Extremely low frequency (ELF) magnetic fields from electric power lines and electric appliances cause errors in the even correct operation of medical and laboratory equipments. High-permeability material plates surrounding all four sides of cables can achieve the best shielding performance. However, the shielding structure reduces the cooling capability and degrades the current carrying capacity of the cable. In a previous report, the authors examined the performance of a magnetic shield using perforated high-permeability alloy plates. In this paper, the authors carried out "the inspection of the heat radiation effect with the opening" and “the relative inspection of the heat radiation effect and the magnetic shielding effect".
This experimental model was consisted of three electric cables connected to a three-phase AC power supply, and a magnetic shield surrounding the cables. To evaluate the heat radiation effect, the authors measured the temperature of the cable surface, in three-phase AC current 400 A. Each cable was horizontally placed 50 mm away from each other. The magnetic shield was made from 0.5 mm thick, perforated Permalloy PB plates and was formed a square tube with 150 mm x 150 mm x 1000 mm in size. Six types of Permalloy PB plates were tested. The opening ratio was 0% (no ventilating hole), 10%, 20%, 30%, 40% and 50%, respectively. The holes were 5 mm in diameter and arranged in straight centers pattern.
Next, numerical thermo-fluid 3-D analysis was performed using FVM software, STREAM (Software Cradle Co., Ltd. Japan). The rise in temperature of the cable surface was calculated by models same as the experiment, and the authors defined the calculating formula to evaluate a heat radiation effect with the opening.
In addition, by the experiment circuit and opening conditions same as the above, the authors measured the magnetic flux density. In this paper, the authors compare the performance of magnetic shielding using the shielding factor, which is the ratio of the measured magnetic flux density to that in non-shielding condition. To evaluate the magnetic shielding effect at the time of the opening, the authors defined the calculating formula of the hold ratio on the basis of a magnetic shielding effect when there was no ventilating hole.
Main results are follows.
A: From the inspection of the heat radiation effect by the experiments and the numerical analyses, it was shown that the rise in temperature of the cable surface decreased when the opening area of the shield plate was increasing.
B: The authors defined the expression of the heat radiation effect, and inspected the difference by the opening rate. In the experiments and analyses, similar characteristics were provided.
C: In the inspection of the magnetic shielding effect by the experiments, it was shown that the magnetic shielding effect decreased when the opening area of the shield plate was increasing.
D: In "the heat radiation effect by the opening ratio" and "the hold ratio of the magnetic shielding effect by the opening ratio ", the authors showed the relations of both clearly.
