Re: Adequate torque from VFD driven motors?

On VFD's , that is a topic I have been most interested in being a
controls engineer forsimulation for some time. While not an industry
expert on machine tools I am learning ( I hope).

I have been looking at numerous retro fits and controllers both for
axis controls and spindle drive. Generally speaking you are correct
it is impossible match the torque gain achieved by gearing without
added capacity. In general most users who have converted from a DC
drive to an AC drive will comment that the very low speed torque is
not there like it once was. Almost all the machines where back gear
was removed with the update to a direct drive have people ticked off
who do the slow heavy work. There are several sitting in the shop I
was just at that they are considering putting a DC drive back on or
gearing.

The Modern vector drives are very good, however it is hard to
generate induction motor torque over a broad RPM range, It is much
easier to generate low end torque from a DC type drive due to the
simplified control required. For a vector drive to be successful the
entire package has to be integrated very tightly. In general .

The motor should be made for VFD use, ideally as a set with the
controller. While a regular motor will work it's dynamic range will
not be as wide. The custom motors have materials and design for wider
frequency operation where the standard units are optimized for their
specific frequency and fall off faster outside this range.

The control and motor should be sized generously. To have the low
end reserve capacity the motor and controller need much higher peak
current capability than a normal run loading. The drive set needs to
be spec'ed out at the low end of normal operation. You will likely
find a significant move up in rating is needed to maintain
performance without the gearing on large work. Also most motor and
control combinations will handle stretching the range to the high end
easier than slowing down. Motors with a lower nominal speed rating
will tend to retain torque at the low Rpm's since you are not as far
off nominal frequency as with the higher RPM motor.

Closed loop control with feedback is required in my opinion for a
wide operating range.The open ended controls do work but I have yet
to see one maintain torque and speed well over a wide operating
range. If spindle tolerance is important find a closed loop
control it will work much better.

If at all possible keep the back gear. If not possible go back to
the spec's again and run the numbers. All the folks I have run into
who use the low end capability and are regretting the upgrade are
undersized on power. I have seen several folks fall for the
marketing campaign but not do the math. If your largest job is
pulling 2H.P. with a 5:1 ratio there is a chance even a 5H.P. direct
drive will not do it. The key here is to ensure the motor/drive
combination maintains enough rating with the direct drive gearing to
pull the low end torque required.

For the small guys again my opinion but don't fall into the trap of
an upgrade just to replace the electronics. Many of the refit folks
insist on pulling the drive to replace it with a "modern" AC drive.
There is nothing wrong with a good DC drive especially for the small
shop. They are fairly simple , provide great power , can be updated
with modern controls easily and run great on single phase power. We
just repaired a Monarch 10EE , it runs great on 220V with a 5H.P. DC
drive , so far I have not seen a refit of this machine that will
run on 220V single phase and pull near the torque this one does down
to a dead crawl at a few RPM. The down side is there is some periodic
motor maintenance required , I can replace brushes and repair power
supply's but have you ever tried to fix a vector drive?

In general it's just a matter of detail. The VFD's work well but
there is no substitute for torque or power. Most of the folks I have
seen who complain are simply geared too high or just plain sized too
small. There are probably some real sharp folks out here who can
provide the math for your requirements. If low end performance is
important start the design there , slower motor, higher peak current,
lower gearing etc.. normally the high speed will fall in place .

Brian
BSP