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Rewind of 3PH core

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Coronafix

Thu Jul 07 2011, 12:49PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

I have a step down 3PH transformer that is 415V/380V. It is for a 2.4kW machine that I have and I
want to rewind it so it is stepping up from 240V to 380V. Question is, will this work? Do i have to consider
another aspect beside window space? I should take a photo of it really so you can get an idea as to how big it is.
How many turns should there be on the primary?

AleSeg

Thu Jul 07 2011, 02:56PM

Registered Member #2727 Joined: Tue Mar 09 2010, 02:39PM
Location: Montevideo - Uruguay
Posts: 33

If you maintain the conexions and the polarity of the windings, you have one side already made.
If you can extract the 415 V side without destroying the isolation of the other winding, you can change it and all will be Ok.
You only must use a conductor section that you can calculate from the section of the conductor that you will extract.
If the winding of 415 V have a section of S_415 in mm2, you will need a section S_220 = (415/220) * S_415. Then you go to a table and get the respective diÃ¡meter.
Of course, the number of turns will be linear, and with the new voltage you have that N_220 = N_415 * (220/415)

Greetings :)

klugesmith

Thu Jul 07 2011, 07:44PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

It's not hard to determine your existing volts per turn, thus the primary and secondary turns counts.
Energize primary or secondary with an available AC power voltage not greater than the rating. Then measure the voltage in a temporary sense winding (say, 10 turns).

It's probably pretty close to the maximum volts/turn that your core can support at that frequency.
We can make a good guess at the volts/turn if you tell us the frequency and the cross-sectional area of the core.

What is the kVA rating of the transformer as is stands? If it's oversized, you might get what you need by just unwinding part of what's already there.

For example, remove 31% of the 380-volt windings, making your transformer 415:262 (or, turned around, 240:380). This can be used conservatively with the original current rating of each winding.

Or if the 415-volt windings are much more accessible, unwind 42% of them to make it 240:380 V.
Again, this can be used conservatively with the original current rating of each winding.

As Alejandro said, if your required current at 240 volts is more amperes than any of your original windings can handle, then you need to remove a whole winding and replace it with thicker wire. The thicker the better, as long as you can fit 240 volts worth through each window.

Finn Hammer

Thu Jul 07 2011, 07:58PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Coronafix wrote ...

I have a step down 3PH transformer that is 415V/380V. It is for a 2.4kW machine that I have and I
want to rewind it so it is stepping up from 240V to 380V. Question is, will this work? Do i have to consider
another aspect beside window space? I should take a photo of it really so you can get an idea as to how big it is.
How many turns should there be on the primary?

If you have 240V and three phases, then you have 380V already!.
240 from neutral to phases, 380 between phases.
If you want to use it as an isolation transformer, you just have to unwind a bit of the 415V coils, and input delta from 380V, delta out from the reconfigured 415 coils.
A more sensible system would be wired star (also called: wye) 240V secondaries, for the possibility of 3 separate 240v outlets, and a 380V 3phase supply, all isolated.

Hope this makes sense.

Cheers, Finn Hammer

Coronafix

Fri Jul 08 2011, 12:17AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Hey thanks guys for the replies, that has helped a lot.
I looked at the transformer again and it is rated for 4kVA which gives me some head room. The machine to run off of it is 2.4kW. Hopefully it is just a matter of unwinding the nearest coil to the correct ratio.
Ironic that the machine is a coil winding machine and I may have to wind coils to get it operational.
Do you think that the rating is too close? 4kVA will be 10A @ 400V, I need 10A @ 240V. Will i need to rewind with a larger diameter wire?

klugesmith

Fri Jul 08 2011, 05:28AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Coronafix wrote ...
Do you think that the rating is too close? 4kVA will be 10A @ 400V, I need 10A @ 240V. Will i need to rewind with a larger diameter wire?

Sorry, you'll have to read a bunch to find my attempt at an answer.

The actual currents are not total VA divided by V, because this is a three-phase transformer. I'll leave the details as an exercise for you.
But the current -ratios- should be the same as if you did the unwinding calculation for a single-phase transformer. (unless the 3-ph 415:380 ratio is figured for a wye-delta conversion, as Finn pointed out). So the wire in 380 V windings of a 4 kVA transformer can handle the current of 2.4 kVA at 240 V, with about 5% extra margin. As you point out, the wire in original 415 V windings would have about 4% margin reduction, if used for 2.4 kVA at 240 V.

The 415:380 ratio is so close to unity (if it's figured for wye-wye or delta-delta) that all windings might be made with the same gauge of wire. Though magnet wire is made and stocked in 1/2-AWG increments for highly optimized applications.

Forced-air cooling with a simple fan is probably good for at least a 10% increase in kVA, if the transformer is rated for natural convection, and you're not far above sea level.

radiotech

Fri Jul 08 2011, 05:24PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Here are some other considerations:

Age and usage of existing transformer., If old, be aware that all
copper windings have a finite lifetime as insulation deteriorates
with thermal cycling. (the copper attacks the varnishes)

If you rewind, you need to re-dip the core in the impregnating
insulant. This will not go well with the older insulation on windings
that exist.

For hobby use, do what you want, but from the viewpoint.of relabeling
and re-rating the above applies.

If you change anything in the connection scheme Y-Delta, Y Y, it will
change harmonics in the 3 phase line to your machine, which may affect
a VFD drive.

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