GB693840A - Improvements in control systems for electric motors - Google Patents

Abstract

693,840. Automatic speed control systems. HEWITTIC SOC. A R.L. Sept. 15, 1950 [Oct. 10, 1949], No. 22770/50. Class 38 (iv). [Also in Group XXXVII] In an automatic speed control system for a reversible, separately excited D.C. motor, the armature current at starting, or upon reversing, is automatically limited and maintained at the limited value throughout the acceleration phase. The motor armature 7 is fed via the reversing switch 90, 91, from the rectifier 1 which is controlled by the auxiliary rectifier 10. This is controlled, as disclosed in Specification 669,850, by superimposing a sawtooth waveform generated at 21, 22, 23, 24, upon a control voltage obtained by comparing the armature voltage with a reference voltage tapped off a potentiometer 93. A battery 41 applies a predetermined minimum bias to the rectifier 10 to prevent overload when the armature voltage is low at starting, or of a reversed polarity (due to the armature back-E.M.F. on reversing). Under these conditions, rectifier 43 blocks and rectifier 42 conducts. The cathode of the rectifier 10 is connected to a point on a potential divider 96, 95, 31, across the motor armature so that a portion of the armature voltage opposes the limiting bias source 41, hence the limiting bias is reduced as the motor speed increases, and the accelerating current is maintained substantially constant at its maximum value. The rectifier 98 connects the resistor 96 into the potential divider when the motor is connected as shown ; but upon reversal, the rectifier 99 connects resistor 97 instead. In a modification, Fig. 2 (not shown), the speed of the motor is measured by a D.C. tachometer generator. In a further modification, Fig. 3 (not shown), polyphase rectification is used, the several controlling valves being controlled by a cathode follower to which the speed &c. control signals are applied.