Patent GB-347248-A (GB)
Title: Improvements in the generation of alternating electric currents for use in induction furnaces and for other purposes
IPC: H05B6/04
Filed: 1930-01-20
Abstract: 347,248. Automatic regulation of frequency. DAVIS, N. R., Park House, Atkinson Road, Ashton-on-Mersey, BURCH, C. R., 12, Derbyshire Lane, Stretford, Manchester, and ASSOCIATED ELECTRICAL INDUSTRIES, Ltd., Bush House, Aldwych, London. Jan. 20, 1930, No. 1925. [Class 38 (iv).] Relates to valve amplifiers impulsed by a small variable-speed dynamo for producing alternating current of frequency higher than that usually supplied by the mains, say from 300 to 1000 cycles, particularly for energizing induction furnaces of the " coreless " type where the effective inductance of the load may vary during heating. According to the invention the frequency of the " drive " motor-generator set is varied automatically by means of a power-factor relay in accordance with changes in the resonance frequency of the furnace load as the charge is progressively heated. The output power from the valve-amplifier installation is also regulated in accordance with the furnace charge by controlling the field excitation of the " drive " through a voltage-operated relay in the load circuit; and the grid bias on the valve amplifiers is varied automatically and simultaneously with the frequency of the output current. Fig. 1 shows a bank of rectifiers 4, energized from a threephase source 1 through a circuit-breaker 2, for supplying the anode voltage to amplifier-banks 13, 16. The motor-generator " drive " 33, 34 is applied to the grids of the amplifiers 13 in pushpull, the output from these being applied in pushpull to the grids of the valves 16, which supply the furnace or other load circuit 31 through a transformer 32. Grid-bias for the valves 16 is derived from a direct-current generator 17, driven by a variable-speed motor 18. Grid bias for the valves 13 is provided by a leak and condenser system 18<1>. A.C. filament current is supplied from sources 6, 19, 25 respectively provided with low-voltage relays 12, 23, 29. An overload-relays 5 is provided for each of the rectifiers 4. The installation comprises pumping gear for supplying cooling-water to the anodes and grids of the power amplifiers ; also " backing " pumps and Langmuir pumps for maintaining the necessary high vacuum inside the valves ; and safety gear for ensuring the proper functioning of these accessories. Fig. 2 shows the control and safety supervision panel. In operation, the switch 43 is closed whereupon, if the permanently-running gear is working satisfactorily, a switch 61 closes to energize the control bus-bars 62. The " drive set 33, 34 is started by hand through a rheostat 64, and the valve filaments are energized by a switch 98. A switch 88 applies the drive voltage to the grids of the amplifiers 13, and the grid-bias generator 17 is then started up. Finatly the circuit-breaker 101 is closed and the whole installation comes into full operation. The field winding 65 of the motor 63, Figs. 1 and 2, is in circuit with a rheostat 66 which is operated by a reversible motor 67 through forward and reverse windings 69, 70 and switches 71 .. 74 controlled by a kilovolt-ampere relay in the load circuit to control the frequency of the drive in accordance with the condition of the furnace charge. Similarly the field winding 78 of the generator 33, Fig. 1, is in circuit with a rheostat 79 operated from the bus-bars through windings, 82, 83 and switches 84..87 controlled by the moving contact 36 of a voltage relay in the load circuit, so as to regulate the power output from the bank of amplifiers in accordance with the furnace load. The armature 68 also operates a rheostat 90 in the field winding 91 of the grid-bias generator 17, Fig. 1, so as to adjust the applied bias as the working frequency rises or falls. The moving arm of the rheostat 64 closes a switch 95 in an auxiliary circuit 96 which comprises protective means, such as switches for the filament supply sources, contacts which are only closed where the cooling-water supply is properly flowing, and contacts associated with the overload relays 5, 12, 23, 29, Fig. 1. Switches 107, 108 reduce the voltage on the anodes and grids of the amplifiers when the main circuit-breaker is operated. Dimming-resistances are also inserted in the filamentsupply circuits when the valves 13, 16 are started up, and are afterwards short-circuited when the voltage of the generator 33 reaches a predetermined value. Switches 118 bring into operation further control gear which allows the valves to be " conditioned " as described in Specification 347,210, [Group XL].