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Mot transformer getting hot when rewound

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I have recently rewound two mot transformers and have found both, to get very hot very quickly with no load connected to the secondary.I proceeded to connect the mains,to the secondary winding of another mot.This solved my immediate problem because the impedance of the winding was a lot higher.Also because of the turns ratio I achieved roughly the output voltage I needed 24.8volts.I have read that the transformer may be or is,in saturation and the only thing that is limiting the current,in the primary winding was the impedance of said winding which is very low about 2.2ohms compared to about 240 ohms when connected in reverse.

I really wanted to wind a secondary that has a centre tap,so I can have two independent dual rail supplies.This can only be achieved I fear,by using a different transformer or trying to add more resistance to the primary,without increasing the inductance and turns ratio of the primary compared to the secondary.I have rewound torodial transformers without an issue and would greatly appreciate,any information or feedback on what I could do,potentially to solve my problem.
 

Les Jones

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Most Helpful Member
Following on from Colin's question in post #3. Are you rewinding the primary or secondary ? Most people just hack off the secondary and rewind that. After removing the secondary they would normally wind a few turns of thin wire just to find how many turns per volt that particular transformer will give so that they can calculate the number of turns they need to give them the voltage that they want.

Les.
 
The shunts are still in place.Its the secondary of the transformer I am rewinding with a centre tap to construct a dual rail power supply.
 

ericgibbs

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Most Helpful Member
hi Colin h,
Measure and post the CSA [ cross sectional area] of the centre limb of the transformer, from that we can give a good estimate of the turns per volt.
E
 
The turns per volt has been measured at 0.7 volts .At the moment 4 turns yield 2.28 volts.However to keep the primary current,with no load on the secondary low,I have wired the primary of another mot transformer in series with the primary winding of the transformer I am rewinding.Without this the current drawn is atleast 4 to 5.5 amps and the voltage induced into the secondary is higher for the same number of turns .The overheating is still an issue.

Curiosity got the better of me and I wired the mains,across the secondary of a non butchered mot.With significantly more turns and a higher impedance,the current drawn by this coil with no load on the now primary used as the secondary winding is acceptable.A lot quieter and the output voltage due to the turns ratio is within the range I need namely 24.8 volts.By wiring two transformers I can achieve a centre tapped voltage of 24.8 or 49.6 volts between windings.After smoothing the voltage jumps to over 60 volts .Which is what I wanted all along.

I cannot understand why in its present state,the transformer is getting so hot with no load on the secondary.I have not measured the inductance of the primary winding but have measured the resistance and it is low 2.2 ohms.The resistance of the secondary on the unbutchered mot is 230 ohms much higher.The core must be saturated and the only thing,that is limiting or not in my case the current in the primary,is the resistance of the winding which in its present form is low.
 

Diver300

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Microwave oven transformers are only rated for short-term use. I think that you have answered your own question about them saturating, as they do, and therefore run hot with no load.

I've no personal experience myself, but I think that others on the forum have added primary turns to reduce saturation and therefore no-load power. Another 10 - 20% would make a big difference, so about another 30 turns would work. On the reversed one, going to that many turns means you won't get much power out, but if you can get enough for your needs, it doesn't hurt.
 

dr pepper

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Most Helpful Member
Mot's have all the e's and i's in the core aligned, they are a high leakage transformer, also the magnetic flux density in a transformer is highest when the load is lowest, mot's are (except when the filament it heating up) always connected to a load, this means the makers skimp on copper and core material so the transformer is just not saturating on load, running one off load will most likely drive it well into saturation, increasing primary current and heating up the trans.
The shunts increase the impedance of the trans to match that of the magnetron, leaving them in will make the transformer less ideal, but more tolerant to a shorted secondary, if anything it'll run cooler with the shunts in place.
 

Colin

Active Member
You should be looking at a higher number of turns per volt for the application you are requiring. Possibly 2 to 5 turns per volt for a 1kVA transformer
 

Colin

Active Member
"I would have expected more like 1 - 2 turns per volt on a 1 kVA 50 Hz transformer. 5 turns per volt would be right for much smaller transformers."

This is not a "normal" transformer. You don't know the cross-sectional area and to keep the core from saturating you may need a higher TPV.
And microwave transformers are always LOADED.
 
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