hysteresis losses in enclosures

[erik_t_viking@xxxxxxxxxxx]

Hello All,

First of all, I'm new to this newsgroup and also to FEA and FEMM in 
particular. Having studied in Germany, I'll do my best to translate 
what little I know of electromagnetics into English.

I'm trying to model hysteresis and eddy current losses we are 
experiencing in certain parts of enclosures of high-voltage 
switchgear our company is developing.

The enclosures are oval, with half-circles (46 cm diameter) on each 
end, connected by parallel straight walls of 90 cm length. Thickness 
is 1 cm. Height varies between 80 and 150 cm for various design 
options. In this are, equally spaced with 45 cm, the three round 
aluminium conductors (diameter 10 cm) for phases R,S and T. Current 
is 4000 Amps (rms) at 50 Hz.
The enclosure is filled with insulating gas (mu rel. = 1), the 
outside is plain air.

Trying to reduce cost, the enclosures are made of low-grade steel 
known as Fe37, which has similar properties as the Steel M-36 from 
the Materials Library of FEMM. (mu rel. approx. 1600, sigma = 9.5 
MS/m)
Experimental measurements of heat production result in 300-600 W/m2 
on the high-stressed left and right sides.
However, results from FEMM show a maximum |B| of 0.15 T, reducing 
rapidly towards the outside. According to the hysteris curves I have, 
this should result in far less than 100 W/m2 hysteresis losses. Eddy 
current losses are then also neglegible.

I'm using the following (planar) geometry:
_______
/ \
| O O O |
\_______/ \_Fe37, 1 cm thickness

R S T

R: I=5657 + j0 Amps
S: I=-2828 - j4898 Amps
T: I=-2828 + j4898 Amps

And now my questions, related to FEMM:

1. Why is the electrical conductivity of ALL steels in FEMM set to 0? 
I'm using 9.5 MS/m as a normal value, about 1/3 of the 34.4 MS/m of 
Al.

2. Can I model various phase-shifted currents by setting the real and 
imaginary parts of the current density? I'm using 0.72 MA/m2 
(resulting in a total peak value of 4000xsrt(2) Amps for the given 
conductors). 

3. What boundary conditions to specify for this kind of problems?

4. Along the inner surface of the enclosure, I've specified a mesh of 
0.02 cm. Is this fine enough given the skin dept of slightly less 
than 1 mm? More nodes seems to crash this machine :-(

5. Does anyone know a steel with a relative permeability of around 
1600 with 'worse' hysteresis properties as the M-36 steel? 

6. If |H| and |B| are known, then how do I estimate the resulting 
heat losess for this situation. I only have detailed curves for 
laminated transformer steels for |B| > 1 T, which of course are 
softer ferrites. 


I would appreciate the comments or experiences of anybody who has 
ever used FEMM for AC current magnetic fields.

Thanks in advance,


Erik Evertz
VA TECH ELIN HOLEC HIGH VOLTAGE BV
R&D Dep.
Amersfoort,The Netherlands