# How to measure electric motor torque?

I need to measure torque output of a PM DC electric motor. My plan was to use a current clamp to log the current vs time, a voltmeter to measure volts vs time, and an encoder to log rpm vs time. Then I would use the equation T = (7.0429*I*V)/n to get torque vs time. Where T is torque in ft-lb, I is current in A, V is voltage in V, and n is angular velocity in RPM. This comes from the link between power, torque, and rpm, and the 7.0429 is just to get units of ft-lb. I don't have the efficiency and I can't get the stall or free load values because its installed in a machine. I do have a plot of RPM vs torque and Amps vs torque on the same plot. Can I use this to back out torque values as I log current and rpm?

• simplemotor.com/calculations Jan 31, 2017 at 20:23
• To make it simple, torque is (roughly) proportional only to the current. It doesn't rely on voltage. Mechanical power is (roughly) proportional to both current and voltage. Multiplying by voltage then dividing by speed does nothing. So you just have to measure the current and calibrate the value to mechanical power you measured outside. Jan 31, 2017 at 20:43
• You are measuring current, and you have a torque vs current plot. Can you not use these to estimate torque? The difficulty with estimating torque based on the input power is you don't know what your losses are. Jan 31, 2017 at 21:18
• "I need to measure torque... I do have a plot of ... Amps vs torque" - if you can measure Amps then that's all you need! (assuming the plot is accurate....) Jan 31, 2017 at 21:18
• I thought about that, but then there is a "rated power" and its listed at a certain rpm, with certain current, and certain torque. If I take that specified current, and use that value on the plot, and then find the matching torque, it doesn't match what is listed under "rated power" values. The plot says its about 3 ft-lb, where at "rated power" the torque value given was 1.91 ft-lb. Feb 1, 2017 at 12:58