GB1379512A - Control systems for dc motors - Google Patents

Abstract

1379512 Control of DC motor SEVCON Ltd 18 Jan 1972 [22 Oct 19701 50343/70 Heading H2J In a control system for a D.C. motor 2, 4, Fig. 1, particularly applied in crane hoisting, switches 22, 32, 36 are arranged so that in one condition they connect the armature 2 and field windings 4 in series with a source 6 for load raising and in a second condition they reverse the polarity on the armature 2 for load lowering and shunt the armature and field windings, the current through the motor being controlled by a pulse control circuit 20. Free wheel diodes 34 connected across the motor permit current to circulate when main thyristor 50, Fig. 2, in a speed control oscillator system 20 is non-conducting. A brake coil 24 is provided in series with the supply for automatically releasing a spring- loaded mechanical brake, and is shunted by a diode 26 to suppress high transient voltages at thyristor cut-off. The motor speed may be controlled through a thyristor pulse control system having a main thyristor 50 in the D.C. supply line with a parallel connected second thyristor 56 in'series with a commutating capacitor 52. In operation capacitor 52 becomes charged to a voltage exceding that of the supply by inductance 18 when the main thyristor is non- conducting and the second thyristor is fired. When the capacitor 52 is fully charged the second thyristor is cut off and the main thyristor is rendered conducting for passing the rectified supply. A third reversed thyristor 58 fired simultaneously with the main thyristor 50 discharges the capacitor 52 through inductance 60 whereby the charge on capacitor 52 is reversed and the third thyristor is rendered non-conductive. When the second thyristor 56 is again rendered conducting the reverse charge on capacitor 52 renders the main thyristor non- conducting until it is again forwardly charged by the second thyristor 56 for repetition of the cycle.