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Stacking MOV EI cores, advantages, disavantages

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LAN_403

Wavetuner

Thu Jun 04 2009, 11:17PM

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

Hello Esteemed HV Readers,

I read somewhere else on this site I think, there are advantages to removing shunts, filament windings, and adding several primary terms.

I am going to try stacking the E cores and placing in series the now installed adjecent primaries for 240v operation and paralelling the now adjacent ouput coils.

In additions, I plan on using the icores stacked on the top and bottom of the E core flats to bolt the cores together with allthread. I am wondering if this is worth the time intensive process and what if any there is any benifits as the core area is now twice as long but still retains the same cross area. The magnitizing capability should be greatly improved but is the core still likely to satuarate? I am in the process of trying this with two identical MOTs. Has anyone else tried this?
Thanks for your input.

klugesmith

Fri Jun 05 2009, 01:07AM

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

Sounds like a fun project. What are you expecting to gain, compared to just running the two MOTs side by side? When the cores are merged, you'll be out of luck if the secondary winding polarities are wrong for paralleling! (no option to reverse the leads to one primary).

Are you planning to add a substantial number of extra primary turns, and end up with a reasonably efficient power transformer? Then I think a critical issue would be the airgap between the mating E's due to varnish, non-flatness, etc. Even a few thousandths of an inch offer more reluctance than the rest of the flux path. That's why tape-wound C-cores come in matched pairs, and why E's and I's are usually interleaved.
On the other hand, there are non-interleaved E-I transformers, shaded pole motor cores, etc. And MOT's ordinarily operate so close to saturation that a bit of air is probably acceptable.

It would be enlightening to measure the magnetizing current of your original MOT's with and without their shunts. Sweep the voltage with a Variac, and pay particular attention around your intended volts-per-turn design point. Then measure it again after performing the unnatural coupling of the two E's. If it actually goes down (when driving both primaries) or less than doubles (when driving one primary) then you did well.

-Rich

Frosty90

Fri Jun 05 2009, 06:35AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

I am going to try stacking the E cores and placing in series the now installed adjecent primaries for 240v operation and paralelling the now adjacent ouput coils.

The power you can put through a core depends on the cross-section, the main benefit of stacking the E cores end to end rather than side by side, would give you twice as much winding window room, but the power you can put through the core will be the same as one MOT.

You could still leave a small air gap to limit the short circuit current I think.

Dr. Dark Current

Fri Jun 05 2009, 07:55AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Frosty90 wrote ...

The power you can put through a core depends on the cross-section, the main benefit of stacking the E cores end to end rather than side by side, would give you twice as much winding window room, but the power you can put through the core will be the same as one MOT.

Nope, the power you can get out of a given core ultimately depends on how much copper you can fit on it (that's for a given frequency of operation), provided, of course, that you don't saturate the core.

Sulaiman

Fri Jun 05 2009, 09:06AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Power throughput of a mains transformer is proportional to

(core cross-sectional area) x (coil cross-sectional area)
=
(volts/turn) x (turns x wire cross-sectional area)
=
volts x amps

Since copper costs MUCH more than steel you will usually find a greater weight of steel than copper, and most (c90%) of the heating/loss comes from the copper.
For economy MOTs usually run at a very high flux density to allow more volts/turn
= less copper

GeordieBoy

Fri Jun 05 2009, 01:13PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Making long stacks of laminations will definitely let you process more power, because you can use less turns on the windings, but those turns now have to go around a larger core perimeter.

The dimensions of transformer laminations are chosen to optimise the core area and winding window areas for an overall efficient use of copper and iron.

You can stack two sets of laminations together for a "short fat transformer" and it will process more power, but it will no longer represent the most efficient design for that weight of copper and iron. The most efficient use of that weight of metal would be to use bigger laminations, not stack lots of small laminations side-by-side. The reason it is sub-optimal is that the winding window should really have increased as the core cross-section increases to give the maximum increase in power handling. When you stack laminations (or complete cores) side-by-side you increase the core cross-section but don't increase the winding window in proportion, so you don't see the full benefit of the added material weight.

When laminations are scaled up with the core dimensions and winding window in proportion, you need less turns because of the increased core cross-section. But, since the winding window is now also proportionally larger, you have much more space to fit in these "less turns". That means you can use much thicker wire, implying much lower copper losses and a profound increase in power handling capability for a given temperature rise.

-Richie,

MinorityCarrier

Fri Jun 05 2009, 11:44PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

I noticed that when I wound a stacked toroid core for a DC-to-DC converter transformer I built.

The number of turns required for windings a two-toroid stack for the voltages I wanted was roughly half that for a single toroid , but I found the length of wire required was about the same for both the single toroid and the double toroid windings.

I was winding 20KVDC RTV insulated wire for the secondary, and needed to go to the two-toroid stack in order to fit all the loops of wire through the toroid center required for the voltage (~9VAC) I was after.

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