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On 4/11/15 11:53 PM, Teslalabor wrote:
Hi Ted,
have you ever checked / measured the voltage drop of the MOT's under
load conditions? Could be a problem because of their strayfield properties.
In my 12kVA DC system I use a heavy duty 3-phase transformer beast and
even there is a significant voltage drop under load. I'm using an
accurate DC-HV meter on the DC output, so I know exactly what is going
on there :-) DC voltage without load is 13,3kV, which drops to nearly
11kV under load, although I even use a 5µF smoothing cap after the
6-pulse bridge. I expect this to be an even larger problem when using
MOT's...
What you are describing is known as the "regulation" of the DC power
supply. And "poor regulation" (e.g. change in output voltage vs load)
can be from a variety of factors..
in single phase power supplies, the filter has a big effect on
regulation: too small a cap and the ripple voltage gets huge as the load
current increases, making the average output voltage appear to get
lower. IN a three phase, 6 pulse, less so: with no capacitor at all the
ripple is about 6-7% , unless one or more diodes has failed.
But there's also the series impedance issue. Some, from leakage
inductance (stray fields) in the transformer (appears as a series L);
some, from plain old IR losses in the windings, and some from losses in
the rectifier (HV rectifiers often have high forward resistance).
There are charts out there that will tell you things like "decrease in
average voltage for load current" as a function of series R(or L) and
the size of the filter capacitor.
Or, these days, you can model it in SPICE pretty easily.
North (see the book reference below) suggests that when modeling
polyphase rectifiers you need to be aware that the three phases won't
necessarily have the same voltage on them. He also describes the issues
that result from the fact that the current (particularly into a
capacitive input filter) is non-sinusoidal, which makes coming up with
an analytical "voltage vs current" equation very difficult.
which has a lot of this kind of information in it, since making high
power microwaves is usually one big tube with a gigantic power supply
and a pulser. There's also chapters on safety, interlocks, etc. You
probably won't need to worry about the stuff on proper configuration of
the substation feeding your transmitter.
The LANL link comes and goes and moves around (after 9/11, it
disappeared for several years while they re-reviewed all the documents
in LANL's "library without walls")