Rewiinding a MOT

Microwave transformer?

I think they are designed to have a lot of leakage inductance.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

I thought the shunts had something to do with that? That's why I removed them.

Mikek

Coupling may still be loose. Measure the coil inductance and leakage inductance.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

This goes back to the 20 month old post that was resurrected about the transformerless power supply for the leaf blower. I decided to try a transformer supply. I need 20V at 8 amps. I think I should try to find how to decrease the leakage inductance and wind for final use before I make the coil inductance and leakage inductance measurement. I posted a picture of the transformer with 10 turns that I used to get a volt/turn measurement. I will insulate, wind properly and probably varnish dip if I get a working unit.

How would I get better coupling, in the same configuration.

Mikek

Well yeah, you loaded it to a teensy fraction of its load capacity.

Protip: put +20% turns on the primary to reduce saturation. All your PF is going into harmonics.

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website:

My favorite surplus outlet has a 24V 10A transformer that may work for you depending on duty cycle and actual load:

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Regards, 
Carl Ijames

You've increased the magnetic resistance by removing the shunts so now it's running further into partial saturation, you can maybe fix that by adding some primary turns, the core is built with E and I stacked instead of interleaved this makes for more gap which probably isn't helping either.

At low loads magnetising current will dominate the current draw, giving low power factor.

If you're running a rectifier into a capacitor PF will be low because conduction angle, This can be improved by putting a choke between the rectifier and the capacitor.

--
  Jasen.

to

it is a mass produced item made with the absolute minimum of materials to not self destruct when fan cooled and used for short times as usual with a microwave

After you end saturation the extra effort is wasted, if you have a variac and a current transformer, or current probe, you could turn the voltage down and look for the voltage where saturation is curtailed, and then scale existing the turns count up from there to at-least meet the maximum line voltage spec (126V) 20% should easily get you there.

As you're going to be running a motor the capacitor is not needed, so neither is the choke.

Be sure to use an over-sized rectifier to handle the stall current.

--
  Jasen.

Many years ago I asked Win for a chapter on this.

===================================

** So you put a 30 watt load on a 1kW, saturating supply tranny and expected a good PF ??? ** Sure thing, but not the Phil you wanted.

As Mrs Cooper famously said: "dumb as soup" .

.... Phil

All your PF are belong to us!

I'm trying to imagine the picture to go with that!

--

  Rick C. 

  + Get 1,000 miles of free Supercharging 
  + Tesla referral code - https://ts.la/richard11209

isn't there a rule of thumb about MOTs that if you have to ask you probably shouldn't be messing with MOTs

I have a confession to make, I don't think I've ever thought messing with MOTs would be a productive use of my time. sometimes I feel like those things are catnip to electronics dudes I don't get it.

The easy way is find a pair of the same part. Rewind the secondaries and connect the primaries in series. No saturation problems.

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** Agreed.

** The secondaries need to be paired too, preferably in parallel.

Ensures the primaries split the applied AC voltage evenly.

..... Phil

I am thinking of a similar thing.

An unloaded transformer is almost entirely inductive, so the PF will be very low. As the load increases, the PF will also increase. This is normal. All transformer are like that.

One thing to look out for with MOTs, is that they are run very "hard", i.e. they magnetize the core well into saturation. That way, the manufacturers can save on iron, which saves on weight, which saves on cost. Since the MOT is normally run at fairly low duty cycle, and with fan cooling, they can get away with that. You may find that the transformer with zero load will draw 100 W or more of real power. If you leave it running for some amount of time, it will get very hot. The remedy for this is to add some additional turns to the primary winding. This will, of course, require you to add some additional secondary turns to reach the output voltage you want.

Intuitively, you might think that adding more primary turns would increase the magnetization, therefore worsen the saturation situation. The reason it does not, is that the added windings increase the inductance of the primary winding, which results in a reduction in magnetizing current. The current reduces relatively more than the increase in turns.

--
RoRo

The point of this statement is to express that they are dangerous. And they definitely are.

But I would not take the statement too literally. How are you going to learn anything about anything if you're not allowed to ask? You end up in a "catch-22" situation.

I think it's about cost. A transformer is attractive to a hobbyist, but buying one with a few hundred VA will cost a pretty penny for a hobbyist.

I have three pieces of advice for those wanting to play with MOTs:

1. Leave the magnetron alone. The magnet is not even very strong.

The magnetron in a microwave oven has a toroidal magnet in it, which might look like fun to play with. The problem as that some magnetrons contain a ceramic-like insulator that is very toxic. It is safe if left intact, but if you crack it, the toxins will go flying. When trying to get to the magnet, it is very easy to accidentally crack that insulator.

1. Watch out for the capacitor.

Inside the microwave oven, there is a capacitor that will charge to a couple kV. Use insulated snips to cut the wires, one at a time. Cut each wire twice, with as much distance as possible between the cuts. That way, you reduce the risk of accidental reconnection. Then, if possible, use an insulated screwdriver to short the capacitor.

1. Do not apply mains power to the transformer until you have removed the secondary winding.

The secondary winding otputs a few kV at practically unlimited power. It is extremely dangerous.

--
RoRo

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** If the same tranny is undergoing core saturation, the current wave is highly distorted and maxima tend to co-incide with zero crossings - exacerbating the poor PF.

** Magnetisation goes up and down with the number of primary turns and applied voltage while inductance follows the square of the number of turns.

..... Phil