Re: [femm] Lorentz force

[David Meeker <dmeeker@xxxxxxxxxxxxxxxxx>]

mercedes sampere wrote:

> Hi femm users!
> Could anybody tell me what does "2x frecuency force mean" when I ask for
> Lorentz force?
> Ex:
> Steady-state force:
> x-component: -3.027513e-002 N/m
> y-component: -1.022735e-004 N/m
> 2x Frequency force:
> x-component: -3.011e-002 - j 9.877e-003 N/m
> y-component: -1.059e-004 - j 5.189e-005 N/m
>
> Thank you very much
> Mercedes.
>

Well, force equals the integral of J X B. If you are evaluating a
time-harmonic problem, current density J varies sinusoidally in time, and the
resulting flux density B also varies sinusoidally in time, with both J and B
oscillating at the same frequency. If you multiply together two sine waves,
you get one portion that is constant with respect to time, and one portion
that varies at twice the base frequency, e.g.:

Sin[t]^2 = (1 - Cos[2*t])/2

Therefore, since the force is the product of sinusoidally varying quantities,
you get a constant component and a 2x component, and what femm is reporting
is the net constant and 2x components. Part of the purpose of making 3 phase
systems is so that the 2x components all cancel out, leaving only the part of
force that is constant with respect to time.

Dave.


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