GB1370942A - Control of electromagnetic contactors - Google Patents

Abstract

1370942 Protective arrangements BALDWIN & FRANCIS (HOLDINGS) Ltd 6 April 1972 [11 Jan 1971] 1246/71 Heading H2K The energization of a contactor coil C/C is controlled by a pulse generator C1, T1, TR1 which generates pulses at a higher frequency in a start condition in which the contactor is closing and at a lower frequency in a run condition when the contactor is closed, and a thyristor TH1 controlling the contactor coil current, the conduction angle of the thyristor depending on the frequency of the pulses so that the average current in the coil C/C in the run condition is less than that in the start condition. The closing of a local start switch or the contacts R1/1 of a relay energized by the closing of a remote start switch completes a circuit from a 240 v A.C. supply through the coil C/C, a capacitor C2, a resistor R3 and a capacitor Cl to rapidly charge the capacitor C1 until a unijunction transistor T1 conducts, the capacitor C1 then discharging through the transistor T1 and this cycle being repeated rapidly to provide the higher frequency pulses. The thyristor TH1 is triggered substantially at the beginning of each positive half cycle of the supply. When the contactor closes, it closes auxiliary contacts AUX1 so that when the start switch is released the pulse generator is supplied from the 60 v secondary winding of a transformer TR4 so that the pulse rate falls and the conduction angle is reduced. An undervoltage release circuit comprises a further pulse circuit C3, T2, TR2 which normally triggers a thyristor TH2 to maintain a relay R2/2 energized. Should the supply voltage fail or fall below a predetermined value, the relay R2/2 releases with a delay determined by a capacitor C4 to open contacts R2/1 to interrupt the discharge current from the coil C/C through a diode D3 which has been able to maintain the coil C/C energized. Automatic reclosing is provided by a circuit 4 which has a capacitor C5 which is charged by the closure of auxiliary contacts AUX2 when the contactor is closed and causes a pulse generator C6, T3, TR3 to produce higher frequency pulses. When the relay R2/2 releases, its contacts R2/2 close to connect this pulse generator to the thyristor TH1 and should the restoration of the supply occur within a time, e.g. three seconds, determined by the charge on the capacitor C5, the re-energization of the relay R2 causes the contacts R2/1 to close before the contacts R2/2 open so that the starting current can flow through the coil C/C.