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.

[Mike Henderson]

I'm going to keep investigating. One report hinted that the starting capacitor in a regular capacitor start induction motor might cause problems to the VFD. I don't know why yet but I'll keep looking.

I sent a message to these people (http://www.anaconsystems.com/text/eagle1.html) to see if their VFD can be used with a regular capacitor start induction motor.

Mike

[Rod Sheridan]

Hi Mike, the issue with a single phase motor is that there isn't a field rotation like a 3 phase motor.

Flywheel effect keeps the motor rotating between power pulses, unlike a 3 phase motor where the rotating field drags the rotor along.

To use a single phase motor with a VFD you would need a motor that keeps the start winding engaged through a capacitor ( cap run motor) or a shaded pole motor to simulate a phase offset.

Regards, Rod.

[Mike Henderson]

Hi Mike, the issue with a single phase motor is that there isn't a field rotation like a 3 phase motor.

Flywheel effect keeps the motor rotating between power pulses, unlike a 3 phase motor where the rotating field drags the rotor along.

To use a single phase motor with a VFD you would need a motor that keeps the start winding engaged through a capacitor ( cap run motor) or a shaded pole motor to simulate a phase offset.

Regards, Rod.

That's not correct, Rod. Single phase AC power can be represented mathematically by two counter rotating vectors. In fact, that's why you can reverse the rotation of a single phase motor. The starting circuitry determines which vector the rotor will follow. There definitely is a field rotation - if there wasn't the motor wouldn't turn.

I did some research on VFD's (and even on inverters) and their use on single phase motors and will write up with I found out in a later post.

Mike

[Mike Henderson]

I did some research on using a VFD with a single phase output to drive a capacitor start induction motor. This post is to pass along what I learned. When I use the term "induction motor" in this write up, I mean a two pole capacitor start induction motor.

+++++++++++++++++++++++++++++++++
I'm going to add this section because of the posting by Rod, above. It is not needed to explain the VFD situation but if Rod has a misunderstanding - and he knows electricity well - I thought others might have it also.

The problem in a single phase motor is the starting circuitry which determines the rotation direction of the motor. Single phase AC power can be represented by two counter rotating vectors and the starting circuitry determines which of those vectors the rotor will follow. A capacitor start induction motor uses a capacitor and a starting coil to "enhance" one of the vectors (this is a simplified explanation) and this causes the rotor to begin following that vector. Once the motor is up to a certain speed, the effect of the other vector is minimal and the motor continues operating in the direction that it was started in. The starting coil is usually then removed from the circuit but the motor would operate with it in the circuit if the components of the starting circuit were designed for continuous operation.

It's sort of like a feedback mechanism. You can prove this by removing the starting capacitor and starting the motor by hand - give the rotor a spin in either direction and it will gradually come up to speed. You can do it in either direction. If spinning it by hand is too difficult, wind some rope around the shaft and use that to give it the initial spin.

+++++++++++++++++++++++++++++++++++++++

A VFD can certainly do speed control of a single phase motor. The RPM of the motor is determined by the input Hz so as you change the input frequency downward the motor will slow down. Raising the frequency above the design frequency has some problems so we will not consider that situation here. To demonstrate this, a synchronous induction motor will operate at 3600 RPM at 60Hz but at only 3000 RPM at 50Hz. Standard induction motors are not synchronous motors - they slip - but the rated RPM of a 50Hz induction motor is less than the rated RPM of a 60Hz induction motor. This is due to the difference in the input frequency.

So - ignoring everything else - if you could control the input frequency to a single phase induction motor, you could control the speed.

But--- now, let's look at some of the other things.

A VFD has to change the output voltage as the frequency changes. At rated Hz, it needs to be the motor rated voltage, of course. But as the Hz decreases, the impedance of the windings decrease so the voltage has to be reduced to prevent excess current.

I believe that a VFD attempts to maintain constant speed and does so by monitoring the current drawn by the motor. As load increases, the motor will slow down, the slip will increase, and the motor will draw additional current. The VFD could compensate for this slow down by increasing the Hz or by increasing the voltage to supply additional current (and power) to the motor.

If the motor was operating slightly above the RPM where the centrifugal switch will cut in, and there was a slight increase in load causing a slowdown, the centrifugal switch could cut in, causing a step increase in the input current. The VFD would see this as an increase load and make changes to bring the speed back up. Once the centrifugal switch disconnected, the input current would decrease, causing the VFD to revert back to the original situation, which could cause the motor to slow down, etc. The motor could oscillate at that condition.

So I thought - why not make the VFD an inverter. Just supply appropriate voltage at a reduced (and adjustable) Hz. An increase in current when the centrifugal switch cut in would just be supplied by the device, exactly as it would if it was supplied by the mains at a different Hz.

And since the impedance of the starting capacitor would increase at reduced Hz, the starting circuitry would not draw the same current as it would at 60Hz and could probably be left in the circuit safely.

Okay, so now we have to look at a few of the applications for such a device. Two that come to mind immediately are speed control of lathes and drill presses.

And that's where the problem comes in. To start a capacitor start induction motor, you really need to start it at rated Hz so that the starting circuitry generates a sufficient vector to get the motor started. So for a 60Hz motor you would have to start it close to 60Hz, but maybe as low as 50Hz.

Now, suppose you're using that on a lathe and you have a big, unbalanced piece of wood in the lathe and you want it turning slowly. You can see the problem of starting it at 60Hz.

So that appears to be the reason they don't sell single phase inverters for capacitor start induction motors - you have to start them at rated Hz.

