Patent GB-516597-A (GB)
Title: Improvements in and relating to electric motor protective systems and apparatus therefor
IPC: H02K11/00
Filed: 1938-06-30
Abstract: 516,597. Protective cut-out arrangement; thermal switches. BRITISH THOMSON-HOUSTON CO., Ltd. June 30, 1938, No. 19445. Convention date, July 1, 1937. [Class 38 (v)] A motor having a winding and an enclosing casing in which the entrapped air is agitated by operation of the motor, has a thermostat responsive to winding-current under all operating- conditions and control means operated by the thermostat. An armature is secured to the movable part of the thermostat and a blockshaped permanent magnet with both poles on one face is mounted adjacent the armature for snap-action of the thermostat. The device is mounted near the stator winding and means are provided for access of the entrapped air to the thermostat whereby the device is jointly responsive to the winding-current and temperature under both stalled and running conditions so as to reduce the winding-current when the winding attains predetermined maximum temperatures. The device is mounted in a chamber 15 in the end-shield 13 of a small split-phase motor so as to be in close proximity to the end-turns 14 of the stator winding 10, and comprises an insulating plate 17 carrying the control switch. A currentcarrying bimetal strip 25, riveted at 26 to plate 17, carries a contact 24 which co-operates with a contact 22 on a flexible arm 20 riveted at 21 to plate 17, the latter having an elongated slot 23, Fig. 2, to allow movement of the arm 20 and provide for air-circulation. The strip 25 also carries an armature 29 co-operating with a permanent magnet 27 in the form of a small disc with a central goove 28 and so magnetized that the projections on either side of the groove constitute poles facing the armatures 29 so as to attract the latter and separate the contacts with a snap action when the strip 25 deflects sufficiently. A suitable material for the magnet is an alloy of 12 per cent. aluminium, 25 per cent. nickel. and the remainder mainly iron as described in Specifications 392,662 and 423,897, [both in Group II]. The magnet is slipped into the space between, and resiliently held by, a tab 32 die-struck from a metal strap 30 and a depression 30a, die-struck into the strap 30, the tab 32 engaging the groove 28 and the strap being secured to plate 17 by rivet 26 and a tongue 31. For current-setting purposes, a projection 33 extruded from the strip by a punch or die, co-operates with a cam 34 frictionally held by, but rotatable about, the rivet 26. The closing action may be effected by the strip 25 itself, or a press-button may act on it, Fig. 4 (not shown), the button being, however, incapable of holding the strip in circuit-closing position. In operation, since certain motor parts, for example the capacitor of a capacitor-motor, may be damaged by the maximum permissible winding- temperature, the strip 25 is chosen of such resistance as to heat up more quickly than the winding under stalled-rotor conditions so as to cut out the motor before it attains its maximum permissible temperature. At first connection to the line, the motor is arranged to have short and substantially equal energized and de-energized periods, whereas during continued heating of the stalled motor, the former periods decrease and the latter increase. During running, the strip is adjustable to the greater permissible current due to air-circulation, by admitting air to chamber 15 through aperture 35. The heating of the strip in the running condition depends mainly on heat-transfer from the windings, and the arrangement is such that the strip must be heated to substantially the same temperature' as the winding in order to operate. Any loss of heat due to a reduced current in the strip is accompanied by reduced heating of the winding, so that the strip opens the motor circuit at almost the same winding temperature for low as for high currents. The Specification as open to inspection under Sect. 91 shows also a form of thermostat in which the armature and magnet have silver facings for making contact and the flux is stated to be normal to the arc so as to exit a blow-out effect (Fig. 5, Cancelled, not shown). This matter does not appear in the Specification as accepted.