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How to determine proper motor torque

  1. Oct 2, 2012 #1
    I would like to make an electric motor powered soil sifter for sifting soil/compost, but I'm a bit stumped at the moment. I'd like to use an inexpensive single phase AC motor from a an old vacuum cleaner or a clothes dryer, but I don't know how to determine what size electric motor I would need for my application.

    Basically, the motor has to have enough torque to move a wire cloth screen fast enough below a maximum 20 lb load of soil to shake/scrape the soil into bits so it will easily fall through the screen.

    I've included a few details below to hopefully help whoever can help me to determine what size motor would be right for my application:

    Motor HP: ?
    Motor torque: ? (is full-load torque what I should be calulating? How to compute how much is required?)

    motor (drive) pulley ": 1
    driven pulley ": 8 (a plate & drive pin/rod will be attached to this pulley)
    belt: A style belt

    inside frame dims: (1.5"H x 24"L x 18"W)
    wire cloth: 1/4" square openings
    Wt: 2 lbs

    wt: 20 lbs
    material: soil/compost
    sift rate/second: 2.2 (driven pulley rpm=132)

    I did some HP & torque formula calculations to determine what size motor I thought I might need for my application, but I am not really confident about my numbers. I don't want to use too small of a motor and discover it doesn't work properly or too large of a motor and waste electricity unnecessarily.

    Can anyone let me know how to solve these questions?

    Thanks in advance for any help you are willing to provide.
  2. jcsd
  3. Oct 3, 2012 #2


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    Hmmmm.....interesting question.

    I'll assume you are in USA and using 60 hz.....therefore your AC motor is going to turn at 3,600 RPM. You show a 8:1 pulley ratio which will drop you down to 450 RPM.....but you say you need 132 RPM....so a pulley ratio of 27:1 is what you really need....or perhaps a two stage pulley system.

    The good news is that you just multiplyed your HP or torque by 27!

    I think this is more of a trial and error thing rather than an exact science. Because to figure the force you need...you will need to know the static friction coefficient of the belt. Also, that static of coefficient is going to change with weight and even moisture in the air and so forth. It will never be quite exact so you are sorta of guessing anyway.

    A vacuum cleaner motor might actually work. Try it...if not big enough....try the dryer motor. Pretty sure the dryer motor will handle it no problem.
  4. Oct 3, 2012 #3
    The motor speed given is making assumptions that a 2 pole motor is being used.

    If using a universal motor or brushed DC motor (common in vacuum cleaners and other appliances), the speed is going to be all over the place depending on load applied. In order to calculate the end speed you will need a speed-torque curve for that motor (or experiment).
  5. Oct 3, 2012 #4
    Hello psparky. Thank you for responding to my posting.

    Yes, my AC is 60 Hz. The clothes dryer motor label shows:

    Therefore, the driven pulley should revolve at 215.625 rpm (+/- some amount due to load & line variations, I guess--although, I don't know how to calculate such a variance--and assume that it probably doesn't matter much--or does it?).

    I understand torque increases on the driven pulley by the pulley ratio multiplier, but I wonder how much because:

    A. Full-load torque is in lb-ft/minute. Correct me if I am wrong, but doesn't that mean the force is measured at a radius of 1 ft from the motor shaft center?

    B. If A (above) is true, then what happens to my full-load torque number if I apply it from a 1 inch pulley (which is 1/12 the diameter of the rate FLT number)?

    I searched the net for "static friction coefficient" for wire cloth and couldn't find anything useful. I called a couple of wire cloth manufactures & they didn't have that information. I found the static friction coefficient for galvanized steel on galvanized steel is .14-.23. I imagine the friction between soil & galvanized steel wire cloth would be less than that. Doing a bit of math, I computed the wire area represents about 28%-40% of the area of a standard 1/4" wire cloth (23-18 gauge wire). Is any of that information useful? When I use the static friction coefficients in the F= μ x mass of solid x g formula, I get a lower F=90.0872 and an upper F=148. I understand those number represent horizontal forces (F) needed to move a solid resting on a flat surface. Unfortunately, I don't understand how those forces are related to the motor torque forces that would be needed for my application. Can you share some insight on that relationship?

    Also, If we assumed the following worst case conditions: soil density= 57 lbs/cf, max load=20 lbs and relative air humidity=80%, how would you use that information to compute the necessary torque to move the screen beneath the load properly?

