Motor torque (momentum) is one of motor ratings which is used to indicate motor rotary force produced on its output shaft. Motor torque rating is usually captured at point when motor is stalled. Torque measurement unit is (Nm) in metric system or (ft lbs) in US system. But principle is the same torque is a multiplication of lever length (r) and force(F) applied to lever:

τ=r x F

So there is a simple experiment on how to measure motor torque quite accurately without any fancy measuring tools.

All you need is a motor of which torque will be measured, nominal power supply of motor, thread long enough, a mass of known weight, ruler and a table or other lifted surface where motor will be fixed.

Attach mass to one end of string and other end tie to motor shaft so that it could turn the windings. Fix the motor at the edge of table so that string with mass would hang. Start the motor and led the rotating shaft to wind the thread while lifting the mass. At the beginning motor will turn very easily but later when radius of winding will increase so the stopping force will increase also. And at last motor will stall. So this i the moment when you have to stop the motor and measure the radius of winding. Then take the formula and calculate the torque. For instance if mass m=0.1kg; radius of winding r=0.01m; acceleration due to gravity g=9.8m/s².

τ=0.01 · 0.1 · 9.8=0.0098 ≈ 0.01(Nm)

if motor will windup all thread try to attach heavier mass. This technique will help to compare different motors or test motors torque and compare to documentation to see how reliable is this technique.

Robots Builder Guide 1992 - Fred G. Martin

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This entry was posted on Wednesday, October 3rd, 2007 at 12:10 am and is filed under Electronics Tutorial. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.