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speed controllers and torque

Started by kelLOGg, November 07, 2005, 04:36:04 PM

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kelLOGg

I made my own motorized sawhead feed on my MP-32 (Cook) consisting of a 12 VDC .07 HP PM motor (2.56 in-lbs torque) and a 100:1 gear reducer with an output of about 15rpm. The final sawhead advance speed is selectable by sliding chain-driven sprockets into position to be turned by the reducer output. The original intent was to provide motorized operation for big logs only. (A mechanical clutch at the sawhead allows for disengagement so I can easily push the head thru small cuts and return the head manually). This has worked OK and gives me lots of force at the sawhead and slow speeds (5 - 10 feet/min) for sawing big stuff. However, now I’m planning to motorize the sawing of small stuff and the return too. Since this will require a broader range of speeds (0 to 120 feet/min or so) I think I need a PWM speed controller and a gear motor with higher rpm.  Using a PWM speed controller confuses me as to what torque/HP/rpm I need. Does torque stay constant over the speed range of the PWM speed controller? If it does and rpm changes, then HP must change too. Can anyone shed light on this and advise me as to what specs are needed in a 12 vdc gearmotor operated by a PWM controller to get reasonable feed and return rates? ???
Cook's MP-32, 20HP, 20' (modified w/ power feed, up/down, loader/turner)
DH kiln, CatClaw setter and sharpener, tandem trailer, log arch, tractor, thumb tacks

D._Frederick

For the carriage drive on my sit-on-the-ground bandmill, I have a 1/4hp, 90rpm, permanet magnet gearmotor with a 3 inch capstan pulling a 3/16 inch cable. It give me about 75 lbs pull @ 0 to 80 ft per min. Since my mill is all electric, I am using a 90V dc SCR drive.

Fla._Deadheader


  We use the PWM on a 12V DC ¾HP motor. It is geared 2" pulley on the motor and 8" on the jackshaft. Slight trick involved with the woring, but, easy as fallin down, and we can flat out stall the engine in a cut.

  We used chain and sprocket drive, just like Wood Mizer.
All truth passes through three stages:
   First, it is ridiculed;
   Second, it is violently opposed; and
   Third, it is accepted as self-evident.

-- Arthur Schopenhauer (1788-1860)

Brucer

Quote from: kelLOGg on November 07, 2005, 04:36:04 PM
Using a PWM speed controller confuses me as to what torque/HP/rpm I need. Does torque stay constant over the speed range of the PWM speed controller? If it does and rpm changes, then HP must change too.

PWM (Pulse Width Modulation) switches your motor on and off at a relatively high frequency. The controller adjusts the proportion of "on" time to "off" time (the pulse width). During the "on" portion of the cycle the system feeds full voltage to the motor so you get the full torque. You get full horsepower during the "on" portion of the cycle, but the average horsepower is proportional to the "on" time.

All you really need to design for is the maximum speed condition. There is usually a slight voltage drop through the controller -- say around 10%.  Size the motor to give you the speed and torque you would need if the motor were wired directly to the battery. Than add 10% or so to the power or torque to compensate for the voltage drop.

Bruce    LT40HDG28 bandsaw
"Complex problems have simple, easy to understand wrong answers."

kelLOGg

Quote from: Brucer on November 08, 2005, 12:35:28 AM
Quote from: kelLOGg on November 07, 2005, 04:36:04 PM
Using a PWM speed controller confuses me as to what torque/HP/rpm I need. Does torque stay constant over the speed range of the PWM speed controller? If it does and rpm changes, then HP must change too.

PWM (Pulse Width Modulation) switches your motor on and off at a relatively high frequency. The controller adjusts the proportion of "on" time to "off" time (the pulse width). During the "on" portion of the cycle the system feeds full voltage to the motor so you get the full torque. You get full horsepower during the "on" portion of the cycle, but the average horsepower is proportional to the "on" time.

All you really need to design for is the maximum speed condition. There is usually a slight voltage drop through the controller -- say around 10%.  Size the motor to give you the speed and torque you would need if the motor were wired directly to the battery. Than add 10% or so to the power or torque to compensate for the voltage drop.


Hey Brucer,
Thanks for the clarity. I dropped the concept of the PWM controller in my design and focused on just the motor. At first, the maximum speed condition as the determining factor seemed anti-intuitive. But then I did the math. See if my concepts are sound:

I have a 1 inch radius sprocket driving the saw head. To return the sawhead at 120 feet/min it must rotate at 240 rpm and exert 40 lbs of force initially to move it: 240 rpm @ 40 in-lbs is 0.15 HP.

When sawing at 4 feet/min the driving sprocket must rotate at 8 rpm and produce, say, 150 lbs of force: 8 rpm @ 150 in-lbs is 0.02 HP!

Just as you said the max speed is the critical design parameter.

Thanks!!!  If I have done this correctly, this is exactly the info I need.

Bob
Cook's MP-32, 20HP, 20' (modified w/ power feed, up/down, loader/turner)
DH kiln, CatClaw setter and sharpener, tandem trailer, log arch, tractor, thumb tacks

Fla._Deadheader


You still outta have variable feed speed. ;D
All truth passes through three stages:
   First, it is ridiculed;
   Second, it is violently opposed; and
   Third, it is accepted as self-evident.

-- Arthur Schopenhauer (1788-1860)

Brucer

Quote from: kelLOGg on November 08, 2005, 09:20:55 PM

I dropped the concept of the PWM controller in my design and focused on just the motor. At first, the maximum speed condition as the determining factor seemed anti-intuitive. But then I did the math. See if my concepts are sound:

I have a 1 inch radius sprocket driving the saw head. To return the sawhead at 120 feet/min it must rotate at 240 rpm and exert 40 lbs of force initially to move it: 240 rpm @ 40 in-lbs is 0.15 HP.

When sawing at 4 feet/min the driving sprocket must rotate at 8 rpm and produce, say, 150 lbs of force: 8 rpm @ 150 in-lbs is 0.02 HP!

Sorry for the slow respoinse, Bob. Been up to my ears in snow and sawdust for the last week ;D.

The math and concepts look good. Don't forget the "real world" ...

  • You will probably want to move the head forward at close to top speed from time to time -- not while sawing but for maintenance, cleaning, blade changes, etc.
  • If the gearmotor is sold as a unit, make sure it will supply the calculated torque at the speed you want. There will be friction losses in the gearbox so the motor will need to supply more power than the calculations show.
  • You will have to overcome some friction in your drive system. Even a chain drive (e.g., similar to Wood-Mizer's) will have some drag. Add a bit extra power to deal with this. The tighter your chain, the more drag you'll have.
  • It sounds like you plan to connect the gearmotor directly to the drive, which is simplest. If you have to use a belt drive somewhere between the motor and the drive sprocket, watch out for the friction losses -- with a small motor they can be as high as fifty percent of the motor horsepower.

Quote from: Fla._Deadheader on November 08, 2005, 09:40:47 PM

You still outta have variable feed speed. ;D

If I understand Bob correctly, he's just ignoring the variable speed controller while doing the power calculations.
Bruce    LT40HDG28 bandsaw
"Complex problems have simple, easy to understand wrong answers."

Fla._Deadheader


OK. Might want to belt drive anyway. We have had the belt slip a bit, when in a bad situation, saving a blade and the adjustable guide arm. As they say, "Stuff happens", and a safety built in sure can't hurt.  ;)
All truth passes through three stages:
   First, it is ridiculed;
   Second, it is violently opposed; and
   Third, it is accepted as self-evident.

-- Arthur Schopenhauer (1788-1860)

kelLOGg

Quote from: Brucer on November 13, 2005, 09:30:29 PM
Quote from: kelLOGg on November 08, 2005, 09:20:55 PM

I dropped the concept of the PWM controller in my design and focused on just the motor. At first, the maximum speed condition as the determining factor seemed anti-intuitive. But then I did the math. See if my concepts are sound:

I have a 1 inch radius sprocket driving the saw head. To return the sawhead at 120 feet/min it must rotate at 240 rpm and exert 40 lbs of force initially to move it: 240 rpm @ 40 in-lbs is 0.15 HP.

When sawing at 4 feet/min the driving sprocket must rotate at 8 rpm and produce, say, 150 lbs of force: 8 rpm @ 150 in-lbs is 0.02 HP!

Sorry for the slow respoinse, Bob. Been up to my ears in snow and sawdust for the last week ;D.

The math and concepts look good. Don't forget the "real world" ...

  • You will probably want to move the head forward at close to top speed from time to time -- not while sawing but for maintenance, cleaning, blade changes, etc.
  • If the gearmotor is sold as a unit, make sure it will supply the calculated torque at the speed you want. There will be friction losses in the gearbox so the motor will need to supply more power than the calculations show.
  • You will have to overcome some friction in your drive system. Even a chain drive (e.g., similar to Wood-Mizer's) will have some drag. Add a bit extra power to deal with this. The tighter your chain, the more drag you'll have.
  • It sounds like you plan to connect the gearmotor directly to the drive, which is simplest. If you have to use a belt drive somewhere between the motor and the drive sprocket, watch out for the friction losses -- with a small motor they can be as high as fifty percent of the motor horsepower.

Quote from: Fla._Deadheader on November 08, 2005, 09:40:47 PM

You still outta have variable feed speed. ;D

If I understand Bob correctly, he's just ignoring the variable speed controller while doing the power calculations.
Quote from: Brucer on November 13, 2005, 09:30:29 PM
Quote from: kelLOGg on November 08, 2005, 09:20:55 PM

I dropped the concept of the PWM controller in my design and focused on just the motor. At first, the maximum speed condition as the determining factor seemed anti-intuitive. But then I did the math. See if my concepts are sound:

I have a 1 inch radius sprocket driving the saw head. To return the sawhead at 120 feet/min it must rotate at 240 rpm and exert 40 lbs of force initially to move it: 240 rpm @ 40 in-lbs is 0.15 HP.

When sawing at 4 feet/min the driving sprocket must rotate at 8 rpm and produce, say, 150 lbs of force: 8 rpm @ 150 in-lbs is 0.02 HP!

Sorry for the slow respoinse, Bob. Been up to my ears in snow and sawdust for the last week ;D.

The math and concepts look good. Don't forget the "real world" ...

  • You will probably want to move the head forward at close to top speed from time to time -- not while sawing but for maintenance, cleaning, blade changes, etc.
  • If the gearmotor is sold as a unit, make sure it will supply the calculated torque at the speed you want. There will be friction losses in the gearbox so the motor will need to supply more power than the calculations show.
  • You will have to overcome some friction in your drive system. Even a chain drive (e.g., similar to Wood-Mizer's) will have some drag. Add a bit extra power to deal with this. The tighter your chain, the more drag you'll have.
  • It sounds like you plan to connect the gearmotor directly to the drive, which is simplest. If you have to use a belt drive somewhere between the motor and the drive sprocket, watch out for the friction losses -- with a small motor they can be as high as fifty percent of the motor horsepower.

Quote from: Fla._Deadheader on November 08, 2005, 09:40:47 PM

You still outta have variable feed speed. ;D

If I understand Bob correctly, he's just ignoring the variable speed controller while doing the power calculations.

That's correct. To simplify matters I ignored friction and the speed controller and computed the HP necessary to saw and return the head. The results were that it takes far more HP to return the head than to saw. (Sorta anti-intuitive but the math don't lie). Now I'm looking for a surplus or used 0.25 HP 12V DC gear motor with an output speed of about 125 - 250 rpm. (It will connected via a chain drive to the shaft). That'e 67% more than the calculated theoretical 0.15 HP and hopefully is enough to overcome friction, etc., and not be so big as to unnecessarily drain the battery operatin :Pg it.
If you know of sources for such surplus equipment - that will be appreciated. Ive been buying from the Surplus Center for 30+ years but they don't currently have it. I surf the net regularly and have found many surplus places but not what I want. (too bad I can't use 90 volts but my mill is portable and can't rely on electricity being available.)
Cook's MP-32, 20HP, 20' (modified w/ power feed, up/down, loader/turner)
DH kiln, CatClaw setter and sharpener, tandem trailer, log arch, tractor, thumb tacks

Brucer

There was a good source of 12 V. surplus gearmotors here in Canada until about 5 years ago. Then their supply dried up. My friends in the amateur robotics field tell me these things get snapped up as soon as they appear on the market. One possible source they suggest is salvaged wheelchair motors (don't know if those have enough power).
Bruce    LT40HDG28 bandsaw
"Complex problems have simple, easy to understand wrong answers."

