Re: Cogging torque

[vecerkaj@xxxxxxxxx]

--- In femm@xxxx, robin.cornelius@xxxx wrote:
> Dear group,
> 
> I am trying to calculate the cogging torque for a brushless rotor, 
> but I am having some fundimental problems (this is not my area of 
> expertiese !).
> 
> What intergral should i look at ? . I have tried a line intergral 
> around the air gap between stator and rotor (with a reasonable 
dense 
> mesh) and looked at the torque from stress tensor. (I am then 
steping 
> the rotor around to see the value at different pole pitches).But 
the 
> magnitude of the results is too large and the pitch/value graph is 
> not quite what i would expect. I remember that David M. mentioned 
> coenergy in a past email in relation to cogging torque? if this is 
> good to use over what areas should this be evaluated 
(iron,air,magnet 
> all?)
> 
> Any advice greatly appreicated.
> 
> 
> Robin Cornelius
> 

Hi Robin

Some time ago I also calculated cogging and I got result about 10 
percent different from data obtained by a measurement. Also the graph 
turn angle of rotor vs cogging torque looked good (sinus). There are 
two aspects, you should consider.
1. Usually permanent magnet motors have slots skewed (to decrease 
cogging) and you can't simply calculate cogging in 2D software (but 
you can at least estimate cogging by integrating of cogging value 
trough corresponding turn angle).
2. There is a limited usage of periodic/antiperiodic boundary 
conditions, because you can't calculate torque on boundary (it 
doesn't work, I tried that once :-) )

Good luck

Jiri Vecerka