transformer 75 KVA Inrush current question

Thread Starter

powerfade

Joined Jan 19, 2022
34
This is kind of a follow up to a thread I started awhile back and was looking for a little input. I have a 75 KVA transformer(wired in reverse to act as a step up transformer (208v to 480v), large inrush current) where the inrush current when the transformer is powered on is blowing past a 100amp breaker in my panel and blowing the main fuse 100amp fuse at the main. I understand that I probably need a service upgrade, but was hoping for some input on an idea a friend of mine mentioned. Here's what he proposed:

Here's the idea:

Install a 20amp 3 phase breaker in the panel and put 3 1000 watt bulbs (70 ohms of resistance) in line on each phase and tie them to the primary side of the transformer to energize the transfomer and "trigger" the inrush current which would subsequently which would then be limited by the 1000watt bulbs(i.e. resistors) on each leg. Wait for a few seconds and then turn on 100amp breaker to fully energize the transformer and then turn off the 20amp breaker.

Any input or ideas would be appreciated. If it's even feasible. We were talking about it, so I figured I would throw it out here and see what the masses say.

Thanks
 
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ronsimpson

Joined Oct 7, 2019
2,540
Is the transformer bigger than you can carry? Much much bigger.
I have used light bulbs to limit current. There is a chance the transformer is bad, and the bulbs will light full on.
 

Thread Starter

powerfade

Joined Jan 19, 2022
34
It's definitely big... Probably 600lbs. I don't think the transformer is bad. I am able to turn it on and off from the breaker at least once with new fuse before it blows. The bulbs are actually infared heating elements.
 

crutschow

Joined Mar 14, 2008
31,128
The amount of no-load inrush current depends mainly are where in the cycle the transformer was disconnected from the mains, and where in the cycle the voltage is again applied from the mains.
After that the current drops to the excitation/magnetizing value.
The excitation transformer current should be less than 5% of the ampere rating.
In this case that would be <18A.
The 1kW bulbs will limit the current to about 3A.
So I'm not sure the bulbs will supply sufficient magnetizing current before the full voltage is applied to avoid a large inrush current, but it should be worth a try.
 

Ian0

Joined Aug 7, 2020
6,711
Definitely worth a try.
When it is running with no load, use a clamp meter and measure the magnetisation current.
 

DickCappels

Joined Aug 21, 2008
9,513
When bringing a large transformer on line, one might have to go through a number of fuses.

If you could taper down the voltage before turning it off you would demagnetize the core and not have that problem anymore. Thinking a little bit more, you could use a triac lamp dimmer circuit to slowly turn the voltage up and down.

1646752646421.png
This circuit, using a BTA41 triac would do (see next sentence). Don't use the switch but change the 13.5 k fixed resistor to a 250k ohm variable resistor.

Edit: Oops! I just rechecked your VA rating. You are going to need a triac rated at much more current of the one I referenced.

I prefer to use the ST Micro sunbberless triacs so this circuit, which controlled a 5 kW 240 VAC load until a plumbing change made it unnecessary ran for years without a snubber and without any problems.

The capacitors should be film and don't need to be rated for any voltage over 40V in the case of the diac and over 20V in the case of the two transistors.

Putting an incandescent lamp between NEUTRAL OUT AND LINE OUT would provide a visual indication of the voltage going into your transformer.

If you can't get the 32 volt diac you can use two inexpensive silicon transistors to simulate a diac (2N2222 is readily available and inexpensive).

1646752967723.png


If you or someone you know can program a microcontroller or Arduino you can make the process automated.
 

Thread Starter

powerfade

Joined Jan 19, 2022
34
So I gave it shot and its still blowing the main fuse. Maybe the current is not sufficient magnetizing current. I'll have to check the magnetization current at no load to see what that is I imagine??
 

Ian0

Joined Aug 7, 2020
6,711
So I gave it shot and its still blowing the main fuse. Maybe the current is not sufficient magnetizing current. I'll have to check the magnetization current at no load to see what that is I imagine??
How bright are the lamps when you switch them out of circuit? By the time you switch them out, the transformer primary voltage should have almost reached the mains voltage. If it doesn't try more lamps!
 

Thread Starter

powerfade

Joined Jan 19, 2022
34
How bright are the lamps when you switch them out of circuit? By the time you switch them out, the transformer primary voltage should have almost reached the mains voltage. If it doesn't try more lamps!
The heat lamps don't actually light but get slighty warm. It's a three phase circuit, so I tied L1, L2 and L3 together after the lamps just to make sure they were working and they get hot, but don't turn red/orangeish, which I thought they should.
 
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crutschow

Joined Mar 14, 2008
31,128
Once you measure the magnetizing current, you can determine what size power resistor you need.
Just a few ohms will limit the inrush current to below the fuse rating.
And the power rating of the resistors should not need to be that high to carry the magnetizing current, once the short inrush current has subsided, for the short period until you disconnect the resistors.

The procedure could be automated if you use a delayed-operate contactor to switch across the resistors.
 

Ian0

Joined Aug 7, 2020
6,711
The heat lamps don't actually light but get slighty warm. It's a three phase circuit, so I tied L1, L2 and L3 together after the lamps just to make sure they were working and they get hot, but don't turn red/orangeish, which I thought they should.
So it's a 3-phase transformer?
 

Thread Starter

powerfade

Joined Jan 19, 2022
34
Once you measure the magnetizing current, you can determine what size power resistor you need.
Just a few ohms will limit the inrush current to below the fuse rating.
And the power rating of the resistors should not need to be that high to carry the magnetizing current, once the short inrush current has subsided, for the short period until you disconnect the resistors.

The procedure could be automated if you use a delayed-operate contactor to switch across the resistors.
So if I could size a resistor to say something that would provide 15 - 18 amps to energize the transformer off of the initial 20amp circuit, do you think that would work??
 

Thread Starter

powerfade

Joined Jan 19, 2022
34
So how can you have L1, L2 and L3 connected together?
L1, L2 and L3 tied together off of the 20amp circuit after the resistors but prior to the transformer just to test to make sure the heat lamps were working. It was just a test to make sure the lamps were good.
 

Thread Starter

powerfade

Joined Jan 19, 2022
34
keep t
Somewhat depends upon the magnetizing current.
So I took an amp clamp to the three legs coming into the transformer with no load, just the energized transfomer and the reading I got were

L1 = 3.7 apms
L2 = 2.3 amps
L3 = 3.6 amps

What size resistors would I be looking at here on the initial 20 amp circuit to energize the transformer??

Thanks so much for everyone's help here.
 
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Danko

Joined Nov 22, 2017
1,492
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crutschow

Joined Mar 14, 2008
31,128
What size resistors would I be looking at here on the initial 20 amp circuit to energize the transformer??
The magnetizing inductance appears to be about 270mH (@ 60Hz).
If you put a 10Ω resistor in series to limit the current to 20A, the resulting steady-state current would be about 2A.
Not sure if that's enough for sufficient magnetizing current to avoid a large current spike when the main breaker is closed.

But the 20A breaker can likely handle a peak surge current of more than 20A so you may be able to use a smaller resistor, if 10Ω is too large.
 
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