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.

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?

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.

Thanks,

John Martin

I posted this message a while back on the OWWM group and got one relevant
response. Responder said he had no problems with plug reversing or with old
motors used with VFDs. Anyone have any other experience?

John

In article ,
says...

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.

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?

You can reverse the motor with the VFD's controls, so
there's no need for a reversing switch/contactor between
the VFD and motor. This has the added benefit of being able
to set the accel/decel rate in the VFD. The accel rate in
most VFDs is set as Hz/sec, so even if you set the accel to
bring the motor up to 60Hz in 1 sec, starting or reversing
at low speeds will be nearly instantaneous. This is how my
mill is set up, making it convenient to drill at high
speed, then turn the speed down to power tap. At 6Hz the
time to reverse is not detectable.

For fastest decel and/or large inertial loads, you need a
braking resistor. Most, but not all, small VFDs come with a
built in resistor. Larger drives will have provision to add
an external resistor.


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.

Though it's true in theory and apparently does happen, I've
never had a problem with this either on my own stuff,
including a 1960s Bridgeport, or on the many drives I've
worked on in industrial settings.

You're in Cumberland or Yarmouth, aren't you? I have both a
lathe and mill running on Mutsubishi VFDs here in
Harpswell. Email me if you'd like to come up and see how
they work in person.

Ned Simmons

All the VFD's I have used have forward/reverse inputs as well as jog
inputs available. The jog speed can be set slower or faster than the
running speed. You can get much smoother control than by bumping a
switch.

A VFD manufacturer told me that it was OK to disconnect the load from
his VFD, but only once. If you did it again, there would likely be
smoke.He said disconnecting the load from a VFD under power is
equivalent to shifting your car into neutral while going up a hill.

(JMartin957) wrote in message ...
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.

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?

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.

Thanks,

John Martin

I posted this message a while back on the OWWM group and got one relevant
response. Responder said he had no problems with plug reversing or with old
motors used with VFDs. Anyone have any other experience?

John

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.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

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.

Older VFDs would tend to smoke, newer ones would trip. As others have
indicated, there is no advantage to doing reversal or run/stop this
way. You should definitely wire up signal switches to the control
input terminals and let the VFD do its job.

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.

Reversing can be handled by a single toggle switch wired into the
control input terminals. "Bumping", in VFD parliance, is called
"jogging". Means the same thing. Very quick reverses can be achieved
by changing two parameters in your VFD's configuration.
"Acceleration" controls how quickly the motor will accelerate to reach
a new, higher setpoint (works generally with either starting the drive
from 0, or changing from, say, 50% speed setpoint to 75% speed
setpoint.) Similarly, "Deceleration" controls how quickly the motor
will decelerate from the previous speed setpoint to a new speed
setpoint, possibly 0 if you're stopping the motor. Clearly, a
reversal involves decelerating the motor to 0, and then accelerating
the drive to the same speed setpoint, but with a sign change (meaning
the motor is turning the opposite direction!). If you experiment with
the "Acceleration" and "Deceleration" parameter values, you can
achieve a very quick reversal.

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?

You're headed down the wrong path here. You might be able to get away
with this, but it is easier and cheaper to follow the manufacturer's
suggestions. Wire a little reversing toggle switch, and start / stop
pushbuttons, and a potentiometer for speed control if you like. I
would really truly NOT bother with plug-reversing the VFD's outputs.
You'll have to oversize the drive, and it won't be anywhere nearly as
reliable as simply using control switches.

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.

Older VFDs had a lot of harmonics in the outputs. Probably some new
ones do too: look for a specification on the VFD called "Total
Harmonic Distortion", or "THD". Lower is better.

On the motor itself: "Inverter Duty" or "Inverter Grade" is a
unit-less indication that the designer of the motor intended it to be
run at less than 100% speed and still not heat up enough to destroy
itself. I do not know of an exact specification for these terms.

The nameplate of the motor will almost always contain a reference to
the "Insulation Class" or just "Class". This is a reference to how
hot you're allowed to run the motor before it fries. The classes a
"Class A" max is 105 degrees C
"Class B" max is 130 degrees C
"Class F" max is 155 degrees C
"Class H" max is 180 degrees C.
Now, ambient temperature of the environment around the motor comes
into this too. In a situation where you're just running the motor at
%100 speed, things are pretty straightforward. You take the ambient
temperature, add to it the "motor rise" temperature off the nameplate,
and if that is less-than-or-equal-to the Class temperature, you're
fine and won't burn up the motor (too quickly). The problem is that
"Motor Rise" assumes 100% speed; the motor will run a heck of a lot
hotter if you run it at 50%. How much hotter is generally not on the
nameplate, but it might be in the spec sheet for the motor, or the
motor manufacturer might be willing to tell you.

Now, here's a few comforting facts:

1. Old motors generally have a lot of iron in them. This is good for
cooling.
2. Modern VFDs (last 10 years +) work good; low THD, better
protected, harder to smoke.
3. As long as you don't go with the "reversing plug between the VFD
outputs and the motor" idea, you're not likely to smoke the VFD.
4. If the motor does fry, modern motors are cheap compared to their
respective VFDs.
5. I've run some pretty old motors off vintage 1982 VFDs and so far
haven't blown anything up (obligatory meaningless anecdotal data
point).

Thanks,
John Martin

Yer welcome! Hope it helps.

Dave Doerschuk

"JMartin957" wrote in message
...

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.



While it is true that old motor's insulation systems are more susceptible to
VFD generated peak voltages, the problem is not as severe as some motor
manuf. would have you believe. One manuf that has made a big deal out of
this is Baldor with their line of motors made with heavier insulation.
Remember that in the typical home shop, the motors are usually rated
460/230V or 440/220V (old motors) and are run on 230V 99% of the time.
This fact alone provides enough cushion in the insulation system so that VFD
peak voltages should not be a problem. I have an BP vert. mill with the
old style pancake motor that has run for 8 years on 2 different VFD's with
absolutely no problems.

Randy

Older VFDs would tend to smoke, newer ones would trip. As others have
indicated, there is no advantage to doing reversal or run/stop this
way. You should definitely wire up signal switches to the control
input terminals and let the VFD do its job.

This is one of the best posts I've seen in a while covering VFDs. I've used
quite a few and have advised others in sizing and using them. They are one of
the best technologies developed in recent years to be applied to the home and
industrial machine shop. The only thing that might be better is the digital
readout on mill and lathe.

Earle Rich
Mont Vernon, NH

You're in Cumberland or Yarmouth, aren't you? I have both a
lathe and mill running on Mutsubishi VFDs here in
Harpswell. Email me if you'd like to come up and see how
they work in person.

Ned Simmons



Ned:

Cumberland it is, and thanks for the invitation. I'll contact you later to see
what our schedules look like.

My horizontal mill is running on the static converter now. I like the idea of
the VFD though, for a little more power and the RPM control. Would hate to
rewire it, though. Have it set up now with a disconnect to static converter to
twist-lock receptacle on wall, with a drum switch on mill.

Thanks for your help,

John Martin