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# 1400w Transformer at 1/2 voltage?

#### gary350

##### Well-Known Member
I know a MOT = microwave oven transformer is NOT rated 100% duty cycle at 120 vac. If I use 60vac or less can a MOT run for 7 days and not over heat? I know the MOT has shunts. I have a 1500w variac.

I need 2.1 vac on the MOT secondary winding. I need 3 vdc 50a output for about 7 days. Rectifier is 100a with a heat sink.

50a x 3v = 150w

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The limitation when the winding ratio and voltages are optimized Pmax is a PF = 0.5 or 50% of the VA rating of the welded steel laminates reach perhaps 80'C max. which will reduce the permeability and input impedance and raise the excitation no load current of perhaps 10% of the VA rating to create a reasonably high M coupling ratio.

Since saturation is determined by primary voltage, you may have to ramp up until the load is triggered.

Implement thermal cutoffs or fuses to prevent overheating and potential damage to the transformer and surrounding components.

Several years ago I did the math, MOT primary windings are 100 turns 1200 watts. 200 turns are needed to be 100% duty cycle at 1200w. I was hoping someone would know the answer without me doing the math again. I seem logical 60v 600w would make a MOT be 100% duty cycle. I only need 150w x 2.1vac=315w. I need to test to see what no load current is for the primary winding. 100 turns will make the primary have a high idle current.

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I need 2.1 vac on the MOT secondary winding. I need 3 vdc 50a output for about 7 days. Rectifier is 100a with a heat sink.

50a x 3v = 150w View attachment 146173
Have you actually replaced the secondary winding to provide 2.1VAC with a 60VAC input?

Do you realize that this AC output level will not produce 3 VDC?

Have you actually replaced the secondary winding to provide 2.1VAC with a 60VAC input?

Do you realize that this AC output level will not produce 3 VDC?
The MOT has a 3 turn secondary with 120v on the primary I get 1.68vac on the secondary. The variac can lower 1.68v to .84v and .42v. A 4t secondary should do about 2v with variac at 60vac A 8t secondary should produce about 2v with variac at 30vac. After I attach the bridge rectifier and cap the variac should dial in exactly 3vdc. Soon as I cut the secondary off in the milling machine I can test 8t at 30v.

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A 8t secondary should produce about 2v with variac at 30vac. After I attach the bridge rectifier and cap the variac should dial in exactly 3vdc.

View attachment 146182
And the two forward diode voltage drops don’t count?

And the two forward diode voltage drops don’t count?
That is what the variac is for I can dial in exactly 3vdc. 3v is not critical but it needs to run about 7 days = 168 hours.

For a low DC output voltage, a center tapped secondary with two diodes will be more efficient. This will make half as much heat as a full bridge rectifier. And, since you're winding a new secondary anyway....

Also, at 50 amps output, your 500uF cap will be about worthless.

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For a low DC output voltage, a center tapped secondary with two diodes will be more efficient. This will make half as much heat at the rectifier. And, since you're winding a new secondary anyway....

Also, at 50 amps output, your 500uF cap will be about worthless.
That is good to know. What do I need to make better DC.? I need to check see if 16 turns of wire with a center tap will fit in the space available. 8 turns of 50 amp wire might not be easy. I will need a heat sink for the 100a bridge rectifier that I ordered.

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That is good to know. What do I need to make better DC.? I need to check see if 16 turns of wire with a center tap will fit in the space available. 8 turns of 50 amp wire might not be easy. I will need a heat sink for the 100a bridge rectifier that I ordered.

As a 'rule of thumb' about 1000uF per amp, so 50,000uF - assuming you need smooth(ish) DC?.

As a 'rule of thumb' about 1000uF per amp, so 50,000uF - assuming you need smooth(ish) DC?.
I can do that I have several large caps. Smooth DC will be better than pulsing DC.

HV secondary winding replaced with 9 turns that read about 2vac on the meter. Do shunts need to be removed?

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I removed the shunts 9 turn secondary is now 9.36v. 2 turns is 2.08v. Now it works like a transformer should work.

I made a 3 turn 60a secondary winding voltage is too high so I changed it to 2½ turns secondary is 3.05 vdc. Perfect. Primary is 120vac the transformer has warmed up 10 degrees after a few minutes of testing. Next I need a thermo switch on the 120vac primary then let it run for several hours to see how warm it gets. When I finish the circuit and put a 50a load on the transformer it might get hotter. I need to check DC output with a scope. I used these 2 caps because they were right in front of me. I have 100s of caps I might be able to find 50,000. uf for 3 vdc. I have a heat sink fan also it might be needed when the circuit is running 50 amps for 7 days.

Try to use a capacitor whose working voltage is closer to the actual rated voltage.

I once read on a Nichicon app note that the ratio should be at least 10%. In your circuit it is 1%.

Mind you, the capacitors will still work, but its effectiveness may be reduced.
Let me see if I can find the note.

After 30 minutes the MOT transformer is almost too hot to touch. With 120 vac on the primary the amp meter shows almost no amps. With load on secondary meter is 2.80 vdc 2 amps = 5.6 watts. 40 carbon rods in salt water need to have .2 amps on each rod but they don't.

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Hello,

Shunts increases leakage inductance which could help with rectified DC filtering but reduce the coupling. They also help to reduce short circuit current. You can think of this as an extra inductor in series with the primary (or secondary).

Anything you do here would probably have to be tested to be sure what will happen because there are too many variables that we don't know about.

Problem solved 1 of the blue caps was bad. I learned 3.5vdc works but 3vdc does not, a 2kw variac connected to 120vac is working good. Amp meter reads 12a at 3.5v = 42 watts. All wires had to be changed to 15a wire. 40 carbon rod electrodes at the anode. Stainless steel pot is the cathode.

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I am curious, what exactly are you doing in the pot?

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