Rewinding secondary on MOT for 50V supply / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Rewinding secondary on MOT for 50V supply

1 2 next

Move Thread

LAN_403

Mads Barnkob

Thu Jul 17 2008, 08:40PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Hi

I have removed the secondary and heater windings from an MOT and want to wind a new secondary for a high current low voltage power supply.

Its a 230Vac MOT, so primary is somewhere between 192 to 230 turns. So roughly I will also have 1V/turn on the secondary, which I will wind in 1.5mm^2(AWG16) to get 50 turns for 50V.

Remove the shunts, by knocking them out carefully with a pin-punch. This improves the leakage inductance for "normal" transformer operation. In the space vacated by the shunts, wind a few extra primary turns, to reduce the primary turns per volt and hence core flux, and take the transformer out of saturation. This improves the magnetising current.

On the HvWiki it is mentioned that knocking the shunts out will make it better for normal transformer operation, and will also give more space. So this is what I would want to do yes? As I wont touch the original primary will it still make an ok supply for shorter runs on various bridge configurations

Dr. Dark Current

Thu Jul 17 2008, 08:52PM

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

MadsKaizer wrote ...

On the HvWiki it is mentioned that knocking the shunts out will make it better for normal transformer operation, and will also give more space. So this is what I would want to do yes? As I wont touch the original primary will it still make an ok supply for shorter runs on various bridge configurations

Actually knocking out the shunts WILL increase no-load current and losses probably too. If you knock them out, I think it's a good idea to add a few (20? 40?) primary turns of the same diameter wire, you don't need to touch the original primary just connect the windings in series (while paying attention to right phasing).

Shaun

Thu Jul 17 2008, 08:58PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I rewound an MOT for my flyback driver and it puts out 60VDC after full-wave rectifier and smoothing cap, with 120VAC on the primary. I removed the shunts for more winding space and I have to say nothing gets too extreme (except my flyback arcs!), in terms of heat.

The longest I ever ran it with full primary voltage was about 15min; the MOT got hot but not too hot to touch.

Dr. Dark Current

Thu Jul 17 2008, 09:02PM

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

The core losses will not manifest in a few minutes, but I won't be surprised if the core was above 100Â°C after an hour! If you want a MOT to reliably operate for hours, you NEED to wind some extra primary turns.
That said, a MOT without shunts will run fine for a few minutes, but I had them draw 3 to 4 amps no load without shunts, from 230V (800VA)

Mads Barnkob

Thu Jul 17 2008, 09:14PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

This MOT is intended for short uses only, I will make another with more turns on primary as you suggest when I find one with a bigger core, this one havent got enough space for extra primary when I want 50V.

Thanks for the help

Dr. Slack

Fri Jul 18 2008, 06:23AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If you want a MOT to operate reliably for a long time with the original number of windings at the design voltage, you may need to cool it, just like it's cooled in an oven. The high core flux results in high core losses and much heating.

If you want lower core losses, add some primary turns.

Shunts do not change the magnetising current or affect the losses at all. Shunts increase the leakage inductance, which is usually undesired in a normal transformer. Think about where the shunts are. With an open-curcuit secondary, the shunts are "bypassed" by the rest of the magnetic circuit through the secondary space. The rest of the core does not have an airgap, so the flux will take this path and the shunts remain with virtually zero excitation. This is the situation when you are determining the magnetising current. It is only when the secondary carries a current that flux is diverted into the shunts, reducing the field and hence the voltage in the secondary.

Dr. Dark Current

Fri Jul 18 2008, 08:11AM

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

The shunts do lower the magnetising current, actually. I have confirmed this by experiment too.
This is because the magnetising current in a MOT is so high that the flux "jumps" the little gap (it is less than a mm) between the shunts and the core. The increase in current without shunts is not dramatic (0.5-1A at most) but it exists.

Steve Conner

Fri Jul 18 2008, 11:06AM

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

Indeed, I think there is a considerable air gap where the I section of a MOT gets welded to the E at the factory, so it's not surprising that some of the magnetizing current wants to flow through the shunts.

quicksilver

Fri Jul 18 2008, 01:36PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Therefore we are in agreement that knocking out the shunts will have a negative effect on losses? I've found it quite easy to knock them out with above stated pin punch but replacing them is a challenge I do not want to repeat.
Speaking of which has anyone found a method of replacement (of shunts) that is worth consideration besides aligning them and using a wider block as a driving alignment tool? They don't appear to like going back.

I've attempted to glue them together with small bits of Cyanoacrylate Adhesive ("super glue"), careful to make a conductive area available in their adherence and driving them in unison back into position: but it's a challenge.

Dr. Dark Current

Fri Jul 18 2008, 01:43PM

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

quicksilver wrote ...

Speaking of which has anyone found a method of replacement (of shunts) that is worth consideration besides aligning them and using a wider block as a driving alignment tool? They don't appear to like going back.

I have done a "super reactance" MOT by removing the shunts, removing the filament winding and cleaning out the place between primary and secondary (with thin knife).
Then I made extra shunts from cut-down "I" sections of a transformer core, taped them to the existing shunts with a heat-resistant tape and inserted them back.
They will insert easily if the place has been cleaned out of any superfluous plastic and paper etc. Oiling them a bit makes inserting much easier if there are any problems.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob