GB113473A - Improvements in and relating to Electric Motor COntrol Systems. - Google Patents

Abstract

113,473. British Thomson - Houston Co., (General Electric Co.). Feb. 20, 1917. Systems for continuous-current motors only; electric braking.-A series motor used for hoisting &c. has reversing means and connexions whereby the field winding may be connected in shunt to the armature during lowering to form a dynamic braking circuit, and the connexions of a resistance in this circuit are controlled bv an electrcmagnetic switch responsive to the speed of the motor. The connexions of the armature A and series field F to the controller C are shown in Fig. 1. With the controller in its " off " position, the motor is connected for dynamic braking and the resistances R<1>, R<2> are connected in the circuit. The switch 3 controlling the resistance R<2> is operated by a solenoid consisting of two differential sections 4, 5 connected with resistances 6, 7 across the line 1, 2, while the junction of the sections is connected to the movable contact of the resistance R<1>. Part of the resistance R<1> is left permanently in the circuit to ensure the proper speed-torque characteristics for lowering and the resistances R<1>, R<2> together are large enough to protect the armature against excessive rushes of current. For hoisting, the controller is moved to the left, the brake coil R is energized to release the brake, and the resistance R is gradually short-circuited while the motor runs as a series motor. For lowering, the controller is moved to the right, and circuits are made as shown in Fig. 2. A complete circuit is formed of the armature A, field coil F, brake coil B, resistances' R<1>, R<2>, and a part of resistance R dependent on the position of the controller. The section 5 of the actuating winding of the switch 3 with resistances 6, R<1> is connected across the armature and its current is responsive to the back electro-motive force of the motor, while the section 4 with resistance 7 is connected across the resistances R<2>, R. The difference in ampereturns between the two sections 4, 5 is not sufficient to lift the switch 3 although the controller may be moved quickly. The resistance R<2> is therefore left in circuit as a protection against current rushes. As the motor speeds up, the voltage on the section 5 rises and that on the section 4 falls until the switch 3 responds and short circuits the resistance R<2>. The resistances 6, 7 may be varied by the controller so that the switch 3 closes with different armature current values during increasing speeds and decreasing speeds. In a modification, the conductor from the junction of the sections 4, 5 includes contacts of a relay 9, Fig. 4, which can only close when the armature current drops to a predetermined value. After closure of the relay 9, the switch 3 is subject to the differential control of the sections 4, 5 as before.