The shielding of permanent magnets from transient magnetic fields

CONSIDERABLE progress has been made toward the development of permanent magnets which are immune to disturbing conditions, such as temperature variation, mechanical vibration, natural aging, and extraneous flux fields. Contributions to this progress include new materials, improved processing techniques, and better manufacturing controls. Constancy of flux is in most applications quite essential. This is particularly true of electromagnetic damping, in which the damping torque varies as the square of the air gap flux. The permanent magnet of a watthour meter is an example. Despite the fact that the modern permanent magnet is remarkably stable, there have been cases in which the magnets showed changes in strength, generally becoming somewhat weaker. Isolated instances of such changes in watthour meter magnets have been found by the electric utility companies. Summary It has been shown that the problem of protecting permanent magnets from transient magnetic fields has been solved in a unique manner by the use of a new high-speed copper-plating process to apply a high conductivity copper shield over the conventional forged steel magnets. Desirable features of this solution are complete interchangeability between unshielded and shielded magnets; forged steel material is used, which is fairly easy to obtain and on which unlimited operating experience is available.