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Making a single phase transformer from a 3P

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LAN_403

Wavetuner

Fri Jun 19 2009, 08:39AM

Registered Member #1500 Joined: Sat May 24 2008, 04:38PM
Location: Ojai, Ca.
Posts: 44

Hello transformer wizards and all,

I have a 5kva 3p transformer that is delta primary wye secondary. I would like to power my VTTC without 3p (requires running my diesel genset â€“ a bit much for a tube coil).****(1.) Can I saw out the center core and coil and run it on single phase 240V (has 6 10v increment input taps). It is rated 2200v/coil out or ~ 3800v in wye 3p. The sqr of 3xV yes? How well do you think this would work? I certainly will have a big winding window if I want to add some secondary turns! It is a really nice transformer and I hate to hack on it though. The cores are 8â€^2 (4widex2) total length is 10â€ x8â€ high. The primaries are wound on the outside. ***(2.)Can I wind on top the primary and series the secondaryâ€™s provided the winding direction is the same? I could use doublers just fine otherwise

Thoughts and critics welcomed.***(3.) I assume either the center winding and core or one side has to go??

Thanks,

Jim Mora

Steve Conner

Fri Jun 19 2009, 09:20AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If you short out the coils on the centre limb (or any one limb for that matter) it amounts to the same thing as sawing it out, but a lot less effort, and reversible. If you then apply 240V to the remaining unshorted terminals, you have a single phase transformer with primaries in parallel and secondaries in series. And the output should be 4400V, I think, if it was originally designed for 120/208.

Wavetuner

Sat Jun 20 2009, 06:05AM

Registered Member #1500 Joined: Sat May 24 2008, 04:38PM
Location: Ojai, Ca.
Posts: 44

Hi Steve,
I was thing on those lines but what abount the magnetic field in the unused core? Isn't that the puesdo equivelent of a 1/3 big arse shunt??

klugesmith

Sat Jun 20 2009, 10:11AM

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

I'm respectfully questioning Steve's suggestion for a different reason, though claiming no real experience with 3 phase cores.

Written second but presented first: a view of symmetric 3-phase transf. with single phase voltage applied to two primary windings in series or in parallel. Depending on their connection polarity, flux in the third leg is double or nothing.
Opt for the latter (no flux) and you can just ignore the windings on third leg.
Opt for the former (double flux) and you can use all 3 secondaries, one with double
the volts per turn. Figuring the saturation voltage rating is a job for another day or another commenter.

Written first, later tweaked: effect of shorted winding when one primary is driven.
To measure the leakage inductance of an ordinary transformer, you short the secondary. Referred to primary side, the "ideal transfomer" part of the model appears as a short, in series with leakage L. Ideally that's very small, so large currents flow with small voltages. Because of the shorted secondary, there's virtually no flux in, so no back EMF from, the coupled part of the primary winding inductance.
With a shorted winding around one leg of a symmetrical 3-phase core, the MMF in that leg from a single primary winding is bucked by the MMF from current in the short. Net flux in that leg is just enough to generate the very small volts per turn of the short. But we have the whole third leg to carry flux motivated by the primary current, and to complete an ordinary transformer as stated by Steve.
Apparent magnetizing inductance will be about 3/4 of the original single-primary, no-shorted-winding case. In words (sorry, no picture), say the original inductance of coil 1 comes from 1 unit of flux: 0.5 going to coil 2 and 0.5 going to coil 3. If coil 2 is shorted it will conduct 0.5 units of current and generate 0.5 units of flux: 0.25 back to coil 1 and 0.25 on to coil 3. By superposition, the net flux is 0 in coil 2, 0.75 out of coil 1 and 0.75 into coil 3. If I had an appropriate core I would confirm this by measurement!

Steve Conner

Sat Jun 20 2009, 03:37PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, my argument is that a shorted coil will exclude magnetic flux from passing through it. Just like an absence of iron will. So shorting out the coils around the centre leg should cause it to vanish from the magnetic circuit, forcing all the flux to pass through the two remaining limbs.

Otherwise, the centre leg acts as a magnetic shunt. Like Klugesmith mentioned, if you drive the primaries in parallel then this shunt inductance may not matter. But connecting two of the three primaries in parallel will short the third one, if they're in delta and you don't open the delta. (I guess this is kind of a proof of my original statement.)

Wavetuner

Sat Jun 20 2009, 06:14PM

Registered Member #1500 Joined: Sat May 24 2008, 04:38PM
Location: Ojai, Ca.
Posts: 44

Another thought... I use a sat reactor in my TC cabinet. Perhaps applying some variable DC on the center leg primary will cause it to "go away" (secondary shorted but no matter in this case). Once I know what voltage it takes, a small transformer and a FW diode/cap could be assigned the duty, retasking my variac.

It is hard for me to visualize what if any loses this would create; but, the leakage should not be an issue. How much the DC affects the outer core flux flow may be? I guess adjusting the variac for max VA output on the seconaries is the goal.

I have made a large CuSO4 resistor with copper end plates and the ability to easily dope from concebtrate to target resistance for these types of load tasks. Given the core size and the copper sizes, 5KVA should be doable pretty easy if the tweaked parameters are working.

I like the idea of of paralleling the primaries and placing in series the secondaries for 4400v. The HV windings look well isolated from the core by a 1/4" shellated former and air space.

In any case, all connections (delta/wye) will be, for this app, disconncted but the connection phenolic plates will be left intact. I'll let you know how it works out.

If the DC is a bad idea, place respond.

Jim Mora

klugesmith

Sat Jun 20 2009, 10:37PM

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

Jim,
I think DC current in one primary is not going to help anything. If you push the flux in that leg to saturation, the other 2 legs get a DC flux at least halfway to saturation. That will greatly reduce their ability to handle AC voltages. And if the DC biased leg is not thoroughly saturated, your DC current source will be back-driven with the primary AC voltage.
The only DC voltage which can easily and safely be forced is zero, in other words short the winding as Steve said, which imposes a condition of very little flux in that branch of the core.

You can learn a lot by measuring and charting the magnetizing current (input current with all other windings open) as you ramp the primary voltage up to your intended operating voltage, or reach a current of a few amps (whichever comes first). The current goes up steeply as you approach saturation. Then repeat with one other winding shorted, and/or with two primaries driven. If you don't have a 240 volt Variac, you could use a split-phase circuit with a 120V Variac on one side of neutral. Or generate a set of discrete AC voltages using 24 to 60 volt transformers to buck or boost the main power. Be careful!

A note about aqueous copper sulphate resistors ... the resistivity goes down significantly as the solution heats up. I just confirmed that by applying 120 VAC to this puppy (which had sat on the shelf for years without leaking or drying up):

-Rich

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