# PWM Motor Speed Control



## old toolmaker (May 22, 2017)

I have an old drill press having a 1/4 hp AC 115 volt split phase motor from the 1950's.
Can I use a PWM motor controller to control the motor speed?  There are several available on EBay.
I am looking for a quick way to adjust speed other than changing belts. Most all my work is with smaller drill bits.

Dick


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## Bob Korves (May 22, 2017)

I am by no means an electronic genius, but I think PWM only works for DC motors.


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## old toolmaker (May 22, 2017)

Bob Korves said:


> I am by no means an electronic genius, but I think PWM only works for DC motors.


Hi Bob, 

Thanks for your response.
Maybe the acronym I used (PWM) is not correct and I just assumed it was.  There are cheap devices on E Bay that are meant for use on AC motors which I have on my drill press.
These devises are advertised for use on AC motors but I realize there are several kinds of AC motors.  Mine is a split phase motor.  I am not an electrician or electronics expert by any definition, so I need some guidance.

Dick


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## rgray (May 22, 2017)

I believe you need a brushed a/c motor to use one of those cheap speed controls on. motor label will say a it's an a/c d/c motor.


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## old toolmaker (May 22, 2017)

I forgot to mention in my last post that unit I am looking at on E Bay is very similar to what Grainger sells for router speed control.  You plug it into an AC 115 V outlet and plug the motor cord into the unit and it has a speed control knob.

Dick


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## old toolmaker (May 22, 2017)

rgray said:


> I believe you need a brushed a/c motor to use one of those cheap speed controls on. motor label will say a it's an a/c d/c motor.



This may sound like like a silly question, but how can a motor be both AC and DC?

Dick


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## markba633csi (May 22, 2017)

Sorry Dick it won't work with a split phase motor, don't waste your money.  It has to be a brush type motor, or a shaded pole induction type (fan motor).
Mark S.


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## markba633csi (May 22, 2017)

There are ac/dc types and they have brushes.  They don't have great speed regulation.  I believe vacuum cleaners are one example.  Some types of power tools. 
Mark S.
ps I think Wikipedia has some info on them, if you're interested.


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## old toolmaker (May 22, 2017)

markba633csi said:


> Sorry Dick it won't work with a split phase motor, don't waste your money.  It has to be a brush type motor, or a shaded pole induction type.(fan motor)
> Mark S.



Thank you, Mark.

I went and looked at my motor information plate and it does not say split phase.  I don't know where I got the notion that it was.  How would one know if it is split phase?
If it is not split phase would the motor control work?

Dick


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## chips&more (May 22, 2017)

old toolmaker said:


> Thank you, Mark.
> 
> I went and looked at my motor information plate and it does not say split phase.  I don't know where I got the notion that it was.  How would one know if it is split phase?
> If it is not split phase would the motor control work?
> ...


Show us a pic of the motor.


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## f350ca (May 22, 2017)

Unless it has brushes, you can't change the speed by dropping the voltage. Induction motors use the line frequency to set they're speed Dropping the voltage may slow them a bit as there isn't enough magnetism generated to overcome the load and you get slip. They generate a LOT of heat then and very little torque. 

Greg


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## old toolmaker (May 22, 2017)

f350ca said:


> Unless it has brushes, you can't change the speed by dropping the voltage. Induction motors use the line frequency to set they're speed Dropping the voltage may slow them a bit as there isn't enough magnetism generated to overcome the load and you get slip. They generate a LOT of heat then and very little torque.
> 
> Greg


Greg,
Would I see exterior brush caps if my motor has brushes or are some internal?


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## f350ca (May 22, 2017)

Your split phase phase motor won't have brushes Dick. Things like treadmills use them to easily get variable speed as do hand drills. Most power hand tools use DC motors as they develop high torque in a small package. 
One more caution Dick. Some really old AC induction motors had brushes. They were called repulsive / induction motors. They won't work with a variable voltage either.
Unless the plate says DC or AC/DC it won't slow down. Or not for long anyway.

