vfd for single phase motor

[Kevin Jenness]

Is it possible to use a vfd to regulate speed on a single phase machine? I have a stroke sander with a 4 hp 1ph motor that would benefit from a slower belt speed at times.

[M Toupin]

The short answer is no, VFD's output 3ph power hence you need a 3ph motor.

Mike

[Mike Henderson]

Well, yes you can use a VFD to control the speed of a single phase motor, with certain constraints:

1. You need to find a VFD that outputs single phase. They are made and someone pointed to one or more in a previous thread.
2. You need to always start the motor at close to 60Hz. A bit under might be okay.
3. You are limited in how slow you can go by the centrifugal switch. You need to always stay slightly above the speed where the centrifugal switch cuts in. Actually, you may be able to run the motor with the starting coil engaged but it depends on the motor and the rating of the starting coil. You can find the speed where the centrifugal switch cuts in by slowing the motor down in increments. You can hear the centrifugal switch cut in - it usually make a "snap" sound.
4. (this is true of any motor on a VFD, single phase or three phase) You need to make sure the motor is being cooled. Most motors used in woodworking are "totally enclosed, fan cooled" (TEFC). When the motor runs slower, the fan moves less air. If you're not using the motor hard you'll be fine, otherwise, get a fan and point it at the motor.

Mike

[Thinking about this some more, it may be quite feasible to run the motor with the starting coil engaged. As the Hertz decreases, the starting capacitor will pass less current so the current through the starting coil will be less as the speed (and the Hz) decreases.]

[Tom M King]

You may be able to find a used one for a variable speed heat pump on ebay. sorry, the one I looked at was 3 phase in, and 3 out.

[Mike Heidrick]

Vfd's output 3 phase so a used one does not help.
Most 3 phase motors do not have a centrifugal switch - there is no start winding.
There is nothing requiring you to start at 60hz. Cooling of the windings will be affected by the fan speed however.

[M Toupin]

I love these threads, they always go sideways and the poor OPs are left more confused than when they started...

Yea, there are special VFDs for 1ph output which can be coupled with special 1ph motors... point taken along with the cravats 1, 2 &3 which really makes it a deal killer...

BUT can we all agree that for the average user who wants variable speed on most WW machinery, a run of the mill 3ph output VFD and a 3ph motor is the path of least resistance?

I'll add my own cravats:
1. assuming the "average user" can wire a light bulb and has a basic understanding of electricity
2 . assuming a standard frame motor or at least a comparable 3ph motor can be sourced inexpensively
3. Assuming the "average user" isn't a electrical engineer which btw, would preclude said user from being "average"

Mike

[Dan Friedrichs]

Vfd's output 3 phase so a used one does not help.

There are a few single-phase output VFDs on the market.

There is nothing requiring you to start at 60hz. Cooling of the windings will be affected by the fan speed however.

I would agree with Mr. Henderson that starting a single-phase motor close to 60Hz would be a good idea. The starter winding, starter cap, and centrifugal switch were all designed for 60Hz operation.


That said, I think others have correctly pointed out that the hassle of using a VFD with a single-phase motor far outweighs the benefits. However, a 4HP 1PH motor is likely worth quite a bit more (on the used market) than a 4HP 3PH motor. I bet you could sell the 1PH motor and buy a replacement 3PH and VFD and still be money ahead.

[Mike Henderson]

I've often wondered why there aren't more speed control devices for single phase induction motors. Seems to me that a single phase input to single phase output VFD connected to a standard single phase induction motor would work well for speed control.

And as far as wiring it up, that seems to be even easier than doing a three phase motor. Basically, you'd plug in the VFD and then plug the motor into the VFD. The VFD could be set up to always start at 60Hz so that you'd get a good start. Dan pointed out that the starting circuit - made up of the starting capacitor and the starting coil - is designed for 60 Hz operation. You might not get sufficient current through it at 20Hz (for example) to get the motor going.

I don't think I'm missing anything, but if I am, please correct me.

I have a single phase lathe that I've always wanted to convert to VFD speed control. Think I'll give it a shot.

Mike

[Rick Lizek]

Consider myself an expert on ss doing wood snd metal for 40 years why you do need to change the speed?

[Tom M King]

some good information here: http://www.vfds.in/ although you can tell it didn't start in English ""Study your application careful before select VFD"

[Mike Heidrick]

Please post up an example of a single phase OUTPUT VFD that will power a 4hp single phase stroke sander motor.

The single phase output VFDs I know of only work with PSC (Permanent Split Capacitor) and Shaded Pole motors and are not going to work with the majority of any power tools I know of.

[Dan Friedrichs]

Ok, Mike, I looked and agree - I can't find a 1PH output drive that big.

Theoretically, there is no reason one can't be built (assuming you're still relying on a starter cap/winding to get the 1PH motor turning). You've got me wondering if a 3PH drive could be connected to a 1PH motor (using just 2 of the legs) (and assuming you could defeat whatever equal-current per phase safety features that likely exist).

But now, as Mr. Toupin suggested, we are rapidly heading sideways

[John McClanahan]

The price of such an item, if you could find one, would make your jaw drop. A more affordable route would be a couple of step pulleys, like found in drill presses.


John

[Kevin Jenness]

Thanks for the responses. It sounds impractical to control the speed on the existing single phase motor. Swapping out the motor is a possibility down the road though not a high priority.

[Mike Heidrick]

Ok Dan and Mike. Whew, I thought you guys had some magic i needed to try out!! Keep your eyes peeled if ever such a thing becomes a practical option. Sorry for OT.