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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