Patent GB-738973-A (GB)
Title: Improvements in or relating to electro-magnetic switches
Inventor: STARR ARTHUR TISSO
IPC: H03K3/45
Filed: 1951-08-01
Abstract: 738,973. Saturable reactors. STANDARD TELEPHONES & CABLES, Ltd. July 21, 1952 [Aug. 1, 1951], No. 18188/51. Class 40 (4). Alternating current is controlled by a choke the inductance of which may be changed by applying a D.C. signal to a control winding, the signal influencing the flux of a permanent magnet so that changed values of inductance persist after termination of the signal. The alternating current control may be enhanced by incorporating several chokes in a # or T filter circuit. As shown in Fig. 5, the choke may comprise a high-permeability internal core 5 of non-permanent magnetic material such as Swedish iron, " Ferroxcube," or " Mumetal " (Registered Trade Mark), and an external core 6 which has a limb P made from high coercivity, permanent magnet material such as " Alnico " or " Ticonal " (Registered Trade Mark) and a limb P1 made from lowcoercivity, relatively-weak, permanent magnet material such as spring steel or cobalt steel. A D.C. signal in the control winding 3 changes the magnetization of the limb P1 and the unbalance flux through the internal core determines the inductance of an A.C. winding 1. In this embodiment, D.C. signal pulses will be induced in the A.C. circuit; this is avoided in the arrangement shown in Fig. 6. A second embodiment, Fig. 7, utilizes a low-coercivity, relatively-weak permanent magnet 10, the magnetization of which is determined by D.C. signals in the control winding 3. A highcoercivity permanent magnet 11 is used to provide a reference flux and the arrangement is initially zeroed by inserting non-magnetic spacers 14, 15 so that no flux passes through the internal core 7. The A.C. winding is arranged in two parts 16, 17 so that the inductive coupling between it and the control winding is neutralized. In a modified arrangement, Fig. 10, the inductances of two A.C. windings 1, 1<SP>1</SP> are controlled in opposite respects by the D.C. signal polarity in a single control winding 3 so that when one inductance is high the other is low and vice versa. A single low-coercivity magnet 20 is used together with two high-coercivity permanent magnets 18, 19, the arrangement being initially balanced by the insertion of non-magnetic spacers 21-24. The arrows in the Figure indicate the flux distribution when the magnet 20 has the magnetic polarity shown, no flux passing through internal core 26 so that the inductance of the associated winding 1<SP>1</SP> is high, while an out-of-balance flux passes through internal core 25 and reduces the inductance of winding 1. In a further embodiment, Fig. 11, the use of a high-coercivity permanent magnet is avoided and the magnetization of a low-coercivity permanent magnet 27 is influenced by control windings 3, 3<SP>1</SP> only one of which is energized at a time. Initially the outer magnetic circuit is balanced so that no flux passes through the internal core 29 and the inductance of winding 1 is consequently high. When winding 3 is energized, opposing fluxes are produced by coils 32, 33 so that flux passes through the internal core and the magnet acquires consequent magnetic poles at its centre. The arrangement remains in this condition until restored to its original state by application of the D.C. signal to winding 3<SP>1</SP>, the coils 32<SP>1</SP>, 33<SP>1</SP> of which are wound to neutralize the consequent-pole magnetization of the magnet. In a practical construction of the arrangement shown in Fig. 6, the outer core 54, Fig. 13, has an adjustable air gap 55 and is formed of Swedish iron with inserted highcoercivity sections 56, 57 of " Ticonal ". The internal core 59 is built up of " Mu-metal " laminations with ground contacting surfaces 60, 61. The air gap is adjusted when the control winding 62 is de-energized to give maximum inductance of the A.C. winding 58. Fig. 14 illustrates a constructional arrangement of the embodiment shown in Fig. 7. This arrangement comprises a core 44 built up of " Mu-metal " laminations, alternate layers of which interleave with laminations of spring steel forming a low-coercivity magnet 47. A solid permanent magnet bar 50 and non- magnetic spacers 48, 49 are also provided.