Single and Double Shading Rings in Single - Phase Magnets: A General Analysis

A mathematical relation between the least force and the shading ring resistance and the ratio of the shaded to the total area is developed. For every value of this ratio, there exists an optimum value of the shading ring resistance which would give maximum least force. In the case of an E magnet, although the middle limb does not have a shading ring, the magnet taken as a whole tends to increase the least force. A general theory of the double shading ring is suggested, and it is shown that the double shading ring develops up to 15 percent more least force for higher ratios of the shaded area to the total area. The theories are confirmed by measurements.