[Amps] Microwave Oven Power Transformer

[Steve Thompson]

I can't pretend my comment came from such detailed understanding 
or analysis - I just wound the primary volts down until the (no 
load) primary current dropped noticeably and the core didn't fry 
without fan cooling. They might have been wound for the days when 
our mains was 245-250V, now it's typically 230V at my workshop so 
the 200ishV I was applying could have been a larger reduction from 
nominal. I don't think I explored how efficiency as a complete PSU 
varied with primary volts.

One of the joys of free materials is that everyone can do their 
own experiments and learn something along the way.

Steve

> 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.