On Thu, 18 Mar 2004 16:51:52 -0800, Jim Stewart
wrote:

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.

Based on my limited experience, it's based more on
the stored energy in the drivetrain and the braking
ability of the VFD. The VFD has to eat the energy
that it takes to stop the drivetrain so a little
VFD and a massive, fast drivetrain takes a long time
to stop without the VFD faulting.

Thats what braking resistors are for, to dump that hysterisis current
into. A standard Hardinge lathe, with a 5 hp motor added, running an
8" chuck, will go from 3000 rpm to 3000 rpm in about 3 seconds with a
good 5hp vfd and proper braking resistor.

Just be damned sure you have a camlock chuck ...when the threated
types unscrew at those speeds..it can get realllllllly interesting.

Gunner

"As physicists now know, there is some nonzero probability that any object will,
through quantum effects, tunnel from the workbench in your shop to Floyds Knobs,
Indiana (unless your shop is already in Indiana, in which case the object will
tunnel to Trotters, North Dakota).
The smaller mass of the object, the higher the probability.
Therefore, disassembled parts, particularly small ones,
of machines disappear much faster than assembled machines."
Greg Dermer: rec.crafts.metalworking