Lamps with nominal current 15 - 18 amps have much bigger inrush current.
More current - faster process of demagnetization.

He's trying to avoid a large inrush current.

And I don't see that speed is a significant factor here.

 

He's trying to avoid a large inrush current.
And I don't see that speed is a significant factor here.

"DEM60C instrument performs fast and reliable fully automatic demagnetization.
High output currents of up to 60 A provides an efficient demagnetization
independently of the transformer core state."

Full article about demagnetization procedure is here: https://www.dv-power.com/demagnetization-transformer/

 

He's trying to avoid a large inrush current.

And I don't see that speed is a significant factor here.

I decided to try 3 - 1500 watt space heaters in line on each line to act as the resistors. I thought putting each in line from the panel to the transformer would turn on each of the heaters, but it doesn't seem like there's enough power to power the fan and the ceramic heater at the same time. The heater is turned on at full heat! Does this make sense?? The resistance on the heater is 19 ohms when I check between the two prongs of the plug.

When I check the voltages where I am feeding the transformer through the heaters, the voltages drop to around 100V and then slowly creep up. L3 seems to be the lowest around 93v.

Maybe that's not going to work?

 

the voltages drop to around 100V

What voltage?
Measure the voltage across the heater plug.

 

What voltage?
Measure the voltage across the heater plug.

So the way I have it wired is the heater is wired as a resistor. On the supply side (before the heater) voltage is 118. After the heater(resistor) drops down to 96 or so. When I measure across the plug while it's powered it's L1 = 14 L2=14 L3=24v.

 

The heating element almost certainly has a positive temperature so while you might measure 19 ohms, in operation it is probably much more.

What you probably need is a negative temperature coefficient thermistor. A really big one.

 

On the supply side (before the heater) voltage is 118.

How come you are not applying the rated transformer voltage?
It's not a valid test with only 118V.

 

How come you are not applying the rated transformer voltage?
It's not a valid test with only 118V.

So I have a heater in line with each leg (so 3 space heaters) of the 3 phase to each coil. So each leg off of the panel is the 118v. Measure across the L1/L2 or L2/L3 or L1/L3 will give me the 208 3phase. So I literally installed a plug where I can plug the heater in. L1 from panel (primary) on one side of the plug and the wire (secondary) from the other side of the plug going to the transformer. I thought that's how I would wire it to act as a resistor?

 

I thought that's how I would wire it to act as a resistor?

Yes that sounds correct.
But I think you need something around 10 ohms or less.

 

Pre-magnetizing transformer reduces inrush current and could be an option if pre-resistor method is not succesful. Main transformer is magnetized with a smaller 3-phase transformer before connecting main supply to main transformer. Voltage of pre-mag transformer should match the main transformer, could be secondary of primary voltage because magnetizing works from either side. About 1.5 kVA pre-magnetization transformer seems to be enough for the 75KVA transformer based on measured no-load currents if the currents were measured on 208V side.

Phasing has to be checked before connecting the main supply but it's enough to check it once if the supply to both transformers is kept the same and wiring is not changed.

If I can recall from a real system, pre-mag time was less than a second but we had two second pre-mag time in a PLC. Size of the magnetized trafo was 5-7MVA.

 

Is the transformer wired star or delta?
if delta, you could start it up on star.

 

Pre-magnetizing transformer reduces inrush current and could be an option if pre-resistor method is not succesful. Main transformer is magnetized with a smaller 3-phase transformer before connecting main supply to main transformer. Voltage of pre-mag transformer should match the main transformer, could be secondary of primary voltage because magnetizing works from either side. About 1.5 kVA pre-magnetization transformer seems to be enough for the 75KVA transformer based on measured no-load currents if the currents were measured on 208V side.

Phasing has to be checked before connecting the main supply but it's enough to check it once if the supply to both transformers is kept the same and wiring is not changed.

If I can recall from a real system, pre-mag time was less than a second but we had two second pre-mag time in a PLC. Size of the magnetized trafo was 5-7MVA.

This sound interesting. So I am not having good luck with the pre-resistor method.

So yes. The currents at no load were measured on the 208 side coming in from the panel to the main transformer. I assume I could use a 1.5kVA or maybe a little bigger depending on what I find. I guess my question would be what would the LV and HV ratings need to be on the 1.5kVA transformer. I would feed the 1.5kVA transformer with 208v 3 phase from the panel, but I would want 208v 3 phase on the secondary side to feed the main transformer correct? Or am I missing something? Which I probably am. Sorry if this doesn't make sense.

For what its worth. The transformer I have right now is 480v to 208v wired in reverse.

Thanks

 

Is the transformer wired star or delta?
if delta, you could start it up on star.

I'm pretty sure it's wire delta.

 

I would feed the 1.5kVA transformer with 208v 3 phase from the panel, but I would want 208v 3 phase on the secondary side to feed the main transformer correct?

Yes, 208v also on the secondary side to match the main transformer voltage. It could be also 480v but then connection would be to the 480v side.

 

Yes, 208v also on the secondary side to match the main transformer voltage. It could be also 480v but then connection would be to the 480v side.

Okay let me see if I understand: The power coming off my panel is 208.

