Patent GB-614304-A (GB)
Title: Improvements in and relating to electric frequency changing systems
IPC: H02P9/42
Filed: 1944-01-19
Abstract: 614,304. Automatic control systems for frequency changers. BRITISH THOMSONHOUSTON CO., Ltd. (General Electric Co.). Jan. 19, 1944, No. 1067. [Classes 38 (ii) and 38 (iv)] [Also in Group XXXVII] In a system for changing the frequency of an A.C. supply by a dynamo-electric frequency changer machine of the induction type, the frequency changer rotor is driven by a Ward Leonard set and means responsive to rotor speed control the field of the Ward-Leonard generator to maintain rotor speed, and thereby output frequency, substantially constant. In Fig. 1 the stators of induction-type frequency changers 5 and 6 are supplied from a common A.C. source 2 and the rotors, in parallel, supply a circuit 1 which has an induction motor 3, driving a fan 4, as a load. The rotors are respectively driven by motors 13 and 14 of Ward-Leonard sets 11 and 12, having generators 19 and 20. In a modification, Fig. 2 (not shown), the rotors of the frequency changers are supplied from the A.C. source and the stators, in parallel, supply the load. Frequency control.-The speed of motor 14 and, in consequence, the frequency of circuit 1, are maintained substantially constant at predetermined values by energizing field 24 of generator 20 from a transformer 25 through a gas-filled tetrode 28, the control grid bias of which is dependent on the armature voltage of motor 14 or the voltage of a tachometer generator 113, driven by motor 14, according to the position of a switch 114. Either of these voltages is connected in opposition to a constant reference voltage, produced from transformer 25 across a glow discharge device 32 by a rectifier 33, across a potential divider 44, 45, 60 or 61 supplying control grid bias to a hard pentode amplifier 37, the conductivity of which controls the voltage to which a condenser 52 is charged by a rectifier 54. This voltage is the grid bias of valve 28. Variable resistances 60 and 61 permit manual adjustment of the speed setting of motor 14 from different remote control positions selected by closure of a switch 65 or 66. Tachometer generator 113 may be driven by motor 3. Division of load.-The speed of motor 13 is controlled so that a predetermined, usually equal, division of load is maintained between the two frequency changers by controlling the energization of field 23 of generator 19 by valve means, associated with a transformer 73, similar to the valve means controlling field 24, the control grid bias of the amplifier valve 79, being, in this case, the difference between the voltages across resistances 100 and 101. According to the position of switches 121 and 122 these voltages are obtained through rectifiers 102 and 103 either from current transformers 96 and 97 associated with the frequency changer input circuits or from current transformers 119 and 120 associated with the input circuits of induction motors 115 and 116 driving the generators 19 and 20. The induction motors 115 and 116 may be supplied from source 2. The division of load may be preset by manual rheostats 108 and 109. Manual or automatic induction regulators 128, 129 control the input voltage to the frequency changers and, thereby, the circulating currents between them. In the modification of Fig. 2, automatic adjustment of such regulators by a reversible servomotor controlled by a relay differentially responsive to the frequency changer input currents acts as an additional control to maintain equal load division. The frequency changer output voltages are also manually adjustable by variable reactors in this arrangement. Protective arrangements.-Supply to valves 28, 37, 54 from transformer 25 is controlled by a time-delayed starting relay 67 which can only be energized, through contacts 64, when variable resistance 60 is in the minimum speed position. After operation, contacts 70 of relay 67 make its energization independent of contacts 64 so that variable resistance 60 can be moved to higher speed settings. Relay 91 controlling the supply to corresponding valves, from transformer 73, is energized through contacts 71 when relay 67 operates. Condenser 115 delays build-up of negative bias of valve 37 during starting to prevent excessive starting currents. Voltage limiting resistors such as 112 having negative resistance-current characteristics are connected across fields 23 and 24. Voltage limiting resistors 110, 111 are also provided across the transformers supplying rectifiers 102, 103. Valves 74 and 29 provide conductive paths to maintain current through fields 23 and 24 during negative half-cycles when valves 72 and 28 are non-conductive. Anti-hunting transformers 126<SP>1</SP> and 127<SP>1</SP> are provided for Ward-Leonard sets 11 and 12 respectively.