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Defeating Micro Wave transformer saturation

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Mosaic

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Due to MOT transformer unloaded saturation significant current is wasted as heat if using the MOT to make a power supply.

My Premise is: If I place a full wave bridge rectifier before the Primary the 120Hz pulsed DC should limit the magnetic saturation. This should then reduce power lost to heat. Power lost to eddy currents is less of a factor.

Sounds good?
 
As Mosaic noted, putting a pulsed DC through the transformer will tend to saturate it, leading to high magnetizing currents and transformer failure.
 
I found that out the hard way too.
Volt seconds are the key factor here, and theres no substitute for turns.
You could put 2 trannys in series, they say you need to worry about physical positioning when you do that, but I dont think its critical, the tranny has a closed magnetic circuit.
 
Cruts, would placing a small air gap by separating the E and I laminations do the job of halting saturation under no load?
 
A air gap does help saturation. The inductance will drop and the no load current will go up.
 
Cruts, would placing a small air gap by separating the E and I laminations do the job of halting saturation under no load?
That's not effective. The way to reduce saturation losses is to add more primary windings.
 
Due to MOT transformer unloaded saturation significant current is wasted as heat if using the MOT to make a power supply.

My Premise is: If I place a full wave bridge rectifier before the Primary the 120Hz pulsed DC should limit the magnetic saturation. This should then reduce power lost to heat. Power lost to eddy currents is less of a factor.

Sounds good?

What is the grid voltage where you are?

What kind of power supply are you planning? The output voltage of the typical MOT is rather high; is that what you want, a high voltage supply?

Also, a MOT is often capable of around 1 kilowatt; is that the power level you want?

When you ask a question on the forum, the more information you give about what you're doing, the more relevant the replies you get.
 
Due to MOT transformer unloaded saturation significant current is wasted as heat if using the MOT to make a power supply.

My Premise is: If I place a full wave bridge rectifier before the Primary the 120Hz pulsed DC should limit the magnetic saturation. This should then reduce power lost to heat. Power lost to eddy currents is less of a factor.

Sounds good?


Hi there,


That's an interesting idea. To place a full wave rectifier in front of the transformer so it gets driven by some level of 2nd harmonic which would be twice the frequency which would reduce the required number of turns for a given voltage.

There's a problem with that however, and it is a big problem. The problem is that the output of the rectifier has a large DC component as well as the desired 2nd harmonic. That's definitely a no go. An air gap is sometimes introduced to stretch the BH curve and thus make it less susceptible to saturation due to some DC current, but in this case the DC current would be so high it would not be effective enough. The only way around it then would be to use an AC rated capacitor on the primary. The problem then however is just the opposite: the cap would charge up to the DC peak level and then pass very little current. So for that to work there would have to be significant resistive load on the rectifier, which would kill efficiency.
For low level signals this technique actually works, but for a power supply unless it is very low power it's not effective.

For power applications you'd have to increase the number of primary turns or else reduce the primary voltage. The formula is:
Bmax=E*1e8/(4.44*F*A*N)

where
E is the rms voltage,
F is the frequency,
A is the core cross sectional area in square centimeters,
N is the number of turns.

From that you can see that to reduce Bmax you can either:
1. Reduce E
2. Increase N
3. Increase A
4. Increase F

Those are your choices.


Perhaps you can specify what exactly you are trying to do here. It sounds like you want to use the transformer for some regular power supply yet reduce the heating of the core. Reducing E a small amount can be done by using some series resistance, although the efficiency goes down of course.
 
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I think mr mosaic wants to build a welder, might be wrong.

I'm with al on the DC issue, I've seen power transformers burn out due to uneven firing characteristics of a thyristor circuit, full wave rectified sine wave will produce much larger amounts of dc.

Valve radio output transformers have to pass dc bias current for the output stage, this is done by either a massive core (A), or an airgap in the magnetic field, both increasing the saturation level of the device, either of these complicates matters on a high power tranformer esp at 50 cycles.
 
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