A friend has acquired an elderly lathe with a dual-wound 3ph motor. It
has 6 wires going in to it, and is intended for 415v 3ph UK supply, but
he only has a normal single phase 240v domestic supply. He has been told
that he cannot use a single to 3ph VFD with dual-wound 3ph motors. Is
this true? If so, why?
What does "dual wound" mean? Is this a dual voltage motor, or a
two-speed motor? Either CAN be used with a VFD, but you have to match
the VFD to the motor voltage. In the UK, I think 415 is the ONLY
3-phase power available, they don't use 208, 230 and 460 V like we
do in the US. If your friend gets a 460 V VFD and programs it for
415 V at 50 Hz, the motor should be fine. Then, of course, you need
to supply the VFD with at least 440 V AC. A suitable step-down
transformer can be wired backwards to step up 240 to 480. That might
be too high for the VFD or the motor. But, it probably will work fine.
Probably a two-speed motor. Brook Motors of Huddersfield made a lot of
these for machine tools in the 1950s and '60s. My power hacksaw has a 1
3/4 hp motor with two completely separate windings (a four pole and a
six pole). I opened the motor, cut the star points and brought out all
twelve wires so I could run in on 240 V. Then I built a static phase
convertor with a changeover switch. I can switch speeds while the
machine is running, but the friction in the belt drive means that it
nearly stops in the process. I seem to recall someone saying that
switching from one speed to the other was the problem with using a VFD
on a two-speed motor. Perhaps someone could confirm this?
I can't see any reason why a two-speed motor would be a problem.
One thing is you don't want to flip any switches that are between the
VFD and the motor when it is running. So, always stop the VFD before
you throw the speed switch. The spark at that switch could pop the
Thanks for the feedback. Yes, its a two speed motor, but I know he won't
be using the speed change at all. He intended (until he was told that a
VFD couldn't be used) wiring the VFD to whatever winding seemed most
versatile, and leaving it like that. He is aware of the need to convert
it to delta so that it will run on 240v.
A VFD can indeed be used on the usual 2 speed motor. You simply pick
which ever winding you want and hook it up.
Some motors have 2 seperate windings that run independantly..others
have 2 seperate windings that run one for low speed, and add the
second winding to the first, for high speed. (some Devore motors in
Hardinge lathes for an example).
In this case, hook the windings up for high speed, then attach to the
"Pax Americana is a philosophy. Hardly an empire.
Making sure other people play nice and dont kill each other (and us)
off in job lots is hardly empire building, particularly when you give
them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you
for torturing the cat." Gunner
I have Taiwanese clone of a Hardinge with a VFD driving the
2-speed motor. The motor would not run on a suitably sized
I do have the VFD connected so it will drive both windings,
but there's probably no practical reason to do so, assuming
the two windings deliver the same torque, which is typical
of two speed motors.
Strange. I wonder why not? I must say that I had to add some extra start
capacitance to mine to get it working satisfactorily, possibly due to
the physical size of the motor, but it works fine now.
I have a similar problem with a 2hp, 3ph Delta Unisaw motor. I looked
into buying a converter (VFD) and the price (260 plus or minus) was more
than buying a new single phase motor ($245 for 1-1/2hp). Also with the
losses developed with using a convertor, my original 2 hp, 3ph would
probably be no more than the effective output of a 1-1/2hp single phase
BTW, anybody want to buy a 2hp, 3ph Delta Unisaw motor? Condition
unknown since I have no 3ph power to test it.
You won't lose any HP with a VFD, but unlike a lathe or a
mill where the ability to vary the speed of the motor is a
big convenience, I don't see any upside for you to install
a VFD considering the price of a single phase motor.
BTW, I have a 3HP single phase Unisaw and have managed to
pop the motor overload a couple times when ripping 2" ash
or oak, or sawing thick aluminum plate. 1-1/2HP is probably
enough 95% of the time, but the saw can take advantage of
It is special in that it has the mounting bracket plates welded to the
motor casing, and it has a 3/4" shaft. The plates are about 2' x 3-4",
and 1/4" thich. If the brackets don't get in the way, it should work for
whatever you should need it for. Besides it would be simple enough to
cut them off.
I bought a 2hp, single phase standard mount motor but after I bought it,
I found out it has a 5/8" shaft. If I really wanted to use it, I could
make a mounting plate and make or buy a sleeve bearing to place over the
5/8" shaft to bring it to 3/4" but I'm a little wary of cobbing up such
a thing since the smaller shaft might be snapped off during some rough