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?
Keep it simple. To measure the torque simply create a transversal arm attached to the motor using a known length. Let the whole motor free to turn (not the shaft, the outside case) along with the attached arm. The arm tip, opposed to where it's attached to the motor, can then be placed on a balance. You can measure the torque now using the balance measure and the arm length.
For example: if length = 0,3m and the balance indicates 100g than your net torque at the specific engine speed equals 0,1Kg x 9,8m/s^2 x 0,3m, in N.m
Be sure to calculate the arm length considering the motor center point to the arm point that touches the balance.
If you can calculate input power vs. speed, then extrapolate to estimate input power and current at stall, that would give you I squared R losses vs current. From that you can calculate a resistance value including brushes and commutator. You could then calculate I squared R losses at any speed and subtract from input power to get a closer value for mechanical power. Without no-load data to determine friction and windage losses, you probably can not get any closer than that to mechanical output power and torque.