Torque of a 393 motor

Over the past weeks I have been doing some motor test on the 393motor. (i.e. speed of a motor, amps pulled and rpm) Well I have done this test to the best of my ability, I have not been able to achieve the stall torque listed online. ( I did not understand this and that gave me the idea to use the info that was listed online provide by Vex. When I inputted that info into the equation, I was given a much different stall torque then what is listed online. That brings me to my question. How does VEX find the stall torque of a motor? And what is the tolerance?

The equation that I have been using for the test is:


Using the info given online this will give a torque of 2.43ft-lbs or 29.16in-lbs. This is not the 14.76in-lbs listed

Thanks for the question! I believe jpearman has done a great job explaining the error from an efficiency standpoint here ( but I’d like to take a crack at a my own explanation. Hopefully after reading them both you’ll have a better understanding of the physics at work.

Let’s start out with the mechanical definition of motor power:
P_mech=ω*T – That is mechanical power is equal to the product of rotational speed and Torque

As well as the definition of electrical power:
P_elec = V*I

And a conservation of energy equation written for electric motors:
P_elec=P_mech+P_heat – Electrical power into the motor is equal to the sum mechanical power produced by the motor and the heat generated in its windings. (Remember, heat is a form of power.)

From these equations, it is easy to see that at stall there is actually no mechanical power being produced by the motor (P_mech=0T=0) but there is a significant amount of heat being generated instead (P_elec=0+P_heat -> P_elec=P_heat=VI=7.2V*4.8A=34.6W)

To put these equations into words, the electrical power put into a motor comes out in two different forms – mechanical power and heat. In an ideal motor, electrical power gets converted to mechanical power by the motor with 100% efficiency – that is no heat is generated. Since we don’t live in an ideal world though, heat is always generated when a motor converts electrical power to mechanical power – the ratio of mechanical power out to electrical power in is efficiency (P_mech/P_elec=efficiency).

A motor’s efficiency varies depending on a variety of factors, but the largest one is the speed the motor is rotating. At stall the motor’s efficiency is 0% (0W/34.5W=0% Efficiency) – in short you’re equations assume 100% mechanical efficiency when it’s actually 0%.

To answer your question about how VEX determined the stall torque of the motor – we attached the output of a motor to a bar of a known length and stalled it. We used a small scale to measure the force at the end of the beam and found torque from that (T=r x F).

Hopefully this has been more enlightening than confusing!

Feel free to ask any other questions you have,


So basic the only way for myself to find the torque of the motor is to use a scale? There is not equation that I could plug my numbers in to? Thanks for all the help, this my first try at physics.

Essentially testing is the only way to determine the stall torque of a motor. You can use a motor’s stall current and torque constant to determine it’s stall torque, but determining the torque constant requires testing.

It is possible to predict motor performance using calculations based on the windings in the motor, but this is very complicated and not as accurate as physically testing the motor.

Here is a good article that explains how to determine & use a motor’s torque constant -

  • Charlie

Thanks i will have a read of that. thanks for all the help