Permanent magnet latch for speed switching device

In an overspeed switch for a motor, an electrically conductive disc supported for rotation with the rotor of a motor, a drag member supported adjacent said disc for limited pivotal movement about an axis at right angles to the plane of the disc and having a permanent magnet linked magnetically with said disc for causing a force on said drag member through electromagnetic interaction between said permanent magnet and the rotating disc, said force varying with the speed of rotation of said disc, switching means connected to said drag member and actuated by said limited movement of said drag member and an improved latching means for restraining said drag member from said limited movement until a predetermined force is developed by said electromagnetic interaction, said latching means comprising a permanent magnet means having a length, height and thickness mounted with its height extending in a direction at right angles to the plane of said disc and its thickness extending in a plane parallel to the plane of said disc, a pole piece on either side of and engaging said permanent magnet means, said pole pieces defining the thickness of said permanent magnet means, said pole pieces projecting towards said drag member and each terminating in a wedge-shaped, flux concentrating configuration, the terminating end of the wedge extending in a direction parallel to the height of said permanent magnet means, the drag member having at least three teeth in an arcuate arrangement about the axis of pivotal movement of said drag member, two of said teeth being separated by at least one intermediate tooth and each one of said two teeth being located opposite to and closely spaced from a respective one of said terminating wedge-shaped ends of said pole pieces, said pole pieces also extending in a direction opposite said terminating wedge-shaped ends and having arm portions extending towards one another and terminating short of one another to define a gap therebetween, and a magnetic shunt means mounted for movement into a position bridging said gap and into a position withdrawn from said gap for adjustment of the amount of shunted flux and thereby the flux linking the pole pieces and the respective teeth of said drag member to adjust the operating level of said switching means with regard to the rotational speed of the disc.