Volume 85, Issue 8, 15 April 1999
Index of content:
- SHIELDING, LEVITATION, PROPULSION
Method for expanding the uniformly shielded area in a short-length open-ended cylindrical magnetic shield85(1999); http://dx.doi.org/10.1063/1.370434View Description Hide Description
A compensation method is proposed by which the uniformly shielded area of the axial magnetic field in a relatively short, open-structure axial magnetic shield can be extended. An open-ended cylindrical magnetic shield of 120 cm in length, 52 cm inner diameter, and a ∼0.5 mm total thickness of the shielding material is used to demonstrate the idea. The shield axis is oriented along the horizontal component (∼320 mG) of the Earth’s magnetic field. A simple way to increase the axial shielding factor is to use a pair of compensating coaxial ring coils set at both open ends of the shield. This increases, however, the radial gradient of the shielded field since the axial compensation field is stronger towards the shield axis. In order to decrease the radial gradient, an additional ring coil is wound around the middle part of the outer surface of the shield. The compensating field generated by this central ring coil is stronger towards the inner surface of the shield, and it helps, therefore, to unify the axial resultant field over a wider area inside the shield. The axial shielding factor obtained with this compensation according to the proposed method is 128, in contrast to only 16.4 obtained with compensation by a set of two ring coils. The field gradients observed are 1.2 μG/cm along the length direction and 2.7 μG/cm along the radial direction, in contrast to the 14 μG/cm axial and 78 μG/cm radial gradients obtained with compensation by a set of two ring coils.
85(1999); http://dx.doi.org/10.1063/1.370435View Description Hide Description
The effect of magnetic shaking on both the transverse and axial shielding factors (TSF and ASF) is investigated using open-ended cylindrical shields made of Metglas 2705M amorphous ribbons. Shaking enhancement is found to be strongly dependent on the orientation of the magnetic anisotropy of the shielding material, that is, the anisotropy axis should be aligned along the corresponding shielding direction to achieve a greater enhancement of the TSF or ASF. Magnetic shaking provides an ∼40-fold increase in the TSF and only an ∼twofold increase in the ASF for a shield consisting of a helical structure of the ribbons. The situation is almost completely different if the ribbons’ structure is axial: ∼twofold increase in the TSF and ∼20-fold increase in the ASF. The shaking field intensity (∼320 mOe at 1 kHz) for axial shielding is found to be about 10 times larger than that for transverse shielding. Experiments with a three-shell axial structure shield show an ∼350-fold increase in the ASF (∼40 000, which is one order larger than that of similar conventional shields). The TSF of this shield is, however, about one tenth of its ASF. Reorientation of the ribbons in the innermost shell, from an axial structure to a helical one, increases the total TSF (∼50 000) while still maintaining a large ASF (∼20 000). Hence, combining shells of helical and axial structures and having a proper distribution of the shielding material between them may allow the construction of an open shield with a large (>20 000) total ASF and TSF.
85(1999); http://dx.doi.org/10.1063/1.370436View Description Hide Description
Two types of magnetic bearing systems employing permanent magnets to be used for vertical-shaft and horizontal-shaft machines, respectively, have been designed and fabricated in our laboratory. In this article we report a comparative evaluation of (i) the permanent magnet configuration and its effect on radial disturbance attenuation, (ii) magnetic losses and their effect on energy storage, and (iii) the off-state position of the rotor magnet in two types of bearing systems. Experimental results are presented.