Patent GB-810299-A (GB)
Title: Improvements in protective arrangements for electrical installations
IPC: B66B1/30
Filed: 1955-07-19
Abstract: 810,299. Automatic voltage control systems. BRITISH THOMSON-HOUSTON CO. Ltd. July 19, 1955 [July 19, 1954], No. 20910/55. Class 38 (4). [Also in Groups XXXVII and XXXIX] A motor 1 driving the cage of a hoist is slipplied from a Ward Leonard generator which is controlled by a motor-driven tachometer generator 2, a protective circuit for preventing excessive speed of the hoist including a rheostat 3 which is subjected to a speed-responsive voltage derived from the armature of the motor and has its contact 5 adjusted in accordance with the position of the cage. If the actual speed of the motor exceeds the value represented by the position of the contact 5 during the fullspeed and retardation periods of the cage, relays 12, 13 respond to reduce the speed of the motor or to stop it, a bridge rectifier 10 being included in the protective circuit to provide unidirectional energization of the relays despite reversal of the motor. If the protective circuit is accidentally interrupted, a control winding 14 of a magnetic amplifier 15 becomes de-energized and an opposed control winding 16 is rendered effective to reduce the output current of the amplifier and to de-energize a relay 20 having contacts in a security circuit 24 whereby the winder motor is stopped. The winding 16 is energized in accordance with the motor armature voltage so that the current passing therethrough normally varies in the same sense as that passing through the winding 14. Additional means are provided for responding to accidental failure of current in the winding 16 to provide a signal and, if necessary, complete stoppage of the hoist. For this purpose, a magnetic amplifier 25 has a control winding 26 in series with the winding 16 as well as an opposing winding 27 supplied from the tachometer generator 2. Any disturbance of the equilibrium between these windings, due to failure of the tachometer voltage or interruption of the current of winding 26, causes the output of the amplifier to decrease so that relay 31 opens contacts 41 in the security circuit 24. The amplifier 25 may consist of two amplifiers connected in push-pull and supplying a further stage with the relay 31 in its output circuit, the relay operating in response to reduction in output current of either of the push-pull amplifiers which each carry portions of the windings 26, 27, Fig. 4 (not shown). The amplifier 25 also has bias windings 28, 29 for adjusting its sensitivity to differences in the ampere turns of windings 26, 27. A relay 43 opens the security circuit if the winding 29 is de-energized accidentally. In another arrangement, a generator 45 is controlled by an amplidyne exciter 46 having a reference field winding 48 and an opposing feedback winding 49 energized in accordance with the generator voltage. A magnetic amplifier 51 has one control winding 50 in the feedback circuit and an opposing control winding 52 energized in accordance with the generator voltage so that interruption of the feedback circuit reduces the output of the amplifier and operates a protective relay 54.