3PH transformer blows breaker on power up
- Thread starter FOG1
- Start date
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gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
220326-2357 EDT
I am reposting my link.
Will see if that works.
.
I am reposting my link.
Will see if that works.
.
Bud I am a tool and die maker I own a tool and die company, I’m not an electrical engineer. I don’t understand why you insist on posting pictures of oscilloscope stuff, I don’t have a clue. I keep telling you I have a 75 kva transformer that works fine and a 51 that blows a breaker at 200 amps. They have sent me two that are 2021 builds and a 2005 they all do it. It’s all the same power, every guess that I have got has not panned out to solve the problem. To run a 1/2 million dollar machine i have to leave a transformer running 24-7. It absolutely makes zero logical sense.
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ptonsparky
Tom
- Occupation
- EC - retired
IMO, You are missing a lot of details in your explanation and I'm not an EE. I hate to think how many more questions they have.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
220327-0603 EDT
FOG1:
Use a higher current breaker, or just use fuses that will hold, or add series impedance in each leg, and short that impedance out with a contractor after 1 to 3 cycles of 60 Hz. I suggest about 0.1 ohm to 1 ohm in each leg might reduce initial inrush enough that you would not trip the breaker.
You could also use negative temperature coefficient thermistors, and possibly leave them in the circuit after power up. However, these would continuously dissipate some power.
If you do not want to be confused by electrical circuit problems, then you need to study some electrical circuit theory.
.
FOG1:
Use a higher current breaker, or just use fuses that will hold, or add series impedance in each leg, and short that impedance out with a contractor after 1 to 3 cycles of 60 Hz. I suggest about 0.1 ohm to 1 ohm in each leg might reduce initial inrush enough that you would not trip the breaker.
You could also use negative temperature coefficient thermistors, and possibly leave them in the circuit after power up. However, these would continuously dissipate some power.
If you do not want to be confused by electrical circuit problems, then you need to study some electrical circuit theory.
.
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ptonsparky
Tom
- Occupation
- EC - retired
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
FOG1: Deep breath. I know this is frustrating, but try to remember that this is a free public forum, and that the folk rubbing your the wrong way are trying to help.
GAR is a fantastic theory guy. He tells everyone that they need to understand more theory. His scope pictures are great if you want to understand the root cause of transformer inrush.
Back to your problem: there is clearly some factor that you don't know is important to describe, that the electricians involved haven't sussed out, and you are hoping that someone here can figure it out. The practical electricians here will give you things to check. The theory guys (myself included) will want you to make measurements.
This thread is long and months old, and most of us don't have our noses in the issue and will have forgotten details.
Could you repost details such as the system voltage and configuration (480V? Wye, delta, grounded, etc.?) Transformer rating and nameplate, pictures of the transformer connection. Is there any other equipment around which seems to affect the transformer operation?
The fact that the transformer no load current is so high makes me think that it is defective. You said that the supplier confirmed that the first one was defective, and it would seem hugely unlikely that the second one is defective. So while it is possible that both were defective I guess we are now looking for common causes that could cause the same symptoms in perfectly fine transformers.
You could have:
Too high voltage for the transformer as connected (are the primary voltage taps correct)
You could have a misapplied transformer (very common when transformers are used in reverse)
You could have unbalanced supply voltage (maybe there is a wiring fault upstream)
You could have high harmonic voltages present on your service.
Something could be injecting DC current into the circuit.
Something external to the transformer could be injecting a large DC magnetic flux into the core.
When you said that the prior transformer was defective, what tests determined this, and where were the tests run?
That's it for my Sunday morning thoughts.
Jon
GAR is a fantastic theory guy. He tells everyone that they need to understand more theory. His scope pictures are great if you want to understand the root cause of transformer inrush.
Back to your problem: there is clearly some factor that you don't know is important to describe, that the electricians involved haven't sussed out, and you are hoping that someone here can figure it out. The practical electricians here will give you things to check. The theory guys (myself included) will want you to make measurements.
This thread is long and months old, and most of us don't have our noses in the issue and will have forgotten details.
Could you repost details such as the system voltage and configuration (480V? Wye, delta, grounded, etc.?) Transformer rating and nameplate, pictures of the transformer connection. Is there any other equipment around which seems to affect the transformer operation?
The fact that the transformer no load current is so high makes me think that it is defective. You said that the supplier confirmed that the first one was defective, and it would seem hugely unlikely that the second one is defective. So while it is possible that both were defective I guess we are now looking for common causes that could cause the same symptoms in perfectly fine transformers.
