On 2012-10-27, wrote:
On 27 Oct 2012 18:30:16 GMT, "DoN. Nichols"
wrote:

[ ... ]

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.

Greetings DoN,
The machine control does indeed need 3 phase power. The Fanuc control
uses a 3 phase xmfr to condition the voltage for the control circuits
and of course that power is rectified but it looks like the servo amps
use 3 phase directly.

I know that the Bridgeport BOSS-3 (and at least through BOSS-6)
used three phase to power the steppers -- but used a single phase for
each stepper. A big three phase transformer to provide the voltages for
the stepper drivers, with a "mag amp" (saturable reactor) to reduce the
voltage when the stepper was stopped or moving slowly, to avoid the
motor overheating. (The idea of a constant current supply to the
stepper had either not yet occurred (1975), or it was too complex for
affordable electronics at the time.) But, if you had enough current
available from single a phase, and replaced that three phase transformer
with one large single phase one, or three smaller ones (depending on
what was available) you could happily run that from single phase. Then
you would only need three phase for the spindle motor, and today a VFD
is the way of choice for driving a single motor.

BTW Running that from a rotary converter increases the chances of
blowing the transistors used to drive the stepper. They were
2N3055 transistors, nominally rated for 60 or 80 VDC. The ones
which I pulled from the old electronics on my Bridgeport test
out on a curve tracer to something on the order of 120 to 180
VCE max. I suspect that they were selected for the higher
breakdown voltage. Anyway -- with the nominal 80V being applied
at full speed to the steppers, and with an unbalance from a
rotary converter, it would be very easy to over-voltage the
transistors at their normal rating. (Of course, I pulled the
stepper motors and replaced them with DC servo motors, and was
pretty close to being ready to add the computer and LinuxCNC to
put it back in service when the lubricator set it all on fire,
keeping me out of the shop until it is cleaned up (a liter of
Vactra No. 2 makes a *lot* of dark sooty smoke, and a *lot* of
heat, too. :-(

Of course somewhere in the amp the ac gets
rectified.

Of course -- and once it is DC -- who cares how many phases were
used to produce it. :-)

The spindle drive will only accept 3 phase power.

The VFD? Others are available, of course. It sounds like you
have a particularly large one, however, so a VFD which will accept
single phase would be rather expensive, unless you were really lucky. I
have (or perhaps had? I haven't been able to test it, and the case is
partially melted) one which was good for 30A input and output at 240
VAC, and it would quite happily run from single phase. This comes up to
just short of 10 HP using single phase ratings. A bit over 15 HP with
three phase power input, I believe. It was all of $100.00 at a hamfest
in the mid 1990s IIRC.

There is
no braking resistor option either. Looking at the manual it says that
the power is dumped back into the 3 phase source.

One which is nice in terms of the power bill (how many HP *is*
that spindle motor?), but which complicates things with a rotary
converter, as others have mentioned. No real bets on what will work or
not.

If I do use 3 buck
xmfrs to lower the voltage will the xmfrs themselves be able to absorb
enough energy to be useful in that regard?

They will try to dump it back into the rotary converter.

Or will their only
advantage be the lower voltage? The xmfrs that Paul Drahn offered to
me will drop the voltage 20 volts. This is fine according to the
manual. When set at the 230 volt setting the drive will operate all
the way down to 198 volts.

That sounds worth trying -- especially at the price.
(Depends on how far you have to drive to get them, of course. :-)

Did he have a current rating on the secondary?

You are getting your three phase from a rotary converter. Is it
wired to give you a Wye "Y" neutral point, or is it delta? I think that
for the buck-boost, you really want Wye to simplify the setup. Delta
would need isolation transformer in each phase as well. If you
connected the primarys to a delta source, and hooked the secondarys
between the corners of the delta and the load you would be applying
correction voltages at a weird phase relationship to the voltages at the
corners. But if you have your rotary converter using a Wye connection
in the idler motor, you should be fine.

Good Luck,
DoN.

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