I am currently taking a course titled "Electric machinery and drives". We have started with the very basics, i.e with the following equation:

$\tau_{motor} = \tau_{load} + \frac{\partial (J\omega_m)}{\partial t}$

where, $\tau$ is the notation for torque and $J$ is the notation for "polar moment of inertia, $\omega_m$ being the angular velocity of the motor shaft.

My question is: what is the source of the load torque $\tau_{load}$?, i.e is there some external agency that tries to rotate the shaft in the opposite direction?

On a side note, please also explain what the equation simplifies to if the load is just simply a massive flywheel


2 Answers 2


The load torque is from what you want the motor to power or drive be it a saw or conveyor or drill which, depending on the process, may vary.

For example, the load can change on a conveyor depending on the amount of material to be moved - keep increasing the material and the load increases.

  • $\begingroup$ But sir, hasn't that been taken into account by the angular momentum term in the equation? $\endgroup$ Jul 25, 2017 at 14:48
  • $\begingroup$ So what is the torque term for then? $\endgroup$
    – Solar Mike
    Jul 25, 2017 at 15:32

I suspect that moment of inertia terms is that of the rotating parts of the motor itself, note that it disappears when angular velocity is constant.

It doesn't make much sense to model the external load on the motor as a moment of inertia as many common types of load are essentially friction of one sort of another.


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