GB709297A - Improvements in and relating to the control of electric motor drives for rolling mills and the like - Google Patents

Abstract

709,297. Control of two D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. Oct. 19, 1951 [Oct. 19, 1950], No. 24455/51. Class 38(3) [Also in Group XXXV] Motors 3 and 4 each drive one of a pair of rolls forming part of a reversible rolling mill and are respectively supplied from Ward- Leonard generators 1, 2, having amplidyne exciters 8, 9. The motor circuits have a common conductor 5-6 comprising an element 7 which is both inductive and resistive, the points 5 and 6 being at equal potential when the motor currents are the same. In the event of unbalance of the motor currents a voltage is developed across the element 7 which causes such energization of field windings 47 48, of the exciters 8 and 9 that the excitation of the heavily loaded generator is reduced and that of the lightly loaded generator is increased, so equalizing the torques of the two motors. The inductance of the element 7 provides a quick reaction to any unbalance of motor current. The provision of rectifiers 50 allows passage of the no-load currents of the motors through the element 7 without appreciable energization of the windings 47, 48. The amplidyne 8 has a control winding 13 which is energized from a constant source 12 through rheostat 15 and reverser 16, an opposing exciter field winding 17 being energized in accordance with the voltage of the generator 1. Further exciter windings 18, 19, 20, restrict the rate of change of excitation of the generator 1 and so prevent current surges in the armature circuit of the motor 3, as described in Specification 698,064. The amplidyne 9 is similarly controlled. The motor field windings 37, 38, are supplied from the source 12 through an adjustable resistor 51 and a step-down amplidyne 39 which is controlled in the manner described in Specification 698,064. When the source comprises a single generator, a connection between the point of interconnection of the motors and a point of potential intermediate those of the generator output terminals includes an impedance across which a signal voltage representing the difference of the motor currents is generated. This signal voltage is used to control the outputs of a pair of exciters connected to control the field excitations of the pair of motors.