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# Re: Cogging torque

- To: femm@yahoogroups.com
- Subject: Re: Cogging torque
- From: vecerkaj@yahoo.com
- Date: Mon, 26 Nov 2001 07:35:51 -0000
- In-reply-to: <9tr1uf+v1je@eGroups.com>
- User-agent: eGroups-EW/0.82

    --- In femm@y..., robin.cornelius@h... 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

- References:
  - Cogging torque
    - From: robin.cornelius@hirst-magnetics.com

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