# PWM Question  Making a power feed for a mill



## outsider347 (Mar 23, 2013)

Just got a Griz 0704 mill & I'd like to make a power feed for it

I have a 12vdc power window motor that I want to use because it has handy mounting bracket/holes 

I  know it has to be a varriable speed set up & not sure how to achieve this. I see on the bay numerous types of PWM / dc motor controllers.
Question is how do I know which one to choose

Thanks for your time


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## iron man (Mar 24, 2013)

I have bought several kits from this outfit there good to deal with this one should work well for you they have others also.. Here is my home built power feed with an in and out clutch.. Ray

http://www.bakatronics.com/shop/item.aspx?itemid=581


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## outsider347 (Mar 24, 2013)

Ray
Good looking build
What kiind of a DC motor did you use?
tks


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## DMS (Mar 24, 2013)

The controller IronMan linked too looks like a good deal. With a DC motor, a PWM controller like that is the way to go. That and a couple switches will get you forward/reverse and start/stop.

The other option is a cordless drill who's batteries have gone bad. The motors are pretty strong, and most have a variable speed controller.


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## outsider347 (Mar 24, 2013)

If using a cordless drill motor, to varry the speed, do you have to use the trigger?
or can a pot be subbed?
if so what value pot?
tks


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## Hawkeye (Mar 24, 2013)

PWM is Pulse Width Modulation. The controller sends out a pulse-train of direct current to the motor consisting of alternating periods of full voltage and zero voltage. The speed of the motor depends on how much of the time the motor is getting full voltage compared to how much of the time the motor is getting no voltage. All this is happening very fast. Depending on the design, it could be turning on and off a few thousand times every second. A potentiometer sets the duty cycle and controls the speed.

Because the motor is getting full voltage every time the pulse is high, it can put out more torque than if you just turn down the voltage using a potentiometer (actually a rheostat - almost the same). You'd have to use one with a very high current rating to control the window motor and the results wouldn't be satisfactory.


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## xman_charl (Mar 24, 2013)

cordless drill motor, to vary the speed, do you have to use the trigger?

I have several of those drill motors triggers. Got 2 drills, motors where shot, $10.

Use chucks on various machines, thingys...

Use triggers to test small dc motors, get fair speed range but not as good as pwm.

Alligator clips on red and black leads from trigger assembly, go to dc motor.

Tested dc voltage as high as 25 volts, on motors. Read somewhere, trigger will
go higher than 50 volts, current of 10 amps or more.

caution, please don't do this if your not familiar with safety, etc...

Charl


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## iron man (Mar 24, 2013)

The cheap cordless drill motor is the way to go there only about $15 at harbor freight and they have tons of torque stick to the PWM board I showed you I converted one of the trigger speed controls to work on a Pot. and was not really satisfied the board I showed you is the way to go and it would work on your wiper motor as well but the cordless drill gives you a clutch.

 The motor I used was a general dynamics motor for an ICBM it controlled the flaps for the guidance system a very small gearhead motor.. Ray


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## iron man (Mar 24, 2013)

Here is what you are looking for I picked this up brand new for $5 because it was a display and they lost the charger it has been laying around my shop for quite some time but it is one complete unit. The chuck comes off by removing a left hand screw in the center of the chuck then the chuck just screws off normally. The shaft is a 3/8 fine thread so an adapter is easy to make. there is tons of these on the internet just google cordless drill power feed and see what comes up.. Ray

 Also the PWM board I showed you can be repaired with common Radio Shack parts they give you a list as to what componets you have with the board.


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## outsider347 (Mar 25, 2013)

Hey Ray
Thanks again for responding
Looks like theway to go


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## xman_charl (Mar 25, 2013)

My testers for small motors...


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## JohnAspinall (Mar 26, 2013)

Hawkeye said:


> PWM is Pulse Width Modulation. The controller sends out a pulse-train of direct current to the motor consisting of alternating periods of full voltage and zero voltage. The speed of the motor depends on how much of the time the motor is getting full voltage compared to how much of the time the motor is getting no voltage. All this is happening very fast. ... A potentiometer  sets the duty cycle and controls the speed.
> 
> Because the motor is getting full voltage every time the pulse is high, it can put out more torque than if you just turn down the voltage using a potentiometer (actually a rheostat - almost the same). You'd have to use one with a very high current rating to control the window motor and the results wouldn't be satisfactory.



Hawkeye is right; an additional reason _not_ to use a potentiometer directly in the power line has to do with energy loss.  

Think of a regulator or switch as a device that goes from zero resistance to infinite resistance.  A good switch goes _fast_ from zero resistance to infinite resistance.  Why do we care how fast?  At zero resistance, no energy is being lost in the switch -- the energy is all going to the motor.  At infinite resistance, no energy is being lost in the switch -- no current is flowing.  But in between zero and infinite, our regulator/switch is wasting electrical energy as heat.  If you do the math, the worst case is when the resistance of the regulator/switch equals the resistance of the load (motor).  

Using a potentiometer/rheostat in the power flow, you are deliberately settling in this energy-losing middle ground.  Conversely, by using PWM switching, you are doing the opposite -- you are getting from zero switch resistance (full on), to infinite switch resistance (full off) as fast as the digital electronics can make the edges of those pulses.  PWM is a good thing.



Hawkeye said:


> Depending on the design, it could be  turning on and off a few thousand times every second.



If the PWM frequency is in the audio range, you will hear it due to minute flexing of the motor windings turning electrical oscillations into mechanical oscillations.  Some people don't care; some people find the whine annoying.
If you think you're in the second category, make sure the PWM frequency is above the audio range (say above 15-20kHz).  I believe, though others should chime in, that these days faster switching is pretty easy to find, and doesn't cost a lot more.


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