# VFDs and single phase motors

• posted
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?
ERS
• posted
Maybe..... 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 works. I did not try that motoro on an inverter, it was for a vacuum pump.
Let us know if it works. Pete
• posted
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, though.
GWE
Eric R Snow wrote:
• posted
Eric, 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 pass up.
Dan
• posted
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 frequency.
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.
Enjoy, DoN.
• posted
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 VFD.
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 speed.
Jim
• posted
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 work. :-) ...lew...
• posted
Dan, 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. Cheers, eric
• posted
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 VFDs).
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 them.
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.
WB ................
• posted
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. Eric
• posted
[ ... ]
[ ... ]
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.
(A)--------------------+ (B)----+ | | | +------)|-------+ | | 3 3 3 3 3 3 3 3 3 3 3 3 | | (C)----+---------------+
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 voltage.)
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 degrees.
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?
Enjoy, DoN.
• posted
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.
• posted
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. Thanks, Eric

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.