GB1251323A - - Google Patents

Abstract

1,251,323. Protective arrangements. BALDWIN & FRANCIS (HOLDINGS) Ltd. 18 Feb., 1969 [18 Feb., 1969], No. 47475/67. Heading H2K. In a remote control circuit for a motor controlled by a contactor and comprising the series circuit of a transformer secondary winding 3, a relay A, a rectifier 9 and a switching arrangement S having three conditions in which the series circuit is open, is closed and is closed but includes a low value resistor 8, incorrect operat ion due to a defective switch is prevented by a second relay B controlled through a silicon controlled rectifier SCR in accordance with the voltage across the switching arrangement Initially with the switch S in the open position, the various relay contacts occupy the positions shown with the SCR conducting and the relay B energized. When the switch S is moved to the closed position (i.e. the " start " position), the junction between resistors 16, 17 is connected to earth through the rectifier 9 to stop a unijunction transistor 15 from producing pulses. The SCR is no longer triggered and ceases to conduct, relay B being de-energized and closing contacts B2. A third relay C is now energized and closes its contacts Cl to complete the series circuit. Relay A is now energized and closes its contacts A1, A2. When the switch S moves or is moved to its " run " position in which the resistor 8 is connected in the series circuit, the resulting increase in potential at the junction of the resistors 16, 17 is such that the unijunction transistor 15 starts firing again, the SCR conducts and the relay B is again energized so that contacts B1 are reclosed to complete the circuit for the contactor closing coil C/C. Various arrangements are described in which the switching arrangement is provided by a threeposition switch S on or near the motor or in the " gate-end box " housing the contactor, or is provided by the contacts of relays A, B in a preceding gate-end box when sequence control is required.