Calculating parameters of dc brushed geared motor

Thread Starter

Saviour Muscat

Joined Sep 19, 2014
Hello everyone,

Trust this thread finds you in good health,

I am trying to approximate the moment of inertia(J) of dc geared brushed motor rotor. I found interesting in a academic paper by rule of thumb that J=10*B, (B is the viscous friction) of a BLDC motor. All I want to know, if this rule of thumb applies as well on a geared dc brushed motor rotor so I could be able to apply my approximated B to the formula J=10*B to find my moment of inertia J.

Thank you and stay safe,



Joined Sep 17, 2013
All I want to know, if this rule of thumb applies as well on a geared dc brushed motor rotor
I would be surprised if it did. For starters, there are several components to viscous friction: windage of each rotating part, bearing friction and gear-tooth friction. Each of these would be dependent on component geometry, clearances and mechanical tolerancess. Even for a BLDC motor the rule of thumb may depend on whether the motor is an in-runner or out-runner type? The full text of the linked paper is unavailable to check the background to this rule of thumb and its origins.
That said, some degree of proportionality between motor size, rotor moment of inertia and losses due to friction etc is to be expected.
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Joined Nov 6, 2012
The question is "why" are You trying to calculate this ?

You can radically change the behavior of the Motor by designing
an appropriate Power-Supply and Circuit to control it.


Joined Jul 18, 2013
The paper mentions determining the number of poles of a BLDC motor. but was cut off.
A very simple method is to short the 3 conductors to the motor and turn the shaft and count the number of 'bumps' per rev.