Re: [femm] Calculation of screen current

[David Meeker <dmeeker@xxxxxxxxxxxxxxxxx>]

Winfried Graupner wrote:
>   Dear All,
>
> First of all many thanks to David Meeker for this great program.
> I use this tool for EMC related issues.
> I would like to address a problem in conjunction to calculation of 
> currents in screened cables.
> The arrangement (see file: Screen current.zip in FEMM file directory) 
> consists of three single screened cables of a three-phase system.
> The cables are laid in parallel at a certain distance. All screens 
> are connected to each other but they are isolated from the domain 
> boundary. 
> The magnitude of the screen current depends on the mutual coupling, 
> the frequency and the impedance of the representative screen loop 
> (see file: Screen current.zip in FEMM file directory).
> For low frequencies the resistor in this loop cannot be neglected 
> against the reactance of this loop. This disruptive factor is the 
> cause, why a cable with e.g. copper-braid screen is not suited at low 
> frequencies.
> For frequencies, approx. four times higher than the corner frequency 
> of the cable the magnitude of the current in the screen reaches the 
> excitation current and becomes independent on frequency. 
> In principle the calculation with FEMM confirms this mechanism as 
> shown in the diagram (see pdf-file).
> For frequencies in this intermediate section below 10 kHz the current 
> in the centre screen is lower than in the right and left screen due 
> to its lower self inductance (higher corner frequency). 
> However, I have no explanation for the current distribution in the 
> right screen. The magnitude versus frequency of this current must be 
> the same as for the current in the left screen due to quasi-
> symmetrical conditions. But the trace does not follow this rule.
> I could establish the following details:
> - increasing of solver precision or refinement of the mesh has no 
> noticeable influence on this phenomenon
> - the current density in all three screens is not only composed of 
> eddy currents but also of a certain share of source current in spite 
> of their isolation from the domain boundary
> - the phenomenon changes from the right to the left screen if the 
> complex space vector rotates in the opposite direction (e.g. exchange 
> of phase currents in two conductors)
> - this phenomenon completely disappears if all three cables are 
> arranged in a triangle with perfect symmetry, in this case all three 
> screen currents have the same amount versus frequency.
>
> What is wrong? Do I violate the scope of FEMM?
> I would be pleased for any help! 
>
> Best regards
> Winfried
If you have three-phase conductors arranged in a line, the problem is
fundamentally just not symmetric--it sounds like you didn't do anything
wrong in setting up and solving the problem. This observation comes up
periodically on the mailing list. See, for example: http://groups.yahoo.com/group/femm/message/1672

Dave.
-- 
David Meeker
Senior Engineer
Foster-Miller, Inc.
350 Second Avenue
Waltham, MA 02451-1196
781-684-4070
781-890-3489 (fax)
dmeeker@xxxxxxxxxxxxxxxxx