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Author Topic: Need advice on 3 phase isolation transformer  (Read 610 times)
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Ichan
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« on: October 28, 2015, 02:38:40 14:38 »

I am trying to salvage an old dead cnc milling machine, planned to retrofit it using a new cnc controller. After removing all the wiring I am now testing the 3 phase isolation transformer, it is 9KVA 3x380V primary 3x220V secondary.

Without anything connected to the secondary, feeding 3x380V via a magnetic contactor this transformer always trip down the wall panel 32A circuit breaker, my questions:

- anything i do wrong?
- is this transformer broken?
- how to check it?
- What winding is blue wires on the picture attached?

TIA, Ichan.
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kreutz
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« Reply #1 on: October 28, 2015, 04:35:12 16:35 »

Normal magnetizing inrush  current  may be up to 10 times higher than the normal rated current of the transformer.

See: https://www.youtube.com/watch?time_continue=19&v=f1HeiO1JU1s
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enzine
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« Reply #2 on: October 28, 2015, 09:50:16 21:50 »

The blue wires may be the connection to an internal  electrostatic  shield that separates primary from secondary.

Remember that a transformer during power-up generates a current peak which can achieve 25x the rated current of the primary of the transformer for a time of about 10 milliseconds.

Protection and disconnection of a power transformer require a specific curve of breaker with delayed curve  (IEC type "D" :above 10 In up to and including 20 In).

Ciao
Enzo
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Ichan
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« Reply #3 on: October 29, 2015, 08:17:25 08:17 »

Thanks, this is a new things for me.

Kreutz, so how to reduce that inrush current? Sure I will ask google soon, but in case you already knows.

Enzo, what should i do with those blue wires? Left them unconnected or something else more useful?

Could you please give me links to sample of that delayed breaker product?

-ichan
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vern
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« Reply #4 on: October 29, 2015, 11:52:07 11:52 »

simplest way is to replace your fuse breaker with one that has a long time lag as enzine mentioned in his post.
The other possibility is to use an inrush current limiter like this one MS35 2R035 from Ametherm, available at Digikey.
Just be careful, high voltage with high power! These things can explode when wrong connected. Stay away when you test the setup. Wear safety goggles!
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enzine
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« Reply #5 on: October 29, 2015, 11:48:48 23:48 »

Let's refresh a bit of  theory Tongue
Transformer Magnetizing Inrush Currents

When voltage is switched on to energize a transformer, the transformer core normally saturates.
This results in a large inrush current which is greatest during the first half cycle (approximately .01 second) and becomes progressively less severe over the next several cycles (approximately 1 second) until the transformer reaches its normal magnetizing current.

To accommodate this inrush current, fuses are often selected which have time-current withstand values of at least 12 times transformer primary rated current for .1 second and 25 timess for .01 second.

Two questions and one answer for Ichan Wink:

1) Which is the mark stating shown in the breaker? (Please indicate model and value , C32, D32, ...)

2) Are you living in Europe? Because the nominal value of three phase voltage in Europe  is 400V and your transformer is 380V rated and  if in your place is near to a power grid,  voltage it is even more high  further increasing  the inrush current.
May you measure your voltages? (L1-L2, L1-L3, L2-L3)


One answer:
In a transformer the shield is used to reduce interference  by reducing the stray capacitance between primary and secondary windings.
Simply connect it to the ground wire.

Ciao
Enzo
« Last Edit: October 30, 2015, 10:32:07 22:32 by enzine » Logged
Ichan
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« Reply #6 on: November 02, 2015, 07:34:27 19:34 »

I can access the machine only on working days, just today i can check it.

Enzo, no i am in asia and our grid is proper for the transformer 380V phase to phase, measured today as around 382V - 385V.

Attached the photo of the tripped breaker there is C32 marking on it, should this one replaced with D32 type?

How to make sure that blue wire is connected to a shield, not another winding?

My fault is removing all the old electronics and wiring blindly (done by my helper boy), no record on how is the wiring before. There is one old contactor interesting me, as the second photo, what device attached on the back of it? Seems like RC filter in delta configuration, could this one related with that inrush current thing? A supressor perhaps?

-ichan
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enzine
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« Reply #7 on: November 02, 2015, 10:27:07 22:27 »

The curve D is just to these purposes.
This may solve your problem of inrush current.

In the picture that you have attached is visible  "HRC3/047". Searching with Google you see that it is a suppressor of interference caused by the opening / closing of the contactor. I do not think it can reduce the inrush current.

For the characterization of a transformer should have available a three-phase variac. You start from 0V and gradually increases the voltage and output and measure the current.  In this way you can also check if the transformer has faults.
But I doubt you have a three-phase variac ...

You have available resistors 1..10 ohm and adequate power to be put in series with the primary in order to do some measure?
This is to ensure that your tranformer have not effectively fault (shorts on  windings ...)

How to make sure that blue wire is connected to a shield, not another winding?

Right question!
I think it's a shield but may be  also another winding!

But I think that this question at this time is of secondary importance.
First we solve the problem of inrush current, then once you powered the transformer you can measure if there is a voltage across the blue wire.

I hope these tips will be useful.

Ciao
Enzo

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solutions
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« Reply #8 on: November 07, 2015, 10:23:57 10:23 »

Hey Ichan,

Why not just run it off of single phase 220V, since you are stripping the machine anyway? Soooo much easier, IMO.

Sell the iron from that boat anchor and pay for the retrofit....
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Ichan
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« Reply #9 on: November 10, 2015, 08:13:06 08:13 »

The machine is rather too big for 1 phase, the AC servo used is 1.5KW for XY and 2.2KW for Z, bought them already.

I am very much not confidence with high voltage, so i decided to ask other people with required experience to handle this transformer thing.

-ichan
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solutions
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« Reply #10 on: November 10, 2015, 11:11:52 11:11 »

I guess it all depends on what wiring you have in the shop more than anything else. I use a 50 amp breaker on my arc welder circuit, so am limited to 10kW or so.

I agree on the high voltage discomfort - high voltage when I was a young pup was 4000 series CMOS on a 15V rail.
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