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Motor Torque Calculation? — Parallax Forums

Motor Torque Calculation?

JohnBot115JohnBot115 Posts: 13
edited 2010-01-17 06:30 in General Discussion
Hi all,

I'm looking at motors that list some specs, but not all, like torque. I'm trying to figure out how to calculate it. I found some formulas, but feel like I'm missing something.

I'm looking at a scooter motor with these specs: 24VDC, 250W, 3000RPM,·rated at 15A.

Now, it's an electric scooter motor, so I know it must be capable of moving 100 to 200 pounds on a simple geared set up. Eventually, I want to bring it down to about 1000 rpm with a chain and sprocket, and therefore increase the torque. But even with no load, something seems wrong. This is my method and calculation for full load torque, assuming 85% efficiency·(I think full load torque what I need):

HP = V x I x Eff·· =·· 24 x 15 x .85·· =· 306·· =· .410
············· 746······················· 746···············746


T = HP x 5252· =· .410 x 5252· =· 2153· =· .71 ft-lbds
··········· rpm···················· 3000·········· 3000

I must be doing something wrong, or using the wrong calculatioin. Because the torque cannot possibly be under 1 foot-pound!? Can it?

Thanks!

John


Post Edited (JohnBot115) : 1/14/2010 8:32:03 PM GMT

Comments

  • AleAle Posts: 2,363
    edited 2010-01-14 21:12
    why ? it means 1 pound at 20 cm rotating at 3000 rpm...(I use SI units most) I see it ok. With the diameter of the weel you can calculate the rpm needed for a determined spped then you will see the torque greatly increased...

    by the way: [noparse][[/noparse]adimensional] != [noparse][[/noparse]ft*lbf], include the units on your formula or it is meaningless.

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  • stamptrolstamptrol Posts: 1,731
    edited 2010-01-15 17:39
    250 W is about a third of a horsepower. and T = (5252 x HP)/rpm, so your numbers look about right.

    If you gear for 1000 rpm at the output shaft (and motor at 3000 rpm), the torque goes up but the HP remains the same. You'll get about 2 ft-lbs at the output shaft.

    The only reason small motors can do the things we ask of them is they have the ability to operate under overload for some period of time before burning up. So, your 250 W motor may actually be able to draw 30 or 40 amps for
    some minutes to get the load moving, or accelerate or go up a hill.

    As we used to say about car racing............there's really no substitute for cubic inches! If you need more power, start with a bigger motor.

    Cheers,

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    ·
  • ercoerco Posts: 20,260
    edited 2010-01-17 01:11
    The rated wattage is the electrical power input. Expect mechanical power output to be about half that.

    I built an e-bike with parts from TNC scooters·that turned out quite nice, here's a video. I used a 250W gearmotor, which has internal gearing and·comes with a·final output sprocket that takes standard 1/2" bike chain, easy to find at any bike shop, even hardware store. I recommend this over non-geared motors·sprocketed for smaller #25 chain that is harder to locate.

    I've posted previously to·recommend using electric bike parts in robots. Cheap, strong and plentiful. Controllers are designed for hall-effect throttles, so they might require some hacking, I haven't tried that yet.

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  • AJMAJM Posts: 171
    edited 2010-01-17 06:04
    I am no expert in DC motors but I do deal with a great deal of AC motors and rotating equipment.

    Is 50% efficiency a reality in this case? It seems a little low to me. Considering the motor is sized correctly.

    Post Edited (AJM) : 1/17/2010 6:10:02 AM GMT
  • ercoerco Posts: 20,260
    edited 2010-01-17 06:30
    DC motor efficiency certainly varies with the load. Peak efficiency can be·near 70%, but only for very small loads. Peak power output occurs at·50% of·the no load RPM (where about half the stall torque is produced) and·power efficiency is typically near 50%. Lots of heating losses. Mechanical losses from dirty chains and driven belts downstream from the motor will reduce it even further. That's why I suggest the geared motor, which has a sealed, lubricated gearbox. More lube is more efficiency.

    http://www.micromo.com/n390432/n.html·has good DC motor info.

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