So I use 1.5kVA 3 phase transformer 480 to 208. Power the 208 (primary) of the 1.5kVa from the panel and then run the secondary 480 side of the panel to the primary 480 side of the main transformer? Would that be any different since I'm wiring the main transformer in reverse to act as a step up transformer as opposed to a step down transformer?

 

Okay let me see if I understand: The power coming off my panel is 208.

So I use 1.5kVA 3 phase transformer 480 to 208. Power the 208 (primary) of the 1.5kVa from the panel and then run the secondary 480 side of the panel to the primary 480 side of the main transformer? Would that be any different since I'm wiring the main transformer in reverse to act as a step up transformer as opposed to a step down transformer?

There actually is an important item to check, vector group of the main transformer and premag transformer has to be the same and they have to be connected "the same way". This applies also when pre-magnetizing on 208v side. Two examples:

1. Main trafo is 480V delta/208V Y and premag trafo is also 480V delta/208V Y. Both gets a supply from 208V and that I mean with "the same way", so both are connected in reverse. Phase of the voltages on 480V should/would be the same which is correct. It should not matter that transformer is wired in reverse when voltage amplitude and phase match. If one of the transformer is delta/Y and another Y/Y it doesn't work because of the phase shift difference.

2. Main trafo is supplied from 208V side and magnetizing is done also on 208V side, but the premag transformer is 208V/208V delta/Y. This doesn't work because magnetizing transformer has 30 degrees phase shift between secondary and primary.

 

I made a sketch in Paint which can help, I assumed that the 75kVA transformer is 480V delta/208V Y but of course it can be something else. Principle can be seen, though.

 

There actually is an important item to check, vector group of the main transformer and premag transformer has to be the same and they have to be connected "the same way". This applies also when pre-magnetizing on 208v side. Two examples:

1. Main trafo is 480V delta/208V Y and premag trafo is also 480V delta/208V Y. Both gets a supply from 208V and that I mean with "the same way", so both are connected in reverse. Phase of the voltages on 480V should/would be the same which is correct. It should not matter that transformer is wired in reverse when voltage amplitude and phase match. If one of the transformer is delta/Y and another Y/Y it doesn't work because of the phase shift difference.

2. Main trafo is supplied from 208V side and magnetizing is done also on 208V side, but the premag transformer is 208V/208V delta/Y. This doesn't work because magnetizing transformer has 30 degrees phase shift between secondary and primary.

This works for me. For my own understanding, would it be possible to give me a brief understanding of why it works??? Does the minimal 2kVA minimally energize the 75kVA coils to limit the inrush current??

Thanks

 

This works for me. For my own understanding, would it be possible to give me a brief understanding of why it works??? Does the minimal 2kVA minimally energize the 75kVA coils to limit the inrush current??

Thanks

Search engine finds commercial premagnetizing transformer and system suppliers and they might have good information. They are typically for higher power and medium voltage but the principle is the same. Power transformer noload current is about 1% of the nominal current and that's why premag transformer can be small. It only has to supply noload current. 2 kVA should be able to nominally/fully magnetize the 75 kVA transformer if my calculation from the noload current is correct.

Optional contactor k3 is closed when premagnetizing starts. Voltage rises when premag transformer energizes so it's not instant voltage step. Many commercial systems seem to have resistors between premag and main trafo which limits premag transformer current. I don't remember resistors in the system that I worked on but it was 20 years ago and the premag transformer was designed for that use and project.

Phasing has to be correct in design as mentioned, but phase sequence has to be correct and checked before energizing the main transformer. With wrong sequence really high inrush will happen.

It's best to disconnect fuses or at least disable contactor of the main transformer for initial check. Premag trafo is energized (for much longer than a few seconds to have time to measure) and about 208v should appear on 75 kVA transformer. Voltage measured with a multimeter across the "empty" fuse holder should be low, lets say within 10% so about 20v. Same check for all three phases.

It's good to remember that the premag transformer has it's own inrush current and too frequent testing should be avoided. I assume that 75 kVA transformer is not turned on and off frequently during normal operation.

 

Search engine finds commercial premagnetizing transformer and system suppliers and they might have good information. They are typically for higher power and medium voltage but the principle is the same. Power transformer noload current is about 1% of the nominal current and that's why premag transformer can be small. It only has to supply noload current. 2 kVA should be able to nominally/fully magnetize the 75 kVA transformer if my calculation from the noload current is correct.

Optional contactor k3 is closed when premagnetizing starts. Voltage rises when premag transformer energizes so it's not instant voltage step. Many commercial systems seem to have resistors between premag and main trafo which limits premag transformer current. I don't remember resistors in the system that I worked on but it was 20 years ago and the premag transformer was designed for that use and project.

Phasing has to be correct in design as mentioned, but phase sequence has to be correct and checked before energizing the main transformer. With wrong sequence really high inrush will happen.

It's best to disconnect fuses or at least disable contactor of the main transformer for initial check. Premag trafo is energized (for much longer than a few seconds to have time to measure) and about 208v should appear on 75 kVA transformer. Voltage measured with a multimeter across the "empty" fuse holder should be low, lets say within 10% so about 20v. Same check for all three phases.

It's good to remember that the premag transformer has it's own inrush current and too frequent testing should be avoided. I assume that 75 kVA transformer is not turned on and off frequently during normal operation.

Thank you very much for your very thorough explanation!!! I'm going to give the pre resistor method one more shot, but if it does not, I will go this route!!!