stop. I don't know much about this, but unfortunately nobody else does either and we are dead in the water here, so any recommendation on places to check for faults. The softstarter appears like everythings cool, no fault code or protection light. I checked voltage between the orange and purple and didn't get any voltage. But, that was after the stuck on condition. Could there be a problem on the internal power source? Just need to find the root cause of that. Sorry for lengthy post, just wanted to fill in all the details I could remember.
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Old March 26th, 2018, 11:24 PM   #8
mhammer214
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Without a load on the softstarter you will have 480 volt on the output terminals because the thyristors always have a small leakage current. The internal resistance of a voltmeter is too high to pull that voltage down.
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Old March 27th, 2018, 10:47 AM   #9
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I did not know that. I assume there would be no output if there wasn't a signal to close. Thanks, so that is nothing to worry about? They symptoms of our actual problem is that it is not consistently ramping up smooth and appears to be doing a hard close...sometimes.
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Old March 27th, 2018, 11:35 AM   #10
James Mcquade
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not familiar with soft starts, so this is my opinion.
transformers should be in front of the soft start units.
they require constant power, not power supplied from a soft start.
transformers are deigned with a laminent / stacking factor in mind to keep the magnetic fields constant.
with a constant power source, you are within the design specifications and the transformer is happy. I do not know what the soft start does, but it does not keep a constant power source to the transformer primary.
if I am wrong, someone please correct me.

regards,
james
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Old March 27th, 2018, 12:40 PM   #11
T Gibbs
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Quote:
Originally Posted by James Mcquade View Post
not familiar with soft starts, so this is my opinion.
transformers should be in front of the soft start units.
they require constant power, not power supplied from a soft start.
transformers are deigned with a laminent / stacking factor in mind to keep the magnetic fields constant.
with a constant power source, you are within the design specifications and the transformer is happy. I do not know what the soft start does, but it does not keep a constant power source to the transformer primary.
if I am wrong, someone please correct me.

regards,
james
I was thinking the same thing. I've never seen a transformer after a soft start.
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Old March 27th, 2018, 12:48 PM   #12
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Transformer is a big inductor with an iron core connecting it to another inductor.

For an inductor, the V = L * dI/dt

a soft start tries to give you a cut off sign wave like this
https://goo.gl/images/1R2i9f

What is the dI/dt at the start of each red section? ~ infinity? yup.

So what could be happening in your transformer? bad things maybe.
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Old March 27th, 2018, 01:28 PM   #13
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This installation has been trouble free for at least a decade, up until recently. I've read around that it is not totally uncommon to have this setup, not saying your wrong, however we've had two instances of the transformer being tested with nothing wrong with it, the output of the soft starter does seem to be erratic in terms of voltage and current.
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Old March 27th, 2018, 01:36 PM   #14
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I have used Soft Starters (and VFDs with sine wave filter) to supply transformer primary with motor on secondary side.
Some tips...
Soft Starter must have all 3 legs controlled by SCRs. Some soft starters have a "wild Leg" that is always "hot" and the soft starter only controls 2 phases. "Wild Leg" scrs do not work well with transformers.
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Old March 27th, 2018, 02:11 PM   #15
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Quote:
Originally Posted by NetNathan View Post
I have used Soft Starters (and VFDs with sine wave filter) to supply transformer primary with motor on secondary side.
Some tips...
Soft Starter must have all 3 legs controlled by SCRs. Some soft starters have a "wild Leg" that is always "hot" and the soft starter only controls 2 phases. "Wild Leg" scrs do not work well with transformers.
Ding ding ding, WINNER! Absolutely correct. Putting a transformer on the load side of a soft starter is not a problem, other than it adds impedance to the circuit so any current limiting effect in the soft starter is going to have even LESS current getting to the motor, but that only counts in measurements and settings, requiring slight compensation.

But, and this is a BIG but, you CANNOT use the "2-phase" versions of cheap soft starters, "2-phase" meaning they only have SCRs on 2 of the 3 phases, the 3rd pole is just a piece of bus bar. The output voltage then is inherently unbalanced. In a motor application, this causes additional heating because of negative sequence currents created by the rotor. But in a transformer, it results in a high DC component in the fields. That might have been the source of your banging sounds.

These types of soft starters are marginally acceptable to motor loads as well; I would never use them on anything other than centrifugal pumps and fans that can accelerate in less than 10 seconds, and then only on loads that were not critical to my operation. Also if this type of soft starter shorts even one SCR, the motor gets power applied to it continuously, even when off, and it burns it up unless someone trips the circuit breaker. the only reason they sell them is because they are a little cheaper, so long as nobody looks at long term operating costs.

What is the model number of the ABB soft starter? The PSR and PSE are both the 2-phase type and should have NEVER been used on a transformer primary application. The only one they still sell that has 6 SCRs is the PST series.
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