GB391475A - Improved means for modifying the braking of permanent brake magnets in accordance with temperature - Google Patents

Abstract

391,475. Eddy-current brakes. FERRANTI, Ltd. and CLIFFORD, C. F., Hollinwood, Lancashire. Oct. 15, 1931, No. 28701. [Class 103 (i).] In order to modify the braking action of permanent magnets in accordance with temperature, a magnetic shunt having a negative permeability temperature coefficient is arranged between a pole of one magnet and an unlike pole of the other magnet, a further shunt connecting the remaining poles. In the construction shown in Fig. 1, two permanent magnets a, b are arranged so that the North pole of the magnet a and the South pole of the magnet b are above a horizontal rotor disc (not shown), the remaining poles being below the disc. One or more stampings c of material having a negative permeability temperature coefficient are provided between the lower poles so as to divert some of the flux from the North pole of the magnet b to the South pole of the magnet a and prevent it from traversing the rotor disc. The shunt c is made wider than the polepieces so as to provide a path for the flux fringes of the lower polepieces. The leakage path is completed by a further shunt d of mild steel or other material having a normal positive permeability temperature coefficient arranged across the upper polepieces. Rise of temperature causes an increase in reluctance of the flux leakage path, so that less flux is diverted and more traverses the rotor disc. A concentration of flux also occurs in the air gaps owing to the increase in reluctance of the path of the fringing flux. Figs. 3 and 4 show the application to a mercury motor meter. The shunt c encircling the lower poles of the permanent magnets a, b is attached to a brass or other non-magnetic plate f applied to the polepieces. The plates c, f and the upper shunt piece d are embedded in moulding material g in which is formed the mercury bath h containing the meter disc i.