    One other question... the vacuum motor doesn't have a motor data label. I don't have a tachometer or a meter that could be used to measure the rpm or any of the motor torque forces. Do you have any thoughts regarding how I might be able to use a multimeter to determine any of the missing motor info (i.e. RPM, HP, Torque, etc)? I could probably measure the winding resistance, although the only way that I think that number might be useful to me is to partially determine how much current the motor would draw when energized. I imagine additional current would be drawn due to drag (e.g. rotor/bearings, etc).
  6. Oct 3, 2012 #5
    Hello mdjensen22. Thank you for contributing to my thread.

    The vacuum motor doesn't have a data label on it. Is there some way that I can determine how many "poles" the motor has?

    I energized the motor and it seems to spin up to a constant speed. As my load will change when I add soil to the sifter and the sifter sifts the soil, what are your thoughts on using such a motor for a variable load?
  7. Oct 3, 2012 #6


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    Sorry I missed the poles on my post there....shoulda known that.

    But in regards to your pulley question....think about your cars transmission and what is going on there. In first gear you have tons of torque....but very little speed. In 4th gear...you have a lot of speed and very little torque. You can barely get the car moving from a 4th gear start.

    The same amount of power is being applied to the transmission by the motor either way. The same thing with your pulley...you are basically going to be stuck in 1st gear which is exactly what you need..

    Here's a nice little dissertation on that:

    So HP is derived from torque. (Torque X RPM)/5252 = HP
    So as you can see....as torque goes up....RPM must go down in this equation. And as RPM goes up...torque must go down.

    The more weight and the more friction you have...the more force you will need to move the belt. If your belt were lubed and there were no weight on it....it would move pretty easy. But you will have plenty of weight and friction going on....so there is going to be an decent opposing force in the horizontal direction....which is just a load on your spinning pulley.

    How about looking up on Google one of these machines? What size motors are they using? Take a looksey and do the same.
    Last edited: Oct 3, 2012
  8. Oct 4, 2012 #7
    I think it would be better to describe it this way: In first gear, the smaller (drive) gear transfers engine torque (rotational force) to the relatively larger (1st) gear. The smaller gear rotates much faster than the larger gear. The available torque is transferred across both gears in proportion to the gear ratio (less torque on the smaller/faster rotating gear & more torque on the larger/slower rotating gear). Power (HP) represents the accumulated work (force x distance) generated over a period of time (e.g. 550 lb-ft/second).

    Last edited by a moderator: May 6, 2017
  9. Oct 5, 2012 #8


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    You should look intor statics and dynamics a bit. Really intersting subjects. You could learn them on-line or get a book or whatever.

    The use of SIN and COS is key for these subjects. They all tie back together with the sin and cos and vectors used with electricity as well. Or you get get the x and y direction vectors all the same. Same, same.

    Without the knowledge of statics and dynamics....anyone will be in the dark about understanding a great many things.
  10. Oct 5, 2012 #9
    I have a textbook on the subject. I'll look into what it conveys on force system resultants. Maybe that will help. It's been a while since I've used trig, integration, or calculus.. It may be over my head at this point ;)

    Thanks for your help.
  11. Oct 5, 2012 #10

    jim hardy

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    you're not in over your head.

    To figure out torques, freeze frame it in your mind and draw a picture.
    Torque is F X D so has same units as work, that can be confusing.
    Horsepower is: 2 X pi X torque X RPM/33,000
    and that's a neat derivation - a hp is 550 ft-lbs of work per minute...

    that should give you ballpark machine size you need

    beware of vacuum cleaner motors, many of them are universal type series field and with no load are capable of tens of thousands of rpm, so if a belt breaks it might fly apart from centrifugal force. I did that as a kid and embedded metal in the walls and floor, fortunately none of the shrapnel hit occupants of the room.

    old jim
  12. Oct 6, 2012 #11
    Hi Jim. Thanks for replying to my thread and for your encouraging words!

    That is true for static torque, but motor torque is expressed as force*distance/time just like horsepower is expressed as force*distance/time.

    Hmm.. I have never seen that equation. How did you derive it?

    That is an error. 1 hp = 550 lb-ft/second (derived from: 33000 lb-ft/min * 1 minute/60 seconds).

