Microwave oven transformer

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High power transformer found in a microwave oven that steps up wall voltage to around 2 kVAC, at power usually between 900 W and 1700 W. Be careful- these are not current limited! They are often known by the acronym MOT.

This is a non-ideal transformer whose purpose is to generate typically 1 kW of pulsed 5 kV DC into a magnetron, by driving a half-wave doubler.

The turns ratio is designed to give about 2 kV AC to the main sedcondary winding, one end of which is bonded to the grounded core. An additional secondary provides an isolated supply of typically 3 V at 15 A for the magnetron heater.

As it is intended to drive a capacitive load, the leakage inductance of the tranformer is deliberately increased by adding a small magnetic shunt between the primary and secondary coils. The inductance is roughly equal and opposite to the doubler capacitance, and so reduces the output impedance of the doubler. This specified leakage inductance classifies the transformer as non-ideal.

The transformer is designed to be as cheap to manufacture as possible, with no regard for efficiency. This is because it is the manufacturer who pays for the copper and iron, but the user who pays for the energy consumed. Thus the iron area is minimised which results in the core being taken well into saturation with result high core losses. The copper area is also minimised, resulting in high copper losses. The heat that these generate is handled by forced air cooling, usually by the same fan that is required to cool the magnetron. The core saturation is not part of the non-ideal classification, it is merely as a result of the economics of manufacture.

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Note that powering a Microwave Oven Transformer outside the original oven carries the risk of injury or death due to electrical shock. This should not be attempted unless you have enough knowledge of electricity to understand the risk and take the relevant safety precautions.


Contents

A MOT up close and personal

650W Microwave Oven Transformer
650W Microwave Oven Transformer

This transformer is from a 650 Watt oven, and is about as small as they come.

Key features are:

  1. Primary - tends to be about 1 turn per volt. This is for 230 V power. A 120 V MOT will have half as many primary turns of thicker wire
  2. Secondary - surrounded by insulation card. The inside of the winding is grounded on the other side of the MOT
  3. Heater Winding - 3 turns of insulated wire
  4. Shunts - several 'I' laminations between inner an outer legs of the core, with an air-gap defined by thin piece of insulation


MOT sourcing

MOTs used to be readily available from municipal dumps, however health and safety concerns have led to the operators being less willing to allow them to be removed from the site. Talk to your local operatives, if you don't ask, you don't get. Small amounts of money or beer have been known to improve the relationship.

Roadside dumpsters often contain dead microwave ovens. The MOT tends to be the last thing to go, so the chances are good that a discarded microwave oven will contain a good MOT.

Try Freecycle or Craigslist, many folks are more than happy for you to cart off their old microwave.


Things to make and do with a MOT

Remove the secondary, and put a few turns of a new secondary of heavy wire on there. It's a typically 1 kW transformer, so makes a good spot-welder or other low voltage transformer.

Remove the primary, and put a few turns of a new primary of heavy wire on there. This allows you to get 2 kV or more at high power from a 12 V battery inverter.

Short-circuit one winding, and use the other as an inductor, seeing the leakage inductance. Typically one MOT with a short circuited secondary can be used to ballast another MOT for drawing arcs.

Use two together to make a saturable reactor. The high turns ratio allows a large AC current in the primaries to be controlled by a relatively small DC current in the secondaries, making a moderate power variable supply.

Grind off the welds that hold the Es and Is together, and introduce an air-gap to make an adjustable inductive ballast. If you have two identical transformers, put both primaries on one, both secondaries on the other.

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.


Things not to make and do with a MOT

Power a tesla coil from 2 MOTs balanced about ground. The available voltage is just not sufficient to run a good coil, too much of the output voltage goes in the spark gap over-voltage.

Power a tesla coil from 4 or 6 MOTs balanced about ground. The voltages are now approaching useful NST levels, however the electrical stress between the core and secondary, or core and primary for floated core operation, now renders the MOTs extremely vulnerable to overvoltage breakdown. Submersion in oil is good, but messy. For applications with little electrical kick-back, such as arc drawing or a Jacob's ladder, series connection of MOTs in air is practical.

More information

A guide on microwave oven transformers

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