# Calculating Brushless Motor's Torque

1. Mar 25, 2013

### iamdenis

Hello,

I was wondering if anyone knows about calculating the torque of a brush less motor (preferably an out-runner).

Is there a way to get the torque based on resistance, number of turns, and number of poles?

Thank you,
Denis

2. Mar 26, 2013

### milesyoung

An analytical expression for the instantaneous torque developed by some brushless DC motor is somewhat complicated and it will probably not be very helpful to you.

What usually matters in the case of brushless DC motors is that they're designed in such a way that the average torque they develop (over an electrical cycle) is directly proportional to the armature current you supply them with. This torque/current proportionality constant is commonly called the torque (or motor) constant and is usually specificed in the datasheet of the motor.

Last edited: Mar 26, 2013
3. Mar 26, 2013

### jim hardy

Do you remember from your basic physics the torque on a loop in a magnetic field?
$T=NIABSin\Theta$

So the torque includes in addition to the things you mentioned magnet strength and geometry.

I'd wager it would be easier to measure than to calculate.

Or as Miles suggested - look up its datasheet.

Last edited: Mar 26, 2013