DanG

Welcome to the Forum, Bob. :)  I hope you'll stick around and let us know how all this turns out. :P

Just one thought on practicality;  If it isn't cost-prohibitive, you might want to go with a new motor instead of surplus.  It might be easier to get an exact replacement if you ever need to.
"I don't feel like an old man.  I feel like a young man who has something wrong with him."  Dick Cavett
"Beat not thy sword into a plowshare, rather beat the sword of thine enemy into a plowshare."

highpockets

I just found this thread and am interested in some electric drive things.  Where are you getting the PWM controllers? 
Louisiana Country boy
homemade mill, 20 h.p. Honda & 4 h.p. for hydraulics.  8 hydraulic circuits, loads, clamps, rotates, etc.

Fla._Deadheader

 Bought ours off Ebay, from a guy in Kaliforney. Had some problems in the beginning, but, a member got us going and we haven't looked back. Bought a spare and it's sittin waitin. 80 Amp rating and you need a drum switch OR 4 solenoids and a double pole double throw switch for forward-reverse.
All truth passes through three stages:
   First, it is ridiculed;
   Second, it is violently opposed; and
   Third, it is accepted as self-evident.

-- Arthur Schopenhauer (1788-1860)

mike_van

Not sure if mine's PWM, but I used a Dart control from Grainger, 6Z386, about 190.00 now.  It powers a 1 hp shunt wound DC motor, direct coupled to a right angle gearbox [can't remember the ratio]  which turns [by roller chain] a 3/4 shaft that pulls the head back & forth from both sides.  Allmost 15 years now, still going.
I was the smartest 16 year old I ever knew.

woodbowl

I'm hoping that there is an application already out there on another product that has both the controller and the 3/4 to 1 hp motor, rather than matching things up or building the unit. It is probably right under our nose.
Full time custom sawing at the customers site since 1995.  WoodMizer LT40 Super Hyd.

kelLOGg

Hey guys,

I'm back on the air with more question about selecting a gearmotor for driving the head advance. Below is a quote from earlier in this thread so you will know the context of my question.

"Now I'm looking for a surplus or used 0.25 HP 12V DC gear motor with an output speed of about 125 - 250 rpm. (It will connected via a chain drive to the shaft). That'e 67% more than the calculated theoretical 0.15 HP and hopefully is enough to overcome friction, etc., and not be so big as to unnecessarily drain the battery operating it. "

I found a wheelchair gearmotor at Surplus Center (item #5-1444-5) but there is insufficient info on it and SC cannot produce more. What I do know is this: 24 VDC, 7 amps NO LOAD, 170 rpm. I can get a PWM speed controller with a max capacity of 49 amps. Under operation do you think the gearmotor will draw more than 49 amps under the operating condition described in the quote above? (It already draws 7 amps at idle and that's equivalent to 0.22 HP electrical input HP). It is probably more motor than I need but if it does not blow the PWM that's OK. I don't think there is enuf info to do a calculation so I'm relying on you guys with more experience for guidance. Thanks

Bob
Cook's MP-32, 20HP, 20' (modified w/ power feed, up/down, loader/turner)
DH kiln, CatClaw setter and sharpener, tandem trailer, log arch, tractor, thumb tacks

woodbowl

Bob, I've been searching for the same info in this thread. There are bits and pieces of info here. Hope it helps.

https://forestryforum.com/board/index.php?topic=17738.msg255338#msg255338
Full time custom sawing at the customers site since 1995.  WoodMizer LT40 Super Hyd.

Kbeitz

Check tarp gearmotors on E-bay.
If you dont want a controler check out the Z-max gearbox.
Collector and builder of many things.
Love machine shop work
and Wood work shop work
And now a saw mill work

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