On 2012-10-27, wrote:
Greetings all you AC Electrical Types,
My shop is supplied with single phase juice (notice my proper use of
technical terms) at 245 volts average. The voltage does vary a little,
usually about 2 volts either way, but sometimes as high as 250 volts.
I've never seen it drop below 240, but then I don't monitor it 24/7. I
have just checked it many times over the years. I'm in a rural area
near the beginning of a line that serves many homes so maybe that has
something to do with it. Anyway, I use a rotary phase converter to
supply 3 phase power to my CNC machines. Setting the xmfr taps in the
machines allows me to supply the machines with power that is within
specs. Except for the Miyano lathe. It has xmfr taps for the Fanuc
control but not for the VFD spindle drive. The Fuji spindle drive has
a switch that allows for either 200 VAC or 220/230 VAC operation. So I
am supplying the spindle drive with voltage that is too high. And the
problem with that is when the spindle decelerates the spindle load
meter goes above 120%, the spindle drive shuts down, and sets an
"overvoltage" alarm.

Hmm ... this sounds like it is missing a braking resistor.
Perhaps it has one and it has fried/opened?

But it is possible that starting from the higher line voltage,
it might be more likely to trip the limits. (And, BTW, have you checked
how well balanced your rotary converter is? It is common for one leg to
be significantly higher than the others, and this can be helped
(somewhat) with tuning capacitors -- though since you are driving
multiple machines with it, it makes the balance just right for *one*
machine, and perhaps worse for others.

The drive uses regenerative braking and the
power supply must be capable of supplying 22kVa, according to this
link:
http://www.fujielectric.com/company/...3-108-1985.pdf

Anyway, after such a long winded description I'm wondering if I could
use 3 xmfrs buck configured to supply power to just the spindle drive.
I don't know what effect, if any, the regenerative braking would have
on any xmfrs connected between the rotary phase converter and the
spindle drive. If I can use buck xmfrs I need to know what xmfrs I can
use and how to wire them. Thanks for reading this far.

It might be easier to use a single buck transformer on the
single phase 240 VAC into the VFD. But your 22 KVA says that it will
need to handle something like 91 A (so 100 A for a round number), which
is a massive transformer. Hmm ... voltage needed to drop your peak 250
V to 240 V for a reasonable maximum would be 10 V (maybe 12 V would be
easier to find), which would be about a 1.2 KVA transformer -- not quite
as bad as it could be. Your VFD might need to be a bit bigger to
provide sufficient spindle output power from single phase (and check
that it will *run* from single phase). Anyway, you would need the power
to be Wye, not Delta format -- at least on the output side of a big
transformer to allow the buck-boost to work cleanly.

BTW It is likely that the rest of the machine (separate from the
VFD and spindle motor) really only needs single phase, which it
derives from one phase of the three phase input, so you likely
don't need the the rotary converter at all for that machine.

Good Luck,
DoN.

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