In article ,
JMartin957 wrote:
I think that I've got a pretty good handle on the relative advantages of
static converters, rotary converters and VFDs for operating three phase motors
on
single phase current. With the VFDs coming down in price and their ability to
control motor speed and provide a bunch of other programmable features they
are pretty attractive. The VFD manufacturers warn of two potential problems,
though:
1. They recommend that the VFD be the last thing between the power supply
and the machine - no on/off or reversing switches between the VFD and the
motor.
They warn that use of such a switch may smoke the VFD.
Although it's probably more applicable to metalworking than woodworking
machines, there are times where I'd like to be able to instantly reverse a
motor,
or to bump it on and off.
As has already been mentioned, the reversing can be done by
commanding the VFD to do that. Three (small gauge) wires from the VFD
to the remote box will do the forward/reverse/stop switching (and can
even be wired to do it using the switch on the machine itself, for a
more natural feel of operation). Right next to that, you would put the
potentiometer, with three more wires, to allow tuning the speed.
Some VFDs even have a "jog" wiring option, so you can move it
just a little from another pushbutton. (Probably only one more wire,
taking you up to seven wires now.)
Ideally -- the wires should be in a shielded jacket to minimize
electrical noise pickup by the wires. It does not need to be
particularly strong cable -- though you will want something which can
survive oil, water-soluble oils, and being trod upon, so a good tough
rubber jacket will help.
I've heard that if you use a VFD that is well oversized for the motor in
question, you can get away with the on/off and reversing switches downstream
from
the VFD. Anyone have any experience with this?
Yes -- but it is a case of a vastly oversized VFD -- the VFD I
have handles 30 A, which is 7.5 HP when run from single phase, more from
three phase power), and the load motor is a 1 HP one. I would probably
not consider anything closer to the VFD's rating than a factor of 1:3
(e.g. a 3 HP VFD for a 1 HP motor). I intend to re-wire this machine so
the VFD is commanded from the switches on the machines.
2. The VFDs - or at least some of them - are reputed not to produce a smooth
output current, but rather one with harmonics or noise that can fry some
older motors. Almost all newer motors are supposed to have insulation that is
rated for inverter use.
How old does a motor have to be before it is cause for worry? How reliable
are older motors when used with VFDs? I'm talking about normal use, and not
using the VFD to run the motor at 30% or 300% of its rated speed.
Well ... the motor which I'm plug reversing from the 7.5 HP VFD
is quite old -- it is on an early Nichols horizontal mill -- one made
shortly after they bought the line from Whitney. I really don't know
the age, but I would expect it to be at least from the late 1930s or
early 1940s, if not older. It is not only a very old three-phase motor,
but it also has built-in gearing to provide more torque at lower RPM.
I posted this message a while back on the OWWM group and got one relevant
response.
"OWWM"? Some form of woodworking mailing list I would guess,
since the naming would not fit a usenet newsgroup. But whatever it is,
it is one with which I am not familiar.
Responder said he had no problems with plug reversing or with old
motors used with VFDs. Anyone have any other experience?
I find the manuals for my Mitsubishi VFDs saying that I should
not switch between the VFD and the motor, and I have read posted reports
of failures induced by switching the motor, but I have no direct
experience with failures -- just the lack of failures with the vast
mismatch in power levels present with the 7.5 HP VFD and the 1 HP motor.
I don't plan to switch the motor leads on the Clausing lathe or
the Bridgeport mill, because I will be using VFDs with only about a 50%
safety factor. However -- I *do* plan on commanding the VFD to reverse
the motor as it was designed to do.
Good Luck,
DoN.
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