Electronics > Beginners

MOT (microwave oven transformer) cooling.

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davelectronic:
Just a thank you for the help,  ideas and data provided. Generally I've knocked out shunts, having read they help in a crude way to avoid overheating in the transformers saturation. Maybe I should have left them in, and the area that they take up in small. Although a couple more secondary turn can be had with there removal.
It's always going to be a poor man's linear transformer, but it's a means to an end.

Wolfgang:
Yeah, right. You cannot expect too much from a junkyard item. If not run in a microwave oven, 300W is realistic with a series inductor and the magnetic shunt out. Still better than nothing if you need some cheap HV.

Happy (and safe) experimenting !
  Wolfgang

Berni:
Id say just wire the primaries of two MOTs in series so they are effectively running on 110V and split your 12V output winding in to two 6V winding on both transformers. Acording to the graph that will bring the magnetizing current down to something normal and save a lot of losses in the primary copper wingdings.

As for the shunt id say just hammer it out. It should make very little if any difference to the magnetizing current on the primary, but will have a big effect on your output load regulation. As you put a load on your secondary coil you don't get all of the flux from it reaching the primary where it accordingly causes the input current to rise and put more power into the transformer. Because of that as the current in the secondary increases and produces an opposing flux it starts to cancel out the original flux and reduce it. Because of that the voltage on the secondary drops.

This makes the shunt effectively a current limiting feature. Without it under a short circuit condition the current is mostly limited by the resistance of the wingdings and this causes a lot of heat. But a magnetic shunt simply hides some of the secondary coils load from the primary causing it to draw less current from the power source. This effect is heavily used in neon sign transformers where they are magnetically shunted so much that you can short the output and leave it running indefinitely without any damage to the transformer.

Cliff Matthews:
Quote from: Wolfgang on Today at 10:58:41 am
Yeah, right. You cannot expect too much from a junkyard item. If not run in a microwave oven, 300W is realistic with a series inductor and the magnetic shunt out. Still better than nothing if you need some cheap HV.

--- End quote ---


A bit more maybe. I got 330 watts applying 78-vac to 120-vac MOT primaries (testing with lamp cord as secondary winding)

Wolfgang:
... some people even use a lamp as a limiting resistor. IMHO the end of the game is reached when the MOT core saturates. From then on it will get really hot. Its also a question of what kind the load is usec (rectifier with cap, resistive, ...). Anyway, as log everything stays cool, why not draw 330W.

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