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rewinding a mot

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Frosty90

Tue Jan 27 2009, 06:36AM

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

hi all,

I need a 160v ct transformer for a project, but I dont want to spend any money, so I have cut the 'I' section off the core of a mot, thinking i could just slip the secondary off, wind my own, leaving the original primary on and have a nice ransformer, but to get the secondary off, I had to get the primary off, and both are moulded on with some sort of glue, and in my enthusiasm with the screwdriver and hammer i damaged the primary, so my question is, how many volts per turn should a core of 20 square centimetres ? if i wind some new coils for it is there anything special i should know? i've never done such a thing before.

Cheers,
Jesse

uzzors2k

Tue Jan 27 2009, 12:01PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I've done this a few times. One transformer was cut open prior to rewinding, the other was left intact. In both cases I've left the original primary on the transformer, as winding the same amount of turns just as tightly by hand is nearly impossible. If you must I think there's about 200 turns on a stock MOT primary, though MOTs are intentionally wound with too few turns.

See "MOT rewinding".

The last MOT I did needed a 225+225V winding, and it had to be silent so I left the core in one peice. (You'll never make it stop buzzing without welding once it's been chopped up.) I used a hacksaw to cut part of the secondary winding off, and a hammer to punch the winding out. Once the transformer was stripped down I started winding a new secondary by wrapping 5 meter lengths of copper wire on pencils. Each length was bifilar wound, saving me half the winding effort.

GeordieBoy

Tue Jan 27 2009, 02:33PM

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

If you are re-winding an MOT for anything that is designed to run for more than a couple of minutes without forced air cooling, you might want to add a few more primary turns.

As Uzzors said, the manufacturers skimp on iron in the core, and skimp on copper in the windings to keep cost and shipping weight down. The downside is a transformer that doesn't have enough primary turns for the core cross-section. It draws a lot of off-load current and gets hot quickly if not in forced airflow.

-Richie,

Tom540

Wed Jan 28 2009, 01:07AM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

GeordieBoy wrote ...

If you are re-winding an MOT for anything that is designed to run for more than a couple of minutes without forced air cooling, you might want to add a few more primary turns.

As Uzzors said, the manufacturers skimp on iron in the core, and skimp on copper in the windings to keep cost and shipping weight down. The downside is a transformer that doesn't have enough primary turns for the core cross-section. It draws a lot of off-load current and gets hot quickly if not in forced airflow.

-Richie,

Yeah good point. even with little or no load they get hot. I rewound a mot for my coil winder power supply. One secondary with 16 turns and another with 4. Even only drawing a small amount of power from the 4 turn it still gets kinda hot.

Frosty90

Wed Jan 28 2009, 01:19AM

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

hi,

Yes i thought i would need more turns, but i just wasnt too sure how many would be appropritate. The existing primary seems to have had about 160 turns, which seems quite low. Is this about right?

Also in regards to heating, and welding, i've noticed that the core is welded together in many places, not just the 'I' section, presumably to hold the laminations together. Could this be one of the reasons the get so hot? i think all these welds mustbe acting like shorted turns. They arent very deep, so they dont cut much of the flux, but im sure they must be eating up some power?

Anyway seeing as the origninal primary had aout 160 turns, would 240 (1 volt per turn) be enough? or should i go higher?

Cheers,
Jesse

edit:

Damn! i just had a look at that link, and the calculations tell me i need 580 primary turns! the manufacturers clearly do like to save a bit of money! No wonder they get so warm!

GeordieBoy

Wed Jan 28 2009, 12:20PM

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

Any amount of primary turns that you add will reduce the off-load current draw of the transformer, provided that you wind them and connect them so they appear as an extension to the existing primary winding. ie. The new turns continue in a clockwise (or anti-clockwise) direction round the centre limb of the core. If you have a variac and AC meters for V and I you could plot an I/V curve with the existing primary, then add another 20 turns and repeat the test. With 20 turns added the magnetising current should be significantly lower at 240VAC. If you have a Wattmeter you can even measure the off-load heat dissipation.

The welds down the outside of the core set are not very clever as they do short together all laminations and form a shorted turn. However, provided that the welds are only on the outside surfaces of the transformer, then the effective "shorted turn" is only around the outside of the core set. The net field outside the core should be low in a well designed transformer that is not operating near saturation (compare this statement with a typical MOT!) Anyway this shorted turn around the outside may not account for as much heating as you think because of it's location. (If the mating surfaces of the laminations were seam welded on the inside near the winding window then I would expect excessive heating though!!!)

I hope this explanation helps,

-Richie,

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