GB591658A - Improvements relating to low and medium voltage 3-phase electric motors - Google Patents

Abstract

591,658. Control of motors. HANNAH, S. H. May 16, 1945, No. 12326. [Class 38(iii)] Safety arrangements. -A system for preventing a three-phase low or medium voltage motor 1 from functioning under single - phase conditions comprises a star connected balanced bank of condensers 2-4 connected across either the terminals A, B, C of the motor )r the starter therefor, with the starpoint 11 of the condensers connected to a relay 12 operable through neutral under single - phase conditions of running to break the circuit of an electromagnetically operated contactor 13 which, under normal conditions of running, maintains the motor in circuit with the supply conductors 8-10 through contacts 18-20. The closure of a starting switch 14 completes an auxiliary circuit for the contactor coil 13, from the line 10, through overload relay contacts 22, over switches 21 and 14, and through coil 13 to line 8, whereupon contacts 17-20 are closed. The motor is thus connected to the threephase supply lines 8-10 and switch 14 can be opened since coil 13 is energized by a holding circuit from switch 21, over relay contacts 15 and 16 associated with the relay coil 12, and through the closed contacts 17 and coil 13 to line 8. If an interruption or disconnection occurs between one of the supply lines and the starter terminals 5-7 the magnetic flux created by the current in the two sections of the motor winding connected across the single-phase supply produces a potential difference between the terminals of those sections and the third terminal, thereby unbalancing the condenser system and energizing relay 12 to open contacts 15, 16. This de-energizes contactor coil 13 and disconnects the motor from the supply.