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Old 16 Jul 2001, 20:39 (Ref:117930)   #1
duckers
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Electric Motor Torque

This question may not be exactly relevant to motorsport engineering in general, however I think this is the best forum for this question:

Does anyone know how to work out how much torque an electic motor has?
From the way I see it, it would be the magnetic force created by the magnet/field windings multiplied by the distance from the centre of the axle to the edge of the armature/coil.
If this is true, how could I find how to calculate the magnetic force applied?

Thanks

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Old 18 Jul 2001, 08:36 (Ref:118698)   #2
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I'm not sure of any forums.An electrical engineer could definitely help you. OR dick smith catalogs have a torque/speed/load/etc graph in the back with a few formulas !maybe that helps??
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Old 19 Jul 2001, 05:50 (Ref:119079)   #3
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Personally, I'd go with the catalog. Or at worst, dyno-test your motor. If you like doing it the hard way, I can try to explain.

It is possible to mathematically calculate the torque of an electric motor. For simplicity's sake, let's take the 3-arm DC motor (the kind you find on your everyday R/C cars, cd players, etc)

For a current-carrying conductor in an electric field, let:

F=BILsinA (scalar eq)
where F=Force acting on conductor(N), I=current on conductor(A), L = length of conductor immersed in electric field(m), and A is the included angle between the direction of I and the direction of B.

We can now take a motor's coil and divide the coil into very short current-carrying wires. Each elemental length of the coil's wire carries the same amount of current but with different directions. In turn, these elemental lengths of the coil will make contributions to the overall force generated by the coil.

Now we know that we can split the coil into very small pieces of wire that experience small magnetic forces on their own. Like you mentioned, calculating torque is now only a matter of multiplying the distance of these elemental wires by the distance from the axis. However, you have to remember that every single piece of these very short wires vary in distance from the axis.

You also have to remember that as the rotor turns, the electric field strength varies. (depending on design, magnet quality etc)

So there, if you can mathematically describe the magnetic force, coil shape and timing, you can take these equations and stuff them into eg MAPLE. However, the number you get (assuming you get the mathematical model correct) would be the GROSS torque rating, and IMHO, not as useful as simply dyno-testing your motor.
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Old 5 Jun 2009, 15:27 (Ref:2476021)   #4
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http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm has the formula. The rated horsepower multiplied by 5252 divided by the rpm number achieved when they got the horsepower rating.
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Old 5 Jun 2009, 18:48 (Ref:2476124)   #5
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Old 5 Jun 2009, 19:04 (Ref:2476132)   #6
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