[Amps] Microwave Oven Power Transformer

[Manfred Mornhinweg]

Hi Steve, Carl, and all,

> Normally, 10 or 15% reduction in volts is enough to bring the 
> magnetising current down,

Not really, with MOTs. It would still have an excessive magnetizing current!

I just took a 1400 watt MOT I happen to have in my junkbox, and ran it 
through some engineering.

To use the windings as they are, the most convenient flux density for 
typical ham amplifier use is reached almost exactly when applying half 
the rated voltage. So the suggestion of using two identical MOTs in 
series isn't a bad one.

Operating under those conditions, the true CCS ratings without forced 
air cooling would be 241 watts input, 91% efficiency, 6.4% voltage drop 
at full load. The output voltage would of course also be half the rated one.

Rating this same transformer for ICAS, of course the power rating would 
go up, with both the efficiency and voltage drop getting worse. I don't 
know the specifications for ICAS, in terms of what percentage of time 
what load must be assumed. I think I remember that some old Handbook 
said that you can take "safely" 40% more current in ICAS. If so, that 
would equate to an input power of 337W, 9% voltage drop, and 89.1% 
efficiency at full load.

>> So if you take a MOT with a 120V primary and series it with an
>> identical MOT and plug it into 120V we have solved the flux
>> problem.... correct?

Yes. And almost optimally. But the power output would improve somewhat 
if we dropped the voltage per transformer a bit less than to one half, 
but that would cost us in idle power consumption, reactive current, and 
of course isn't as convenient to implement.

>> Now the issue is what is the resultant individual secondary
>> voltages,

One half the original ones.

> how are we going to connect those windings, 

Depends on the desired voltage! For a typical ham amp, you would connect 
two secondaries in series. And then you can go on and add more 
transformer series pairs in parallel, until reaching the desired power 
level. If you happen to have six to ten identical MOTs lying around, 
that would indeed be a quite decent way to make an inexpensive HV power 
supply for a legal limit amp.

 >> and what rectifier circuit do we want?

Full bridge.

>> when something is free it begs a solution.

True. That's ham spirit!

Manfred

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