Patent GB-666853-A (GB)
Title: Improvements in and relating to alternating current motor control systems
IPC: H02P7/26
Filed: 1948-02-09
Abstract: 666,853. Automatic control systems. BRITISH THOMSON-HOUSTON CO., Ltd. Feb. 9, 1948 [May 2, 1942], No. 3748/48. Class 38 (iv). To maintain constant the speed of a load device driven by an A.C. motor, the load on the motor is varied by a control winding energized by electric valve means controlled in dependence on the speed of the load device, further means being provided to prevent the motor supply current from exceeding a predetermined value. A three - phase synchronous motor 1 drives one member 9 of an electromagnetic slip coupling 8, the other member 10 of which is coupled to a load such as fan 12. The motor is started by manipulation of switches 5 and 6 to connect armature windings 2 to a source of reduced voltage 4 until a predetermined speed is reached and then to a source of normal voltage 7. Motor field winding 3 is energized at a predetermined high speed by the closure of a relay 112, responsive to the frequency of currents induced in the field winding, to bring the motor to synchronous speed. A control winding 14 of the slip coupling is energized from a three-phase A.C. source 16 through a transformer 18 and a fourphase rectifier including grid-controlled gas or vapour discharge type rectifiers 25-28, a resistance 42 having a negative impedancecurrent characteristic being connected across winding 14 to prevent excessive voltage transients. A.C. potentials derived from source 16 through transformer 38 and applied to the grid circuits of rectifiers 25-28 lag 120 degrees relative to corresponding anode potentials and the instant at which the rectifiers fire during each cycle and thus the degree of energization of winding 14 is determined by the magnitude of a D.C. potential 43-44, applied to the grid circuits of the rectifiers, derived from a D.C. source 45-46 produced from A.C. source 16 through a transformer 47 and diode rectifiers 51, 52. In the circuits described below, parallel-connected valves are employed to reduce the risk of failure. Speed control. To maintain constant the speed of fan 12 the output of a tachometer generator 68 driven with the fan is applied to the grid circuits of triode valves 65, 63 in opposition to the output of a remote speed-setting potentiometer 123 or 124, selected by operation of switches, e.g. 126, 127, which is supplied with the stabilized voltage across glow discharge valves 58, 61. Deviation of speed from the set value, the maximum value of which is limited by an adjustable resistance 128, thus results in grid control of triodes 62, 63, which form with a resistance 65 a potential divider across D.C. source 45-46, to control in turn the grid potentials of a further pair of triodes 55, 59 which form with resistances 57, 79, 80, 81 a bridge circuit, the output of which is the D.C. potential 43-44 controlling the energization of winding 14 and thereby correcting the speed deviation. An anti-hunt circuit involving a condenser 75 is included in the grid circuits of triodes 62, 63. Under extreme conditions should the D.C. biasing potential 43-44 attain a predetermined high negative value a pair of triode valves 76, 77 conduct at a point determined by the setting of a potentiometer 80 so that rectifiers 25-28 remain conducting and are in a condition to function as inverters in the manner described in Specification 529,577 to reduce the energization of winding 14. Current control. To prevent the current supplied to the motor armature from exceeding a predetermined value the output of current transformers 97 is rectified by diodes 101, 102 to control the grid potentials of a pair of triode valves 87, 88 in parallel with triodes 55, 59 so that at a predetermined armature current determined by the setting of rheostats 108 the increased conductivity of triodes 87, 88 overrides any attempt by triodes 55, 59 to maintain speed constant by increasing the energization of winding 14. An anti-hunting circuit involving a condenser 92 is included between the grid circuits of triodes 87, 88 and triodes 55, 59. Excess voltage limiting resistances 105, 106 similar to resistance 42 are connected across the outputs of current transformers 97. Starting arrangements. A relay 20 controls the connection of transformer 18 to source 16 and, during starting, a time delay relay 119 prevents the actuation of relay 20 before the cathodes of rectifiers 25-28 have reached a safe temperature. Even then, relay 20 is only actuated if a manual switch 121 is closed and the selected speed-setting potentiometer is at its low speed setting, e.g. potentiometer 123 is in a position to close contacts 131. Self-holding contacts 121a, however, permit adjustment of the speed-setting potentiometer after relay 20 has been actuated. Until relays 20 and 112 have operated, the grids of triodes 55, 59 are maintained by a potential divider 95, 111, associated with relay contacts 109, 110, 114, at a potential which ensures reduced energization of winding 14 and therefore reduced load on the motor during starting or if the motor falls out of synchronism. Specification 292,110, [Class 38 (iv)], also is referred to.