If you had a RPM feedback from the motor to the inverter, it would work because the inverter could reduce the Hz as soon as the motor reached the speed specified by the chosen Hz. That is, suppose I had chosen 25 Hz on the inverter control. That would correspond to a bit below 1500 RPM. Once the motor got to close to 1500 RPM, the inverter could reduce the input Hz from 60 Hz to 25Hz. But there's no feedback from a standard capacitor start induction motor.

Anyway, that's what I came up with. Other suggestions will be appreciated.

Mike

[M Toupin]

I believe that a VFD attempts to maintain constant speed and does so by monitoring the current drawn by the motor. As load increases, the motor will slow down, the slip will increase, and the motor will draw additional current. The VFD could compensate for this slow down by increasing the Hz or by increasing the voltage to supply additional current (and power) to the motor.

Some VFD's do. Simple Volts/hertz drives (AKA FM50) have absolutely no feedback, they simply put out the volts/hertz it's set for. A vector control drives on the other hand utilize feedback to adjust on the fly so to speak. Sensor less vector drives (ie Tenco CV7300 series) do as you suggest by monitoring current draw while a true closed loop vector drive utilizes a shaft encoder to provide feedback and provides even better monitoring and control.

Is this a purely academic exercise or is there some other reason to consider a system that is more complex, has more moving parts and more expensive to produce?

Mike

[Mike Henderson]

Is this a purely academic exercise or is there some other reason to consider a system that is more complex, has more moving parts and more expensive to produce?

Mike
I'm not sure exactly what you're referring to but the question of whether you can speed control a capacitor start single phase induction motor comes up frequently. The research I did was an attempt to answer that question.

From what you said, a simple volts/hertz single phase output VFD would work to speed control a single phase induction motor but it would have to start at close to 60Hz. If the user could tolerate starting at rated speed, they could accomplish speed control of a capacitor start induction motor with a simple single phase output VFD.

The problem is that few VFDs are offered with single phase output.

Mike

[Dan Friedrichs]

The problem is that few VFDs are offered with single phase output.



What if a "simple" VFD was oversized 3x (relative to a 1PH motor) - could you just hook the 1PH motor to 2 legs of a 3PH VFD?

I guess the only problem would be that the L-L voltage would be less?

[Mike Henderson]

What if a "simple" VFD was oversized 3x (relative to a 1PH motor) - could you just hook the 1PH motor to 2 legs of a 3PH VFD?

I guess the only problem would be that the L-L voltage would be less?

I'm not a power guy, but aren't 3 phase voltages measured line to line? So if you had a VFD with 240V three phase output, wouldn't the voltage across any two wires be 240 volts (at rated Hz)?

If you had four wires coming from a Wye transformer setup, the line to neutral would be the L-L voltage divided by the square root of 3. This is going back to my power classes, which I took about 50 years ago. I spent my career in electronics.

Mike

[Rod Sheridan]

I'm not a power guy, but aren't 3 phase voltages measured line to line? So if you had a VFD with 240V three phase output, wouldn't the voltage across any two wires be 240 volts (at rated Hz)?

If you had four wires coming from a Wye transformer setup, the line to neutral would be the L-L voltage divided by the square root of 3. This is going back to my power classes, which I took about 50 years ago. I spent my career in electronics.

Mike

yes Mike, a 240 volt 3 phase VFD would be 240 volts single phase ( line to line).

Thanks for the info Mike, I hadn't considered starting the single phase motor at near rated speed, not normally what I think of for a VFD.

Interestingly, the single phase motors on my Hammer machinery would be ideal for this as they don't use a centrifugal switch for starting. There are two contactors, the run contactor which latches closed, and a start contactor which engages the start capacitor only while you hold the start button. The run capacitor is always connected.

Regards, Rod.

[Steve Kohn]

If anyone finds such a magic solution let me know. I've got a PM 2 HP edge sander that I would love to slow down and reduce burning while giving more control.

[Mike Henderson]

If anyone finds such a magic solution let me know. I've got a PM 2 HP edge sander that I would love to slow down and reduce burning while giving more control.
Maybe you could be the test case. Look for a VFD with 3 phase output that will support a larger motor than 2HP and only does voltage/Hertz (no attempt at speed regulation). Connect your motor to two of the output wires and let us know how it works. This assumes that you can start the motor at rated speed, and then slow it down.

Mike Toupin may be able to give you a recommendation on a VFD.

Mike

[Most motors in woodworking equipment are operated at significantly less than their rated HP so you wouldn't need a VFD for a 6HP motor.]

[Duane Meadows]

Do remember though, a 2HP 3PH VFD is rated 2 HP for the 3 phases combined, not for 2 HP on 1 phase. Just something to think about!

[David L Morse]

The square root of three is the derating factor for operating a single phase load from a three phase source (the same, not surprisingly, as for a three phase motor operated from a single phase source). For example, to run a 2HP motor at full power requires a 3.5HP RPC or, looking at it the other way, a 2HP RPC can power a 1.2HP load.

[Bill Adamsen]

Not a single-phase VFD, but perhaps functionally an alternative. I use a Dart 250 DC reversable speed control (120/240 vac input) on my lathe with a Baldor DC motor. The control is available from many sources. These motors are readily available from worn out treadmill units - cheap. I use it with a foot pedal. I did not put this configuration together but it works ok. Theoretically there are advantages to DC motors in this application since they have maximum torque at stall.

I suspect changing your 4hp single-phase induction motor is out of the question - so likely this solution is of low interest to you. However, I did see one of these on ebay in a 3-1/2hp configuration for significantly less than $200. Original manufactured price was $3000+. They may have some custom specifications (frame, speed, shaft length or diameter) that make use challenging, but not impossible.

(I retained the original Delta pulley speed control just because I am averse to throwing out something that might be required in the future.)