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Sawing the MOT open

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Dr. Dark Current

Fri May 02 2008, 06:49PM

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

As you may know a MOT has the E and I section welded together and the weld "extends" ca. 1-2mm into the core. Some questions:
1) Does this weld take part in carrying the magnetic flux?
2) If I saw the MOT open by sawing off the weld, will it work just as well after I put the core sections back together?

I'm just asking so I don't mess up a perfectly good transformer...

Chris

Fri May 02 2008, 08:55PM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

It will work just as well as before if the joining surfaces to meet almost perfectly, yes. Make sure to file it carefully along the direction of the laminates if at all, so the burrs don't short the laminates together, creating a path for eddy currents. I would apply a thin coat of laquer on the ends too if there is bare metal exposed.

Dr. Dark Current

Sat May 03 2008, 06:53AM

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

Ok thanks. Just to clarify what I meant: The core will be thinner in the place of sawn off welds. Will this have any significant effect on decreased inductance/lower saturation voltage and increased no-load input current?

Dr. Slack

Sat May 03 2008, 07:56AM

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

A MOT is already heavily into saturation, so reducing the core area *will* make the magnetising current slightly worse.

However, failure to get the inevitable air-gap between the Es and Is back to what it was before will have a far more significant effect (what a manufacturer can acheive with pristine metalwork and a hydraulic press is going to be far better than an amatuer can with hand-buggered-about-with metalwork and a vice). This is where you need to apply your care and skill.

So with these changes, you are unlikely to "mess it up" with care, but you will degrade it slightly.

You could probably ignore the increase in magnetising current due to both sources, probably without extra cooling. Or you could mitigate it a little by throwing a few extra primary turns on while you have the core open. This will reduce the peak flux, mitigating the reduction in core area. It will also increase the inductance, mitigating the increased air-gap. It will however reduce the turns ratio, reducing the output voltage.

Dr. Dark Current

Sat May 03 2008, 08:34AM

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

Dr. Slack wrote ...

A MOT is already heavily into saturation, so reducing the core area *will* make the magnetising current slightly worse.

yeah, this thing sucks 600VA just sitting there plugged in! (Chinese quality with aluminium primary)

The only place where I could put the additional windings is above the shunts where the heater winding used to be. I would really like to wind it under the shunts so the inductance actually increases. Maybe I can move the shunts a bit up, I'll see...

Any guess as to how much worse will the magnetising current be? Maybe someone who had actually done this can comment?

I wouldn't mind messing it up if there were free microwaves next to the roads, but it is impossible to obtain a free one here.

Dr. Slack

Mon May 05 2008, 07:47AM

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

You could lose the shunts as well. The leakage inductance they add is pretty low, rarely enough to comprise all of the ballast you need. Without them it will give you the flexibility to operate it like a real transformer. When you need additional inductance for ballast, add it externally.

Lakeowner

Mon May 05 2008, 05:58PM

Registered Member #1370 Joined: Mon Mar 03 2008, 09:01AM
Location: Finland
Posts: 56

I've done core opening with an angle grinder. I took shunts out and wound new secondary, 4 turns of 16mmÂ² grounding copper. Regardless thick bolts and wood that clamps MOT, system is VERY noisy. It takes about 4A (@230V) magnetising current, and sharp 50Hz roar is audible very loud and clear. Secondary gives >300A short-circuit current. I also made new shunts from normal sheet iron and duct tape (yes, where is silicon steel...). I think epoxy could solve the noise problem...

GeordieBoy

Tue May 06 2008, 04:08PM

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

The welding of transformer stacks really is a cheap and dirty trick. Transformer cores are laminated to stop eddy-currents that would circulate if the core was solid iron. Running a welder down both sides of the stack of laminations just shorts them all back together again at that point allowing current flow between the laminations.

Dr. Dark Current

Tue May 06 2008, 04:54PM

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

I have sawn it open already (everything went as planned), and I was able to see that the laminations do not short together. They are just "bent" in the place of the weld but not shorted. How would the transformer otherwise work? It would trip the breaker.

benbradley

Wed May 14 2008, 05:00AM

Registered Member #312 Joined: Mon Mar 13 2006, 01:50AM
Location: Georgia, USA
Posts: 27

I was going to suggest measuring the inductance of the primary before cutting it open, and measuring it again after putting it back together to get an idea of how much inductance is lost in the process. This won't tell you anything about saturation (unless you measure voltage, current, phase, current waveshape and such before and after), but will give some idea of how much things changed with the weld opened up. Perhaps do all that with your next MOT.

Do you have a "Variac" (TM), (R) of sufficient current rating? That would be useful to determine what primary voltage it starts to saturate at (before and after cutting it apart/reassembling).

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