Single Phase Motor Speed Control: Advantages/Disadvantages, Cost, Etc.

[Paul Alciatore]

Talking about 1/2 to 2 HP range, single speed motors used as main power on machines. Not many people do it, so there are probably good reasons why. But then, a quick internet search does show that units are available, some described as VFDs and others not. So how about a discussion on it. Thoughts and experience, if any? Advisability?

https://www.google.com/search?client=firefox-b-1-d&q=vfd+for+single+phase+motor

and

https://www.google.com/search?q=speed+control+for+single+phase+motor&client=firefox-b-1-d&sxsrf=AJOqlzV1zeV08aShYVRcGPh1WTHJVfNikA%3A1673847723196&ei=q-PEY9nKC4yiqtsP78aNgAs&ved=0ahUKEwiZzMvXsMv8AhUMkWoFHW9jA7AQ4dUDCBA&oq=speed+control+for+single+phase+motor&gs_lcp=Cgxnd3Mtd2l6LXNlcnAQDDIMCAAQgAQQhwIQDRAUMggIABAIEAcQHjIICAAQCBAHEB4yBggAEAgQHjIGCAAQCBAeMgYIABAIEB4yBggAEAgQHjIICAAQCBAeEA8yBQgAEIYDMgUIABCGAzoKCAAQRxDWBBCwAzoHCAAQsAMQQzoGCAAQBxAeOggIABAHEB4QDzoLCAAQCBAHEB4Q8QQ6BggAEB4QDzoICAAQBRAHEB46BggAEAUQHjoKCAAQCBAHEB4QDzoHCAAQgAQQDToICAAQCBAeEA1KBAhBGABKBAhGGABQ9AlYkSlg7EdoAXABeACAAW2IAasJkgEDOS40mAEAoAEByAEKwAEB&sclient=gws-wiz-serp

[Noitoen]

The way a single phase motor is wound will never compare to a three phase motor and won't ever have the efficiency of the latter. Systems I've seen in the past were a lot like universal motor controls where the slight speed reduction was achieved by reducing the motor voltage and use some kind of speed sensor as feedback to maintain low speed at different loads.

[Mike Burch]

I fitted a single phase in/single phase out VFD to slow the motor on my bandsaw down to an appropriate speed for cutting 316 dry. What I had stupidly forgotten was that at low revs the motor's centrifugal switch kicks in. So I can slow the motor from the mains frequency (50Hz in this part of the world) to only about 33 Hz before it all goes pear-shaped.

One could, of course, mess around with the spring tensions on the switch, but that's above my pay grade.

Fortunately, the optimum blade speed for 316 requires about 39Hz, so from that point of view the experiment is a success. The reduced power output is unlikely to be a problem for me.

I have not altered the saw in any way, so on the rare occasions when I plug it into the VFD rather than directly into the mains I must be careful not to let the auto switch-off remove the load from the VFD at the end of the cut.

So, Paul, as a way of changing speeds on a single-phase machine it's pretty limited.

[mikey553]

Talking about 1/2 to 2 HP range, single speed motors used as main power on machines. Not many people do it, so there are probably good reasons why. But then, a quick internet search does show that units are available, some described as VFDs and others not. So how about a discussion on it. Thoughts and experience, if any? Advisability?

I think I understand where are you coming from. I have several single phase machines, which would benefit from the motor speed control. My drill press has a 1 HP motor, harsh start and its lowest speed is still too high for some applications. My lathe has a 2 HP motor, it is a good one, but it would be nice if I can reduce its speed 2-3 times. Both motors are capacitor start/capacitor run motors with a centrifugal switch. Most likely I would not be able to use a speed control on them.

My band saw came with a 1/3 HP motor, which was not good from day one. I am in the process of replacing it with a 3 phase motor and a VFD. So far I did not feel the need for a speed control there, but 3 phase motor was dirt cheap and I already had a VFD. As a side benefit I get a speed control.

Another thought is on using 1 phase output from a VFD. I have 3 VFDs with single phase input, but the output on all of them is 3 terminals with 3 phase on them. There is no neutral terminal, only ground. I suppose you can use 2 phase terminals as your output, the voltage and frequency can be adjusted. Not sure how the VFD would react to the loss of 1 phase. And both phase wires will have a potential above ground, so we need to keep this in mind.

[J Tiers]

Ther are perfectly standard available single phase VFDs. They work like any other VFD, just outputting single phase.

However, as noted they will not work with regular start-switch type motors. They ONLY work with "PSC" (single capacitor start-run) motors, and shaded pole motors. They may work with some other rare types as well, but I have not looked into that.

I have used single phase VFDs to control PSC motors. We did one project with a variable capacity oil burner, one with a cooling fan for a military radar, and one with a pump motor. Those I designed the VFD for. We also did some with commercial single phase VFDs.

Single phase VFDs work quite well when applied properly. But, one oddity is they need to start at full speed, and then slow down. Since the major application is with "PSC" motors, the start torque would be very low due to the capacitor being high impedance at low frequency, so the full speed start is to ensure it does in fact start.

Because they are limited application, you won't find a lot of applications outside of cooling fans. But, european companies have traditionally liked PSC motors, so many single phase motors on european equipment could be used with a VFD.

[Tundra Twin Track]

Most of my machines have been converted to 1ph single speed motors.Both Lathe and Mill are industrial size machines,I haven’t noticed the problems that is constantly mentioned with issues with 1ph motors.I had the motors on hand from a grain handling facility.

