Patent GB-796541-A (GB)
Title: Improvements in or relating to alternating current stand-by equipment
Inventor: BENNETT GORDON CHARLES
IPC: H04M19/00
Filed: 1954-03-10
Abstract: 796,541. Stand-by systems. STANDARD TELEPHONES & CABLES, Ltd. June 10, 1955 [March 10, 1954], No. 6970/54. Class 38 (4). Stand-by equipment used in conjunction with an alternating-current supply comprises an inverter and means for stabilizing the load thereon comprising a circuit arrangement for rectifying and feeding back to the direct-current input circuit of the inverter alternating-current power surplus to the instantaneous requirements of the load circuit. A change-over switch is adapted to detect a failure of the normal alternating-current supply and effects a rapid change-over to the stand-by equipment whilst isolating the normal supply to the load. The inverter may be held ready for operation but isolated from the main supply until it is triggered into full operation upon detection of a failure; alternatively, the inverter normally floats in idling condition across the normal supply and load. In another arrangement, the inverter supplies the load at all times, means being provided for rectifying a normal source of alternating-current and feeding the rectified power to the inverter in parallel with a directcurrent source therefor, the change-over switch detecting a failure of the normal source and connecting the alternative source in circuit. As shown, the stand-by supply is derived from battery SB by means of an inverter comprising discharge valves V1, V2, and transformer TR3. A bridge rectifier MR2 with a choke output filter L2 feeding the battery, and supplied from an auxiliary winding on the inverter output transformer TR3, is so arranged that, with the inverter providing slightly more than full-load current, the rectifier is just conductive. With loads less than full load, the output voltage increases until it is limited by means of the rectifier circuit. The load on the inverter is thus maintained substantially constant and the output voltage varies between narrow limits. The inverter is normally held quiescent by negative bias applied in series with the alternating voltage to the grids of the valves V1, V2. During normal operation, the alternating voltage at terminals N1, L1, is compared with a voltage derived from a synchronous motorgenerator set MG, so arranged that at the normal supply voltage the output from a comparison circuit is substantially zero. The comparison circuit comprises transformers TR1, TR2, and rectifier MR1, current-limiting elements R1, LN1, being provided. The motor-generator also supplies the alternating voltage to the grids of the valves V1, V2. On mains failure, the output voltage of the comparison circuit increases rapidly and is applied to the grid of a cold cathode tube V4 which discharges capacitor C6 through a coil of change-over contactor CO, and also triggers a thyratron V3 which removes the disabling bias from the inverter valves, so starting the inverter. After the inverter has been put on load, the motor-generator is switched from the mains to the inverter output and continues to operate. On restoration of the main supply, a relay MF detects when the supply is within its normal voltage limits, and shortly afterwards the motor-generator is switched back to this supply. The comparison circuit is now used to detect lack of synchronism, and, as long as there is such a lack, a relay SYN in the circuit of thyratron V5 is held operated. When the output from the motor-generator is in phase with the mains, the relay releases and connects a coil of the contactor CO to a charged capacitor C10 which discharges through the coil and re-connects the load to the mains. In order to ensure that full output of the stand-by source is reached more quickly after mains failure is detected, a capacitor charged from the alternating-current supply may be used to boost the battery voltage during the starting period of the inverter. Specification 679,831 is referred to.