Greg


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## old toolmaker (May 22, 2017)

Thanks, Greg

I will probably use the drill press as it is. It is an old Toro that I rebuilt and it is wired for forward and reverse, but it is inconvenient to change belt positions on the step pulleys.  If it was a Walker Turner or old Delta it might be worth the effort.

Dick


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## f350ca (May 22, 2017)

Not cheap to do but you could replace the motor with a 3 phase one and use a VFD variable frequency drive to change speeds. 
Or there have been many posts with people using a treadmill motor and the PWM controller to get variable speed.

Greg


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## silence dogood (May 23, 2017)

old toolmaker said:


> This may sound like like a silly question, but how can a motor be both AC and DC?
> 
> Dick


 That is not a silly question, but an excellent question.   AC/DC  or also known a universal motors are usually  small motors that you find on portable tools and small appliances such as vacuum cleaners.   Universal motors are basically a series wound DC motor with some modifications. The main mod is a  added compensating winding so it will run on AC better.  Usually a thyrister controller is used.  You know those lamp dimmers for regular light bulbs. Same type of control.  Since the motor is only 1/4 hp. you may be able to use an old appliance motor with a dimmer switch.  Just make sure the dimmer can handle the current.


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## silence dogood (May 23, 2017)

PS an old sewing machine motor might work.


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## hman (May 23, 2017)

silence dogood said:


> PS an old sewing machine motor might work.


I'll offer a cautionary note here.  Picked up a relatively new looking Singer sewing machine at Goodwill a couple years ago, just to check out the motor and speed controller.  The motor's nameplate says 120V 60W (or about 1/12 HP).  That's way too small for a drill press (which is what the OP was asking about).  Might be OK for a micro drill press, though.

<Edit added> - My used sewing machine motor, previously offered, has been given away.


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## silence dogood (May 23, 2017)

hman said:


> I'll offer a cautionary note here.  Picked up a relatively new looking Singer sewing machine at Goodwill a couple years ago, just to check out the motor and speed controller.  The motor's nameplate says 120V 60W (or about 1/12 HP).  That's way too small for a drill press (which is what the OP was asking about).  Might be OK for a micro drill press, though.
> 
> If anybody wants to experiment with it, I'll be happy to send them the motor and foot pedal for just the cost of postage.  PM me.


 Red face department.  You are right. Should have check it out.


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## rolleikin (May 24, 2017)

old toolmaker said:


> I have an old drill press having a 1/4 hp AC 115 volt split phase motor from the 1950's.
> Can I use a PWM motor controller to control the motor speed?  There are several available on EBay.
> I am looking for a quick way to adjust speed other than changing belts. Most all my work is with smaller drill bits.
> 
> Dick


No. You cannot. AC motors are trickier to control. Some AC motors (like fans) can have some kind of crude speed control.  3 phase AC motors can be controlled very accurately. Same goes for DC motors. Your drill press is not one of them. Use your belts to change speeds. 

Sent from my SAMSUNG-SM-T217A using Tapatalk


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## ammomfg (May 25, 2017)

Unless you don't care about possibly burning up the motor, I don't recommend using PWM, VFD, etc on any older motor, they most often do work, however because of the way these systems work a newer motor design that's better able to deal with the inductive heat is a better deal.

PWM - Pulse Width Modulation is a fancy way of averaging a lower voltage to control a DC motor while still getting the higher torque of using the higher voltage, it's just on average the voltage is lower. This is the simple answer, there's more to it than this, but most simple implmentations will have a pulse rate (usually a few khz) and a duty cycle of whatever the voltage is. So 12V w/60% duty cycle is an average voltage of 8V. The sometimes encountered problem if you're running it quite a bit, is the PWM freq is constantly turning the motor on and off, which can lead to inductive heating in the coils armature, etc. 

VFD - Variable Frequency Drive is much more complicated and is kinda the go-to for most AC motors, the VFD system can change the pulse frequency (so from 60hz AC line to 30hz) it can change the voltage, etc. Unlike a PWM system that operates in open loop control, most VFDs are closed loop. meaning they can "read" motor state, either by measuring back EMF through the feedlines, and a lot of other complex means. I've never seen a VFD burn up a motor, however I won't say it can't happen.


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