# Variable speed for single phase motor?



## lazylathe (Mar 2, 2011)

Hi,

I am wondering if this is possible to do?

On my Myford i have a 1/2hp AC 110V motor, single phase.
I do not have access to 3 phase in the house.
Is there any way to add a variable speed control to the motor without first having 3 phase power and then go through a VFD?
I am thinking there must be some form of solid state variable speed control that can be used for this purpose.
Or would i have to change over to a DC motor for this to work?

Any help or suggestions would be greatly appreciated!

Andrew


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## Maryak (Mar 2, 2011)

You can use a 3 phase motor and an inverter from your single phase supply but you need to be aware of the torque requirements at low speed on a lathe, i.e. a bigger 3 phase motor than your current 1/2 hp. 

Hope this helps.

Best Regards
Bob


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## GailInNM (Mar 2, 2011)

For up to about 1-1/2 HP there are many VFD drives available for single phase input. Some of them offer the option of either single or three phase input. Of the single phase input type they can be either 120 or 240 volt input types and for a little bit more money some units will accept either 120 or 240 volt input. I personally think the dual voltage input units are worth the extra money as you can then move the equipment around easier as your shop expands, and it will over the years. Regardless of the input, the most common voltage out is 240 three phase. 

Bob has a valid point about the lower torque when you reduce the RPM and increasing the HP on the new motor is worth while. If you keep all your existing speed change pulleys it is not quite so important as you can change the speed range by changing the pulley arrangement, but with the larger motor you will not have to do so except for unusual circumstances.

Gail in NM


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## Kermit (Mar 2, 2011)

The DC motor would be better than the single phase AC motor. A three phase motor combined with a Variable Freq Drive would be the grand prize choice.


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## metaldestroyer (Mar 12, 2011)

Hi Andrew; You may want to look at Minarik drives they sell a unit that will control the RPM of certain types of single phase motors. I did test one or their 3 phase units and it worked OK.

Regards Jack


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## Paulsv (Mar 12, 2011)

When looking at VFD's you should be aware that there are different systems for controlling motor speed. The standard VFD has what they call a V/F control, which stands for Volts/Frequency. Then there is a "Sensorless Vector," also called "Open Loop Vector," system which has the ability to increase power to the motor in response to load, and it gives better power at lower RPM's than the V/F system. Finally, there is the "Closed Loop Vector" drive, which requires an encoder on the motor that provides feedback to the VFD, kind of like a servo motor on a cnc system. That has the best low speed performance, but additional expense and complication. Sensorlesss Vector is now available in some of the smaller (110 volt input), less expensive VFD's such as the TECO-Westinghouse EV series.

Also, when looking for a motor, you will probably be looking to match or exceed the rpm's of your current motor, since that is what the gearing of the lathe was made for. In this regard, you should be aware that it is not uncommon to "overspeed" a 60 HZ motor by running it at a higher HZ, since the rpm increases with the frequency. Most VFD's are programmable to allow you to do this. Obviously, the bearings in the motor should be rated for higher speed if you are going to do this, but lots of folks do it with no problem.


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## Twmaster (Mar 12, 2011)

If you shop carefully you can buy a VFD and motor for less than $150. Surplus Center has new 3PH motors at very attractive prices. 

I've never heard of a cost effective variable speed control for single phase AC motors. DC is another option but can get spendy unless you can find used/surplus gear.


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## Tin Falcon (Mar 13, 2011)

my little edelstaal has a motor that plugs into 120vac. but it is a dc motor there is a full wave bridge rectifier in the switch box the motor is simply labeled 115 volts. 
so check to see if you motor is really AC.
Tin


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## Stan (Mar 14, 2011)

There are many different types of motors called single phase AC. The one we are all familiar with, as a power source for equipment, uses a start winding which is switched out of the circuit by a centrifugal switch when the motor picks up speed. There is no way to operate this motor at a reduced speed as the centrifugal switch would cut in, and the start winding would go up in smoke. If it happened to be a capacitor start, the capacitor might explode before the start winding burnt out.

A second common type which has recently appeared on import tools, but long used on portable power tools,is the universal motor. This is the ubiquitous wound armature with brushes found in every drill, router, portable saw etc. These motors will run at full speed on AC and can be run on variable voltage DC over the full speed range.

For various reasons, none of the other types are a practical consideration for powering a lathe.


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## lazylathe (Mar 14, 2011)

Thanks for all the informative answers!
I will just stick with what i have for now.
Maybe in the future i will change over to VFD.

