In article ,
Rick Cox wrote:
I hope to bring and old Rockwell lathe back to life this weekend.
The lathe is a Rockwell Delta 46-450 1HP 3phase monster with the Reeves
speed control on it.
First question: What is the service factor of the motor (labelled SF
on the label)? If you have a 1HP motor with a SF of 1.3, you in
reality have a motor that can run at 1.3 HP for a short time. If your
VFD has only enough oomph to run a 1.0HP motor (the real oomph of the
VFD is expressed in Amps, not in HP), then the VFD will be overloaded.
Which is OK - most VFDs handle overload very gracefully, and have
internal safety circuit that will shut down cleanly. In a tool, this
is easy to deal with, because you can control the load by controlling
feed pressure etc.
We had a 1HP water pump converted from single phase to 3 phase. The
pump shop insisted that they have to put a 1.5HP motor on, because the
old single-phase motor had a service factor of 1.35, and a 1HP 3-phase
motor with a serivec factor of 1.0 would not be able to spin the pump
at full speed, and would overload. It then turned out that a 1HP VFD
was marginally able to power the 1.5HP motor, but only at a line
voltage of 240V (at which voltage the motor uses fewer amps); when
running on the emergency generator, at a line voltage of 220V, the
motor draws a few more amps, and the VFD went into overload shutdown.
I replaced the VFD with a 2HP model, and everything runs excellent.
I got a used Toshiba VFD on ebay for $80.00 and a great manual with wiring
diagrams.
Warning: not all VFDs are able to run on a single-phase input, but
some are. Look at the spec or manual for it. Some VFDs absolutely
require a 3-phase input.
My father has the JET 1642 and I noticed that it also
has a brake resistor. Would this be a good thing to get before I try the
VFD, or is it all that necessary?
Read the manual for the VFD. The brake resistor is needed if you
program the VFD for rapid braking - the energy stored in the motor and
in the mechanically moving parts has to be dumped somewhere. If you
program your VFD for coasting to a stop, or for a very gentle rampdown
without braking, the resistor will not be needed. You might want
ultra-rapid braking for an emergency stop, though.
I saw a "Hint" in the manual that you might want to set the Maximum
Frequency for an older non VFD designed "General Purpose" motor to max out
at 80hz. Has anyone done this?
It's not just about the motor. If you have a system that's designed
for 3600 RPM at 60Hz, and you run it at 80Hz, everything will spin at
4800 RPM. There may be parts of the drive that don't survive this
(whether immediately or long-term is an interesting question). Also,
to spin all that stuff at a much higher RPM might require much more
power from the motor, and you might overload it and fry it. If you
forget to program your VFD to honor the maximum current from the motor
nameplate, you might burn out the motor.
Another warning: Some old motors don't like running at low frequencies
either. For a motor to deliver high torque at low RPM on a VFD, it
has to be built quite differently: more cooling, better voltage
insulation in the coils to handle the foltage spikes from the
high-frequency drive of the VFD, and so on. It seems to be pretty
safe to take any old 3-phase motor, and run it at 60Hz, maybe with a
few seconds of rampup and rampdown. Whether it is afe to a run an
older (non-VFD-rated) motor at underfrequency is a topic of a lot of
discussion.
One HUGE advantage of 3-phase motors on VFDs: you can program the VFD
to ramp the motor up and down slowly (over a 5-second period). This
gets rid both of the electrical startup surge of the motor, and all
the mechanical stress of the motor jerking on or off.
To start out, I plan on using the 0-60hz and control the vfd from its own
control panel, how ever, i would like eventually use a remote potentiometer
and run stop switch.
Run/stop switch is easy, as most VFDs have inputs for those. I would
go with a mushroom-style emergency off. Note that by using a VFD, you
also get the functionality of a magnetic starter: You can program the
VFD to not restart after a power interruption.
A remote potentiometer is a little harder, as the good ones (with big
dials) are hard to find, and not all VFDs have an analog input. I
want to do the same thing to my drill press: I'll replace the existing
single-phase motor with a 3-phase motor, and replace all the control
wiring.
Last warning: The high-frequency output of a VFD is considerably more
dangerous (in the sense of electrical shock) than regular 60Hz. Make
sure the frame of the motor is really well grounded. Being the
paranoid type of person myself, I install VFDs in metal enclosures,
and use non-flexible metal conduit to connect the VFD enclosure to the
motor itself, making sure everything is really well grounded. Better
safe than sorry (or dead).
If everything goes ok I will have a great heavy lathe,
If not....I have to get rid of a heavy lathe.....
Unless you really screw up, you will at least have a heavy lathe with
a fixed-speed motor, and gentle rampup/rampdown.
--
The address in the header is invalid for obvious reasons. Please
reconstruct the address from the information below (look for _).
Ralph Becker-Szendy