Patent GB-422202-A (GB)
Title: Improvements in or relating to electrical frequency-responsive devices
IPC: H01H51/32
Filed: 1933-07-08
Abstract: 422,202. Electromagnetic switches. BIRD, W. G., 11, Cranbrook Road, Redland, Bristol. July 8, 1933, No. 19384. [Class 38 (v)] [See also Group XXXVI] A frequency meter, tachometer, or other frequency-responsive device has a plurality of magnetic circuits having a magnetizing coil to be connected to the supply, a plurality of mechanically connected movable members, of which at least one is of magnetic material, acted on by the fields of the magnetic circuits, and means for causing a relative variation of these fields in accordance with frequency variations comprising one or more closed electrical circuits coupled magnetically with one or more of the magnetic circuits. In the construction shown a magnetic core has three arms 2, 3, 4, the central arm 2 having a winding 1 connected to an alternating current supply. The flux divides between two paths formed by the arms 3, 4 and air gaps 5, 6, which may be bridged by magnetic shunts 7, 8. A coil 9 is wound on the arm 3 and is short-circuited through a small resistance 10. Alternatively the coil 9 and resistance 10 may be replaced by a band of conducting material. A spindle 11 carries two camshaped members 14, 15 which move into the airgaps 5, 6. Both members 14, 15 may be of magnetic material, or one may be magnetic and the other of conducting material such as copper or aluminium. The members 14, 15 are so shaped that their torques oppose one another, the moving system taking up a position of equilibrium depending on the frequency and indicated by a pointer 16 moving over a scale 17. The movement of the spindle may produce a mechanical response such as the operation of a relay. To compensate for temperature errors, the coil 9 may have a negligible temperature coefficient, or the resistance 10 may be controlled by a thermostat such as a bimetallic strip. The position of the magnetic shunts 7, 8 may be varied by a thermostat, or they may be made of material having a high temperature coefficient of magnetic permeability. The resistance 10 may be replaced by a condenser 18. Coils 9, 19 may be wound on the arms 3, 4 and each connected in series with an impedance, for example condensers 18, 20, such that the circuits are resonant to different frequencies. The impedances in series with the coils 9, 19 may be changed by means of switches 21, 22 for changing the range. The coil 1 may be replaced by two separate coils, one wound on each of the arms 3, 4. The two magnetic circuits, in this case, need not have a common part. The exciting coils may be combined with the coils 9, 19 as auto-transformers. The moving system may be constrained to a zero position by means of a solenoid with a spring-controlled armature or plunger, the solenoid being energized from the supply. On cessation of the current the armature or plunger is released and acts on a cam on the spindle 11 to return this to zero position. Alternatively a permanent magnet may be arranged to act on one of the members 14, 15 or on a separate magnetic member on the spindle 11. The apparatus may be employed as a tachometer by connecting it to the output of an alternating-current generator driven by the shaft whose speed is to be determined. The output may also be connected to a synchronous motor operating a revolution counter. More than two magnetic circuits may be employed with a member movable into and out of the field of each magnetic circuit. The movable member may move in a straight line or other path. Instead of shaping the members 14, 15, the poles of the magnetic cores may be shaped.