Thanks,

Andrew


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## BillC (Mar 14, 2011)

Andrew,

Look on eBay for 'treadmill motors.' These are usually 90V DC (which is a standard for the lower line voltage DC motors) and variable speed systems are readily available and can be a simple as a triac/diac drive with load feedback. I've built a couple of the controllers and one is still running a small drill press that I built. (need to show that one off!)

Anyhow, These treadmill motors are plenty powerful, very popular and would work very nicely on a lathe. Take a look and if you need some help with a choice, or for a controller, I would be most happy to help.... I believe this would be an economical as well as very adequate answer to your drive system.

BillC


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## Twmaster (Mar 19, 2011)

While on this subject of variable speed on AC motors I thought I would ask this...

I have a Skil 3 Wheel 10" table top band saw. It has a variable speed control. It will not go slow enough to cut steel safely. I'd either like to hack the controller to go slower or can I remove the speed control and replace it with one from a router? I'm ASSuming it is equipped with a 'universal' type motor. Or even better, if it -is- a universal type motor I could put a DC variable speed controller on the thing.

Or should I just give up and get the shrink to put me on meds?  Thoughts?


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## BillC (Mar 20, 2011)

It - is - probably a universal motor and the speed variation is within the requirement for the design/intent of the band saw - wood. There are many controllers for universal or DC motors, some with speed compensation and some that don't do too well in that department. The 'gearing' is probably set-up for the high blade speed for wood and slowing it enough with motor speed would reduce the power and efficiency of the motor an undesirable amount. You could give another controller a try but increasing the drive ratio is probably the best way to get the blade speed down enough to cut steel. Maybe you could add a jack shaft and a couple of step pulleys?

I had a large old wood cutting band saw that I restored. It had babbit bearings and had been through a fire. Anyhow, to reduce the speed and keep the drive system as simple as possible, I used a Mercedes Benz diesel car transmission in granny gear to reduce the motor speed. Kind of a waste of a good transmission and its other four gears but it made a great reduction for that old 36" band saw.....reminiscing a bit there....

BillC


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## rklopp (Mar 20, 2011)

The Brits have a VFD that can be used on single-phase motors, but only of the "permanent split capacitor" (PSC) type. My company imported one for a test. The PSC motor has a starting capacitor, but, unlike most single-phase machine-tool motors, the capacitor stays permanently in the circuit, rather than being switched out by a centrifugal switch. I don't think the PSC type of motor is very commonly used on machine tools.

You'd probably be happiest with a VFD/3-phase motor combination. That will be closest to plug'n'play, both electrically and with respect to the motor frame dimensions.


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## lordedmond (Mar 21, 2011)

Twmaster  said:
			
		

> While on this subject of variable speed on AC motors I thought I would ask this...
> 
> I have a Skil 3 Wheel 10" table top band saw. It has a variable speed control. It will not go slow enough to cut steel safely. I'd either like to hack the controller to go slower or can I remove the speed control and replace it with one from a router? I'm ASSuming it is equipped with a 'universal' type motor. Or even better, if it -is- a universal type motor I could put a DC variable speed controller on the thing.
> 
> Or should I just give up and get the shrink to put me on meds?  Thoughts?




The motor is a series wound one these have a poor speed control unlike a shunt wound one , to alter the speed you need a pulse width controller 


to slow your saw down to cut metal why not use a smaller blade after all its the cutting speed that matters not the rpm of the spindle


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## Twmaster (Mar 25, 2011)

How does a smaller blade fix this? As I said, it's a band saw...


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## Chazz (Apr 22, 2011)

Hi lazylathe, this is what is in my future; Check out Dart controls (if you're considering new) They have a complete package for a 1HP DC Motor, Drive and Tach\Encoder for about $700.00. BTW Most of their Drives will do both 120v and 240v. Also, if I read the litrature right, some of the drives will allow you to encorporate 1 or 2 limit switches, jog function and eStop.

Cheers,
Chazz


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## Hilmar (Apr 23, 2011)

Andrew, 
   See Gails and Maryaks posts.
   Keep your existing Belt system and go for a Variable Speed drive, single Phase to 3 Phase.
 But also stay with the lower RPM motors. The tork is better on the lower speed.
  Motor Pols:  2 Pol = 3600 RPM
            4 Pol = 1800 RPM
            8 POL = 1200 RPM
  A FM50 Series Sub-micro AC Drive 1/4 to 3HP cost below $100.00 Motor 3/4 HP 3Phase about US $50.00 used.
 This system will have all the gimmicks You would ever need.
 I use this on my 9x20. 3/4 HP
Happy Easter
Hilmar


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