GB782109A - Improvements in or relating to electric motor control systems - Google Patents

Abstract

782,109. Control of D.C. motors. IGRANIC ELECTRIC CO., Ltd. Dec. 14, 1953 [Dec. 26, 1952], No. 34743/53. Class 38 (3). In the first hoisting position of a master switch 14, a circuit is completed to a motor armature 10 and series field winding I through relay coil 15a and over-hoist limit switch contact 16a, the circuit being completed through brake release winding 45 and accelerating resistors 47, 48, 49. To provide for stable low speed operation when hoisting light loads, contacts 23b are closed in the first hoisting position so that resistor 34 is connected in shunt with armature 10 and resistor 37. Actuation of the over-hoist limit switch opens contacts 16a, 16b and closes contacts 16c, 16d to form a dynamic braking circuit through resistor 63. The opening of contacts 16a de-energizes relay 15a, the contacts 15b opening to de-energize relay coil 23a. Thus, the contacts 23b open and the motor is prevented from driving the hook downwardly when the over-hoist switch is actuated, the brake being de-energized to hold the hook in the over-hoisted position until the master switch is moved to a lowering position. This arrangement permits the use of resistor 34 as an armature shunt in the first hoisting position, and, at the same time prevents the oscillation of the hook in the over-hoist limit zone which would be obtained if the circuit through the contacts 23b is not broken. The coil 15a may be placed in other suitable parts of the motor circuit, and it may be replaced by a coil of the voltage-operated type. This relay is not included in circuit for lowering operation, during which the motor is connected as a shunt machine with the field excitation controlled by means of resistors 47, 48, 49.