    From what I have read, Watt and some of his contemporaries agreed to round up ~32572 lb-ft/min to 33000 lb-ft/minute--which is our current definition for 1 horsepower.

    It is my understanding that ~32572 lb-ft/min was calculated by Watt as follows: 180 lbs carried by horse around a 24 ft diameter wheel for 144 revolutions in 60 minutes. Therefore,:

    1 horsepower = work / time
    work = force * distance

    1 hp = f * d / t
    1 hp = 180 lbs * (24 ft * pi * 144 revolutions) / 60 minutes
    1 hp = 180 lbs * ~10857 ft / 60 minutes
    1 hp = ~1954322 lb-ft / 60 minutes
    1 hp = ~32572 lb-ft/minute

    If we adjust for the current 33000 lb-ft/min horsepower definition and use the original distance (~10857 ft), we can then compute the new initial force is:
    Initial Force (in lbs) = 33000 lb-ft/min * 60 min / ~10857 ft
    Initial Force (in lbs) = 1,980,000 lbs-ft / ~10857 ft
    Initial Force (in lbs) = ~182.4 lbs

    Doing those calculations also allows us to see the new work (i.e. F*D) = 1,980,000 lb-ft = (~182.4 lbs * ~10857 ft). This a nice even number that represents the total work done in 1 hour.

    I'm not sure what you mean. The essence of my original question is regarding how to determine the amount of force I will need to drive a screen under a 20 lb load of soil/compost to sift it properly. The solution method should result in a minimum force (torque) required to sift the 20 lb load properly. That seems to be a shear force problem to tear through the soil with each driven pulley revolution + some force to overcome initial inertia & friction (bearings, wheels, etc).

    Thanks for the tip on universal type vacuum motors. I guess the only way to see whether it is a universal type motor is to fire it up without a load & check the rpm. Would you know how I might be able to measure the motor rpm without a tachometer? :)
    Last edited: Oct 6, 2012
  13. Oct 7, 2012 #12
    Why dont you take a sewing machine, turn it on its side weld your sifter to a shaft and the shaft to the sewing machin. or use the sewing machine motor.
    I used one for a water pump and boy did it shoot water
  14. Oct 7, 2012 #13
    I don't have an old sewing machine to use. I can see how it might be a good thing to use--especially because it has a variable speed controller.
    Last edited: Oct 7, 2012
  15. Oct 8, 2012 #14


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    Variable speed is the ticket. Look on Ebay or classifieds for a sewing maching motor. You could prob find one for $10 I bet. Now you are going to ask.....what HP sewing machine motor should I get.....right:)
  16. Oct 8, 2012 #15


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    In basic statics and dynamics....there is little to no integration or calculus. Simple algebra will do the trick quite nicely. Setting up the problem with the correct x and y equations will be the only challenge. And that gets real simple with a little practice. Again, you can either use sin and cos....or you can use trigonometry to get the sides of the triangles or angles.

    Don't forget the old Indian name for this trig handed down generation to generation: SOH-CAH-TOA

    Prononced: So-cuh-toe-a

    SOH = Sin ∅ = Opp/Hyp
    CAH = Cos ∅= Adj/Hyp
    TOA = Tan ∅= Opp/Adj

    Great phrase to memorize. Catchy too.
    Last edited: Oct 8, 2012
  17. Oct 8, 2012 #16
    Haha... not really. It will either have enough power or it won't. But, I would still like to know the minimum oscillation speed of the sifter basket :). Assuming I found such a motor & got it setup, do you have any idea how I might be able to easily determine the shake speed if I don't have a tachometer?
  18. Oct 8, 2012 #17
    I have already calculated the torque per S and torque per revolution at the drive pulley & driven pulley. I just thought I might need to integrate the torque per 1/2 revolution in order to get the full amount of work done per screen oscillation. As for calculus, I only mentioned it because I thought I might use it to determine the instantaneous torque at a particular moment--which I thought might useful for comparison purposes to my other torque figures.

    Thanks for the basic trig mnemonic reminder. I remember the basic trig functions with the phrase "Oscar has a handful of apples." O/H=S∅, A/H=C∅, O/A=T∅
    "SOH-CAH-TOA" seemed harder to remember for me because phonetically I always wanted to spell it as "SO CAW TOE" ;)

    Since I have the .2 hp motor (which will be way overkill) and the pulleys, I'm going to make my 1st design/prototype using those components.
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