[Noitoen]

Other than the starting difficulty, single phase motors for small applications are (almost) as good as any other. When you want variable speed then, the only reliable way in my opinion, is to use, without electronics, a repulsion motors, which has the ability start and run with high torque and speed, in both directions just by varying the brush timing. The other ways would be the famous universal motors or dc but these, although simpler, require electronics.

VFD electronics has become so reliable and "cheap" these days that even in the heavy industry for the sake of power savings it is applied. At this very moment we are in the market for a 1500 Kw 750 rpm 6000 volt motor with a VFD to run a new metal shredder which, just a few years ago, would run on a a combination 1500 rpm motor, variable fill fluid coupling and gear reducer to achieve the same 750 fixed rpm. All this just to get the 22 ton heavy hammer mill rotor up to speed. Today, the VFD starts the motor with a direct coupling and with the added bonus of being able to reduce the high start currents and lowering the working speed to reduce power consumption, adapting for the material being processed.

On small motors, other than special format or size, it's much easier and reliable to use single to three phase VFD's and replacing the motor to get variable speed.

[Mike Burch]

"On small motors, other than special format or size, it's much easier and reliable to use single to three phase VFD's and replacing the motor to get variable speed."

Amen to that! I've done it on both my lathe and my mill/drill, and couldn't be happier about it.

[lakeside53]

Interestingly, we have many "single phase" motors at work (1-2hp) that are variable speed. BUT...these are actually 3 phase (more like BLDC) motors with the "VFD" integral to the motor. To the end user - just single phase though. The entire "electronic commutaton" portion is about 1 inch thick and mounted like an end-bell, the motor casing is part of the heat sink. These are for hvac fan control, and required by code here now.

Unfortunately the drive portion is potted so no repair possible and at about $1000 for a 1hp, stupidly priced for the home shop. Keep your eves open for them in when commercial remodeling is underway.

Like these Vari-Green Motors | Greenheck

Here's a video comparing the effciency ("code required") of vari-green with PSC

https://greenheck.wistia.com/medias/3o48p0vndb



I'll dig up some photos of the internals

[Noitoen]

Those integrated vfd motors are very common in the parcel distribution centres to drive the conveyor belts

[FinnoUgric]

I am by no means an expert on any motor, yet an electric one. All I have is some experience on different types of motors and how to vary their speed.
I assume that this discussion is about modifying the speed with electronics and not with some mechanical means.

This discussion is about single phase motors so let's stay on those. Or ... can we?
The "normal" single phase motor that has this starting capacitor, running capacitor and some sort of switching to connect/disconnect them is actually a highly modified three phase motor. The speed of those can be changed only marginally as the capacitors and circuitry is designed for a specific frequency only. Those might run at different frequencies but their effiency will fly straight out of the window when the frequency deviates from the designed frequency. Reconnecting those as a "genuine" 3-phase motor might be possible but that is probably a bad idea because the windings were not designed that way.

Unfortunately quite many hobby-level lathes have this type of a motor. If one wants to have speed control on them, I suggest replacing the motor with a real 3-phase motor and a VFD.
Speed control brings in also the question about torque. At some point a 3-phase motor looses most of it's torque when the speed is brought down. Some VFD:s can compensate for this but the torque will always be smaller on smaller revs. Trying to get a high torque with low RPM may cause thermal damage to the poor motor even is there is auxiliary cooling available (separate fan).
I recommend modifying the speed ratio (smaller pulley on motor) if a motor+VFD modofication is done on a lathe or some other equipment. This allows for lower speeds.

There is one more thing to consider when modifying the speed of some equipment. Is the equipment designed in a way that it survives the speed changes? There might be lubrication issues if the speed deviates from what the equipment was designed for. There might be oil spill or not enough or in worst case no oil at all. Cooling might also be dependent on speed.

[Paul Alciatore]

Well, it seems to be going pretty much the way I thought it would. But I had to ask. I saw a lot of ads and sometimes things change.

[Noitoen]

Well, it seems to be going pretty much the way I thought it would. But I had to ask. I saw a lot of ads and sometimes things change.

Unfortunately, single phase motors are a no good but needed thing. We should adapt and move on. If you buy a new electric forklift today, they all have either low voltage VFD driven three pase motors or the more recent ones smaller higher efficiency magnet rotor BLDC motors. With these, the physical size of them has reduced to less than a quarter of their size.

[IanPendle]

Well, it seems to be going pretty much the way I thought it would. But I had to ask. I saw a lot of ads and sometimes things change.

So, you start a post and then dismiss the replies in this off-hand fashion. People put in an effort to reply only to be brushed off like this. Why do they bother? Why am I even writing this? I hope that North America can warm up soon and people get back to their workshops. Huh!

[FinnoUgric]

So, you start a post and then dismiss the replies in this off-hand fashion. People put in an effort to reply only to be brushed off like this. Why do they bother? Why am I even writing this? I hope that North America can warm up soon and people get back to their workshops. Huh!

This sort of discussions with quite predictable result may also be quite fruitful.
I personally learned a few things about controlling single phase motors. One was a repulsion motor. Didn't know those existed. I had always believed that early days motor control was done with resistors and wondered how they got away with heat dissipation in San Fransisco. I assume they used those repulsion motors in street cars there.

In the Finnish language if a person is really cranky, it is said that (s)he is like a bear that has been shot in the ass.Maybe Paul succeeded in shooting a North American bear straight into the butt while it was having the wintertime hibernation.