Calculating Brushless DC Motor's output Torque

Discussion in 'General Electronics Chat' started by Terp, Jul 29, 2008.

  1. Terp

    Thread Starter Active Member

    Jun 6, 2008
    I want to calculate the torque of the brushless DC motor for my ornithopter. This is what I wanted to do:

    Measure the in-coming current into the motor, measure the out-going current going from the motor to the Electronic Speed Controller (ESC). The difference in current multiplied by the voltage would be the output power. Measure the rpms and then -- (torque = output power / (rad/s)).

    I tried this out yesterday. I had two multimeters in current-measuring (mA) modes: One that measures current into the motor, and the other that measures current out of the motor. With this setup, the motor would hardly move. Like the shaft rotates, but not at its full speed -- I mean it makes one complete rotation in like 2 seconds. Then, I got rid of the multimeter that was supposed to measure the current going out of the motor to the speed controller. So, with only one multimeter, the motor did operate at a full speed. However, this way I cannot get the current going out of the motor into my speed controller, and this defeats the purpose of calculating the output power.

    So, my question -- is it because of high combined internal resistances of two multimeters, that very little current was going into the motor (causing bizarre 1 revolution in 3 seconds)? Please also let me know if my measurement method is flawed, and suggestions will be greatly appreciated.

    Thanks :)
  2. scubasteve_911

    AAC Fanatic!

    Dec 27, 2007
    Yeah, you're probably right. If your multimeter isn't mean to measure really high currents, like those experienced with hobby RC brushless motors, then you'll either blow a fuse or not have enough current. It depends on how they did it inside the multimeter.

    If you aren't doing it for a long time, you can use a small resistor in series with the load. Then, you can measure the voltage across the resistor and calculate current. Beware that you will experience a predictable loss across the resistor and factor that into your equation. P = I^2 * R

    You are also assuming a torque coefficient that is constant, which may mess with your readings. Unfortunately, you can only infer a torque measurement.

  3. jpanhalt


    Jan 18, 2008
    I am not sure what your set-up is when you refer to current going into the motor and from the motor to the controller. Most set-ups have the controller between the motor and the battery. Also, remember that although it is called a brushless DC motor, it is actually similar to a 3-phase AC motor. There should be three wires to the motor. Your DC milliammeter may simply not respond. The current you should be seeing is several amps. The internal resistance of your meter is certainly part of the story too.

    As an alternative, put a regular propeller on your motor. Go to any one of several sites that provide on-line calculation of power in watts based on rpm and propeller size. I find Neu Motors easy to use, but there are others.

    Once you know the output of your motor, you can back calculate to current draw based on the efficiency given, or just use a number like 85%.

    Last, you can put an ammeter between the battery and ESC to find power in. It will need to be a very low resistance meter. You do need a load on the motor to get meaningful numbers.