Brian,

If you are planning to use a VFD to start your rotary converter then
reversing is not an issue. Rotarys always run in the same direction, i.e.,
that direction in which they are started. IMO there would be very little
effect "felt" back at the VFD whenever a load machine connected to the
rotary phase converter is reversed. Sorry, but I have no expertise with
VFDs. I can only imagine a VFD could be a way to start your 25 HP idler ---
however, it would seem to be an expensive proposition to use a VFD in that
fashion. A VFD rated for 25 HP would be adequate to directly operate the
load machine(s), I would think. IMO, a pony drive is your best bet. Get
some local electrical "talent" to help with the pony arrangement.

Bob Swinney

"Brian Lawson" wrote in message
...
Hey Bob,

Yep. My thought was that the VFD would drive the rotary converter,
and the question was whether doing an "instant reverse" of lathe or
mill would give the VFD an undesirable spike. Seems to me I had heard
that somewhere here on RCM some time back. I either have to come up
with a "pony drive" or use a VFD to get the 25HP idler cranked up to
speed. I dunno. Maybe a VFD to ramp it up, then switch the VFD out
of the circuit??

My first choice would have been for a maybe 10 HP motor, but elevators
seem to jump that power range. The 25 was close, available, and
shipped here, all free! so I guess that's what I'll use. Hmmm..
I've got a "soft-start" system. I wonder if that will function like a
VFD. I know I'd have to do some cheating because it has a built in
reverse/low phase sensor.

Take care.

Brian Lawson,
Bothwell, Ontario,.
XXXXXXXXXXXXXXX
On Thu, 18 Mar 2004 18:34:08 -0600, "Bob Swinney"
wrote:

Brian,
The "speed" of reversing a 3-phase motor would be a function of how stiff
the 3-phase source is. A genuine 3-phase line being the quickest. I
have
done this and it is hard to tell when stop-then- reverse occurs. It is
about as close as you can get to instantaneous. Following, I would
guess,
is a good stiff, balanced, rotary phase converter such as the 25 HP one
you
propose. Though I've not tried reversing with a VFD, I believe the
smaller
the power rating of the VFD, the longer it would take to reverse a
3-phase
motor.

Bob Swinney
"Peter T. Keillor III" wrote in message
.. .
On Thu, 18 Mar 2004 07:51:58 -0500, Brian Lawson
wrote:

On Thu, 18 Mar 2004 06:55:28 -0500, Peter T. Keillor III
wrote:

On Tue, 16 Mar 2004 15:57:06 -0800, "Paul"
wrote:

SNIP

I like vfd's, but wouldn't argue with the guys building converters
from low cost or free motors. But for that price, I would have gone
for a vfd.

Pete Keillor


Hey peter,

I thought there was at least some controversy about being able to do
"instant" reversals with a VFD attached. Any comments from anybody?

This comment/question from somebody who hopes to be starting a 25HP as
a rotary convertor some day pretty soon.

Take care.

Brian Lawson,
Bothwell, Ontario.

Brian, I don't know about instant, but you can do 'em pretty quick.
It depends on how you program the accel and decel times. I don't need
fast, so I set mine for 10 seconds. When I flip from forward to
reverse, the motor decelerates to a stop, then instantly changes
direction and accelerates in reverse.

Update: I just went in the shop and programmed 1 sec. accel - decel
times. It changes direction damn quick.

Pete Keillor