You could have:
Too high voltage for the transformer as connected (are the primary voltage taps correct)
You could have a misapplied transformer (very common when transformers are used in reverse)
You could have unbalanced supply voltage (maybe there is a wiring fault upstream)
You could have high harmonic voltages present on your service.
Something could be injecting DC current into the circuit.
Something external to the transformer could be injecting a large DC magnetic flux into the core.
When you said that the prior transformer was defective, what tests determined this, and where were the tests run?
That's it for my Sunday morning thoughts.
Jon
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
I agree that the characteristics of the supply voltages may be causing or aggravating the problem. On this transformer the primary winding configuration is the same for 240V and 208V, and so there will be less headroom till saturation when used at 240V. Although disappointing if true, maybe the design of this 51 kVA transformer has a relatively small margin before it saturates due to insufficient core material or some other issue.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
It seems to me the installation is marginal and doomed to be quirky. Probably have to add some impedance to the primary side of the transformer to reduce the inrush. Reactors or conductors. Neither will be cheap.
Otherwise replace the 125 a cb with one that has a much higher it trip setting. This will likely cause the 200 a main to trip now and then so it will likely need a higher it trip setting too.
Likely op has a very high available short circuit current from the poco, at least some of the time. Not a lot you can do about that.
Otherwise replace the 125 a cb with one that has a much higher it trip setting. This will likely cause the 200 a main to trip now and then so it will likely need a higher it trip setting too.
Likely op has a very high available short circuit current from the poco, at least some of the time. Not a lot you can do about that.
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
Is the 75kVA transformer located further from the main service panel than where the 51kVA is installed? If so then that might be providing additional resistance which limits the inrush current such that the breaker does not trip.
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
Although an unusual application for it, a solid state soft starter (used for starting motors) would also reduce the transformer's inrush current.
But these measures are not going to significantly change the transformer's steady state current without a load, however.
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they set about 10 ft apart one on the floor and the 75 is on a shelf, power comes in building right between the two.
I was actually looking at that yesterday
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
2203227-1741 EDT
I don't like the idea of a solid-state switch when it is not needed. These are subject to high voltage failure.
If you used a 10 ohm resistor in series with each lead for startup, and then shorted out these resistors after possibly 1 to 3 cycles of 60 Hz, then maximum inrush current would be in the range of 12 A. Resistors are very reliable, even when overloaded, so you might get away with just a 25 W resistor.
An electromechanical relay with ordinary contacts will have very low voltage drop, 10 to 50 mV, at full load compared to a solid state really of possibly 1 to 2 V.
An electro mechanical relay with mechanical contacts can tolerate very large voltage transients without damage.
.
I don't like the idea of a solid-state switch when it is not needed. These are subject to high voltage failure.
If you used a 10 ohm resistor in series with each lead for startup, and then shorted out these resistors after possibly 1 to 3 cycles of 60 Hz, then maximum inrush current would be in the range of 12 A. Resistors are very reliable, even when overloaded, so you might get away with just a 25 W resistor.
An electromechanical relay with ordinary contacts will have very low voltage drop, 10 to 50 mV, at full load compared to a solid state really of possibly 1 to 2 V.
An electro mechanical relay with mechanical contacts can tolerate very large voltage transients without damage.
.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I agree with Gar. A resistance 'pre-charge' makes more sense than a semiconductor system.
But this assumes that inrush current is the only issue, and IMHO there is still something strange going on.
There are no large magnets near this transformer? What about DC power supplies?
I'd really like to see a scope trace of the no-load current.
Jon
But this assumes that inrush current is the only issue, and IMHO there is still something strange going on.
There are no large magnets near this transformer? What about DC power supplies?
I'd really like to see a scope trace of the no-load current.
Jon
No magnets, nothing in my shop is DC, put the new 2021 build transformer back in that 2005 drawed stupid amps with no secondary load. 2021 is back to 3-4 amps per leg. Noticed that the disconnect when thrown trips breaker but I can leave disconnect on and reset breaker and it holds.
ptonsparky
Tom
- Occupation
- EC - retired
Have you looked closely at that disconnect for proper operation?
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Are you saying that the no load current of the transformer is changing? Back in your original post you say 9 amps no load.
Jon
The first transformer was around 9 the 2nd transformer has been in the 3-4 amp range. Someone posted also that I said the 1st transformer was defective, DMG thought it was defective. I don’t believe it was as #2 did the same thing as the first one.
That’s the first transformer
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