Patent GB-384703-A (GB)
Title: Improvements in or relating to electrical frequency responsive devices
IPC: G01R23/02
Filed: 1931-08-07
Abstract: 384,703. Frequency measurements ; electrostatic meters. BIRD, W. G., 11, Cranbrook Road, Redland, Bristol. Aug. 7, 1931, No. 22430. [Class 37.] A frequency-responsive device such as a frequency meter or electric tachometer has a movable system which is acted on by opposing torques produced by an inductive and a capacitative element respectively. As shown in Fig. 1, the shaft e carries a cam-shaped soft iron armature d, moving between the poles of an electromagnet b, and conducting plates g moving between fixed conducting plates f. The current of which the frequency is to be measured is supplied to the primary winding l of a transformer k having a low-tension secondary winding h, which supplies the winding a of the electromagnet b, and a divided hightension winding j, which may be of high resistance wire, connected to the two sets of fixed plates f of the capacitative device. The junction of the coils i is earthed, so that the moving system is at earth potential. The coils h, j are provided with tappings so that their ratio may be adjusted. Wave-form errors may thereby be compensated for. The torque of the capacitative device may be increased by immersing the plates in a liquid of high dielectric constant. Fig. 5 shows a tachometer having a rotating permanent magnet n which produces an alternating flux in the iron core k, on which are wound the windings h, j supplying the electromagnet a and the plates f. An autotransformer m may be included in the circuit of the coil h and provided with tappings for facilitating adjustment of the ratio of the voltages applied to the winding a and the plates f. Fig. 6 shows a construction of the inductive torque-producing element suitable for a scale length of 270‹, the rotating armature d being cam-shaped as shown and a quadrantal groove v being milled out of the edge of the electromagnet b to take the shaft e. For a .scale length of 180‹ the groove v is situated in the centre of the outer face of the electromagnet core, the armature being shaped to cover an annular width of 180‹. Fig. 7 shows a modification suitable for a scale length of 90‹, the magnetic circuit being formed of two members b<1>, b<2> stamped out of soft or silicon iron so as to enclose the winding a. Opposing internally projecting sectors p<1>, p<2> form polepieces between 'which rotates the armature d stamped with 90‹ sectors. For a 45‹ scale the sectors cover 45‹. Terminals t<1>, t<2> in insulating bushings serve to position the members b<1>, b<2>. A third dummy " terminal may also be provided for this purpose. Fig. 8 shows a construction of capacitative device suitable for a scale'length of about 270‹. The fixed plates comprise two similar blocks f<1>, f<2> die-cast from aluminium alloy in the shape of sectors covering 90‹ in plan. These blocks are fixed bv terminal screws t, t<2> within a sheath r of moulded insulating material. This is cylindrical with one end open and with a 90‹ sector omitted to allow insertion of the moving system, which consists of two groups of vanes g<1>, g<2> mounted on the shaft e. These vanes may be stamped out of sheet aluminium or of mica covered with aluminium foil or spluttered with aluminium. In a modification, Fig. 10, for a scale length of 90‹ or 45‹, the insulating sheath r is cylindrical and open at both ends and is divided midway by an internal circumferential flange q, which may be milled as shown to increase the leakage path. Each half of the sheath contains two similar blocks f<1>, f<2>, each pair of blocks being connected by strips of thin aluminium sheet or foil o<1>, o<2> wrapped round the interior of the sheath r. The casing of the instrument is of cast or pressed metal, earthed to act as a shield, and virtually extended over the glass face of the instrument by painting this with a transparent conducting varnish. Instead of the moving members d, g being mounted on the same shaft, they may be coupled by gearing, link motions, &c. The motion may be linear instead of rotational. The instrument may be employed as a null instrument by varying the ratio between the coils h, until a standard reading is obtained. The frequency is then proportional to this ratio.