Thanks for the suggestions. I have a couple of those transformers
but not big enough... I am currently using a 1 kva transformer
with taps 210 through 250 vac and a nominal 18 volt secondary at
60 amps. Although our line voltage (unloaded) is closer to 250,
with the 210 tap connected I have 28.5 dc with no load. I figured
that 50-60 amps DC would be enough at least for a half-load test,
but as you see, I am having problems.
I hate to be negative, but I see a few issues with your ideas:
Even a pair of 1 kva transformers is only about 60% the size it
needs to be if I want to load this inverter to its capacity, which
requires 115 amps at 28 dc. The math is left as an exercise :)
Another problem with your doubler suggestion (besides the price
of BFC's in that size!) is that 30-40% of the power would be
dissipated as heat in addition to the 55% full load efficiency of
the inverter itself (ok in the winter, not so much in summer), and
would have to be drawn from the line. To get 3 KW dc I'd need a 30
amp 240 volt outlet, and more iron than two 1 kva cores too.
As I'm discovering, making dc supplies in the kw range, even
linear ones, is not a trivial exercise.
...
Meanwhile, the plot thickens!
I shortened the 8-ga wires (from the transformer to the bridge)
from 2 feet to one foot each, which helped a little.
I also discovered why there wasn't any 400 Hz output. The transfer
relay (24-28v coil) was pulling in at first, but with the inrush
(I measured 220 amps ACat the transformer secondary, with an
Amprobe clamp meter) pulling down the dc, it would drop out, and
with the voltage not coming back up enough to pull it in, the
contacts were open. Sigh.
So, actuating the relays via the Switch On terminal from an
external power supply, then I had 400 Hz on both A and C phases.
My Triplett 630-A showed 185 volts ac when it stopped rising. At
first I thought it may actually be 200 (as in 115/200 three phase)
by the time I get it spinning at full speed with 24 volts. Or
maybe the regulators aren't regulating when unloaded, since A is
allegedly 115 volt single phase, not 200... but that's for another
day once I have enough power supply.
But even unloaded, the input current never dropped below 110 amps
AC (again, measured at the transformer secondary) although I only
had 16 volts on a panel meter at the inverter terminals, and with
lots of ripple.
Will the motor current drop when the voltage comes up and it spins
faster? I don't remember all the details of compound-wound DC
motors...