GB858974A - Improvements in or relating to electromagnetic devices - Google Patents

Abstract

858,974. Electric clocks. HORSTMANN CLIFFORD MAGNETICS Ltd. March 11, 1959 [May 2, 1958], No. 14088/58. Class 139 [Also in Group XXXV] A constant speed electric motor, e.g. for driving a clock, comprises a permanent magnet rotor and a field coil energised from an amplifier whose signal input is derived from a second coil influenced by the rotor flux. The speed is determined by the resonant frequency of a resilient member, which may be either the rotor or the signal coil assembly. As shown in Fig. 1 the rotor has resilience and comprises two permanent magnet discs 1, 2 attached to a thin shaft 3. Voltages induced in the coil 15 due to rotation of the disc 2 between soft iron pole pieces 16, 17 are amplified by a battery powered transistor amplifier 50 to energise the field coil 7 on similar pole pieces 8, 9, both coil assemblies being attached to a rigid frame 25. Twisting of the shaft 3 allows relative oscillation between the discs which decreases the driving effect of the field until a rotational speed is reached at which the oscillations are at resonance. A fine adjustment of speed is obtained by arranging a spring of magnetic material adjacent to one rotor with a screw to adjust the air gap (Fig. 10, not shown). The "runaway" speed is limited to #2 times the desired speed by a capacitor connected in parallel with the signal coil. A known type of device (not described) to prevent reverse rotation may also be incorporated. Alternative forms of rotor are described, in one of which (Fig. 7, not shown) the discs are connected by three leaf springs which are secured to the shaft by a collet at a nodal point, and in a further modification (Fig. 8, not shown) one disc is many times heavier than the other and becomes the collet. Fig. 11 shows an arrangement in which the signal coil 15 and its pole pieces 67, 68 form the oscillating member, the rotor in this case being a single disc 60. The pole pieces are attached to those of the driving coil 7 by crossed leaf springs 76, 77 which also serve as electrical connections to the signal coil. The driving coil assembly is rigidly secured to a base plate 65. Three alternative amplifier circuits are described, one employing a single transistor, the others, two connected in push-pull. It is stated that the device may be used as an escapement mechanism. It is stated in the Provisional Specification that the motor may be synchronised with a.c. mains by applying a small mains voltage to the amplifier, and that the speed can also be finely adjusted by varying the phase of the driving current relative to the signal voltage, or by angular displacement of one rotor relative to the other, in the first embodiment.