Sequential switching with delay for controlled rectifier circuits

TLDR: In this paper, the authors describe an A.C. switch with two pairs of contacts which are operated sequentially to apply the voltage of the source to the anode-cathode circuit of a rectifier in series with the load before applying it to the gate of the rectifier and to remove the voltage from the gate before its removal from the load circuit.

Abstract: 1,058,526. Arcing preventing in switches. SINGER CO. July 29, 1965, No. 32389/65. Heading H2B. [Also in Division G3] To prevent arcing at the contacts of a switch controlling a load, e.g. a portable electric tool, supplied from an A.C. source through the anodecathode circuit of a controlled rectifier, the switch has two pairs of contacts which are operated sequentially to apply the voltage of the source to the anode-cathode circuit of the rectifier in series with the load before applying it to the gate of the rectifier and to remove the voltage from the gate before its removal from the load circuit. The switch has an inherent delay time between the operation of the two pairs of contacts of at least one half-cycle of the A.C. supply so that the load current is always interrupted by the controlled rectifier becoming non-conducting and not by the opening of the switch contacts. Several embodiments are described including one in which the load is a motor having its speed controlled by a centrifugal switch driven by the motor and with its contacts in series with the gate of the rectifier.