I've asked one person who should know, and Automation Direct, and
haven't gotten the answer I want. What I wanna do is run a single
phase motor, without a centrifigul starting switch, with a VFD. I'm
told it can't be done. You can't even hook up 3 single phase motors
to one VFD. Oh well. But is there some way to fool a VFD into thinking
it's controlliong a three phase motor so the speed can be varied on a
single phase motor?
If it is a capacitor run motor. Maybe....
I've ran capacitor run motoros on three phase power, without the
capacitor by connecting the run windings to phase A and B. The C phase
connected instead of the capacitor.
Worse case, you burn out the inverter, the motor or both. Best case it
I did not try that motoro on an inverter, it was for a vacuum pump.
Let us know if it works.
I suppose you could hook up 3 identical motors, each running off of
one phase. Theoretically you should be able to speed control a single
phase squirrel cage AC motor just the way you speed control a 3 phase
one, by varying the frequency. I haven't ever seen a single phase VFD,
Eric R Snow wrote:
Why do you want to run a single phase motor on a VFD? I mean is the
single phase motor something super special that is the only motor that
will fit the application? Or is it you have this kind of ordinary
single phase motor and you want to vary its speed. The reason is you
are kind of local and I can keep an eye out for a really cheap three
phase motor. Might even have one on hand that was too good a deal to
The problem is not that the VFD will be unhappy, but that the
*motor* will not work well at frequencies much different from the design
In the case of cap start motors, the synthesized phase
difference to start it is a function of the inductance of the start
winding and the capacitance of the start cap. Both have a reactance
which is a function of the applied power line frequency. With
capacitance, as the frequency decreases, the reactance increases. With
inductance, as the frequency decreases, the reactance decreases. Thus a
start cap and winding, which offer a reasonable phase shift at the
design frequency will not do so properly at other frequencies. Thus, it
will not start at either high or low frequencies.
If you could apply the third line to the start winding *without*
the capacitor, at a voltage appropriate to that winding (usually
different from the run winding voltage), you could probably do pretty
well at a somewhat wider range of frequencies.
As for induction motors with no capacitor start (such as is
found in small motors like phonograph motors and Rotron muffin fan
motors), the starting phase is generated by a shorted turn around *part*
of each pole. The ratio of the field strength there to that in the main
parts of the poles is also frequency sensitive. I have *tried* a
Rotron fan, and it truly does not start at low frequencies, though it
will start at the normal frequency -- even from the VFD. However, as
you either increase or decrease the frequency, the motor loses power,
and eventually stalls.
So -- the answer you got really means that it is not *practical*
to run single phase motors from a VFD -- you don't get nearly the
variability which you would like to have.
In principle, once a single phase motor has been
electrically or mechnically run up to its normal operating
speed, it can thereafter be speed controlled by a VFD in
exactly the same way as a 3 phase machine. However the
practical difficulties are pretty horrendous.
Firstly the VFD needs to be rated for at least 3x the
motor power AND of a type that is not discommoded by a 100%
unbalanced load. Although the semiconductors are not
necessarily overloaded by this condition, the control
circuits may interpret this as a load fault and shut down
The second problem is starting - the usual single
phase starting arrangements are optimised for operation at
normal power supply frequencies and will fail miserably at
widely different VFD frequencies. +/_ 10% is probably
possible but much outside this gets pretty doubtful. You'll
probably be forced to start the motor at normal supply
frequency and only then adjust to the required operating
Well back in the "dark ages" of 1950s We ran 2 phase motors with a
variable speed by amplifing the signal ( Note! the amps were vacuum
tube ) and splitting the phase, second amp. and driving the motor over
a reasonable speed range. This was to follow a radar antenna scan,
the signal was generated by a clock motor geared to the rotating
yoke in a PPI display. SO if the motor is "appropiate" it will
Here's the deal. It's a grinder motor where the motor is the grinder.
So replacing the motor means replacing the grinder. Otherwise I'd
already have put in a 3450 motor. But thanks for the offer.
This won't be practical, and most likely wouldn't work, and could possibly
damage the components. The windings are different (not identical) in a
split-phased cap-start motor, and aren't placed properly for the motor to
operate using both windings (only during normal startup).
Also, the insulation of the windings isn't adequate for VFD use (which is
the big difference in inverter-duty motors which are designed for use with
I think you will need to use the existing grinder/motor as-is.
For a moment, I thought you might be able to try to locate a stator from a
3450 motor to use with your rotor, but the rotor laminations are different
in a 3450 motor (the spiral pitch change).
There are a few methods to change the speeds of some types of induction
motors from zero-to-full speed, but doubling the designed speed isn't one of
A half-assed kludge would be to use the existing unit as an arbor, and belt
drive it with another motor to obtain the speed you want.
Once again I failed to be precise. The motor is a permanent capacitor
split phase machine. So it has two identical windings. The speed can
be easily lowered by lowering the voltage. But raising the voltage
only increases starting torque and improves speed regulation. Since it
is a 1725 rpm motor I suppose I could find the ends of the windings
and connect them so that it has two instead of four poles. I've never
done this and only know about it from books. The motor is a dual
voltage motor but I don't know how this would affect changing the
number of poles by re-wiring. Furthermore, I don't know if the pole
numbers can be changed just by changing connections. Will the
laminations be made so that the motor can only be a 4 pole motor? BTW,
it's a Baldor grinder. I bought it from Grant Erwin for a good price.
He has always been very fair and honest to deal with.
[ ... ]
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Hmm ... if this is one of the type where the direction is
selected by which of the two windings power is applied directly to (and
thus which gets the power through the capacitor), you might be able to
run it -- though I'm not sure how efficient or how reliable it would be.
If it is wired like this, with connection of the power line to
(A) and (C) running in one direction, and connection to (B) and (C)
running in the other direction, I would suggest simply removing the
capacitor and seeing what it does with the three terminals to the VFD.
(You might start with the VFD set for lower than standard voltage, and
check current and heating in the motor before risking it at full
Certainly the accuracy of the phase shift for the second winding
(when the motor is used as designed) would be a function of the
frequency and the accuracy of the capacitance. Since such capacitors
are often quite low tolerance, I suspect that the windings would
tolerate a phase shift of 60 or 120 degrees instead of intended 90
If you want to feed it a precise 90 degree phase shift, you
could build up a Scott-T circuit -- but the transformers might not work
that well at lower frequencies.
But I am perhaps still puzzled as to *why* you want a VFD on
this motor, instead of using it as it was designed to be used.
And normally (this is a grinder, right?), the motors are
selected for a safe speed for the size of wheels which will fit within
the guards -- so increasing the speed is generally a bad idea.
I like VFDs, but this does not seem to be a project crying for
one to be applied.
So -- why do you want to speed it up? Or to slow it down?
aaah, Eric - 1750 RPM grinders go for a premium - why not just sell it and
get a 3450 RPM unit and pocket the difference? many folks who do wood
turning get rid of 3450 grinders and go for the lower speed - it is a lot
less damaging to a tool that's sharpened every few minutes. Offer your
grinder up to local wood turners and make a trade.
I don't want to make it any slower than it is. I just posted that for
sccuracy of description. The wheels can go faster. Which means
sharpeneing drills faster. The slower speed is also good for grinding
some things. So, like always, I just wanna tinker and mebbe get the
higher speed for some jobs.