Patent GB-585171-A (GB)
Title: Improvements in or relating to control systems for direct-current motors
IPC: H02P7/26
Filed: 1944-07-21
Abstract: 585,171. Control of D.C. motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. July 21, 1944, No. 13977. Convention date, July 24, 1943. [Class 38 (iii)] [Also in Group XXXV] Varying electromotive force.-A control system for a direct current motor which is energized from an alternating current source comprises electric discharge valves of the arclike type interposed between said source and the motor armature and additional discharge valves for rendering the first valve means conductive in each half period of the source following operation of a starting device, said additional valves being effective to vary the firing-point of the first valve means to limit the armature current to a predetermined, maximum. A separately excited direct-current motor 43 is supplied from alternating-current mains through a transformer 11, a secondary winding 13 of which is connected to two rectifying discharge valves of the arc-like type 17 and 23, the cathodes of these valves being connected to the armature of the motor through a resistor 29 and contacts of relays 33 or 37. When a valve 101 conducts a maximum current, the potential across a resistor 73 which is in the grid circuit of the valves 17, 23 is such that these control valves do not fire. The conductivity of the master valve 101 is determined substantially by a potential, which is proportional to the armature voltage, across a resistor 87 opposing a potential impressed in the control circuit of the master valve through a potentiometer 79. In order to compensate for armature resistance drop a potential proportional to the armature current is placed across a condenser 169 and is amplified through an auxiliary thermionic valve 161 and impressed through a potentiometer 147 in the control circuit of the master valve 101, so that an increase in armature current causes the firing- point of the control valves 17 and 23 to be advanced. A switch 209 is closed to start the motor in a forward direction. A condenser 191 is momentarily discharged and then charged through contacts of a relay 33. The circuit through the armature 41 is closed for forward operation and a dynamic braking resistance 303 is disconnected from the armature. A relay 255 is de-energized as the armature voltage is low so that the potential across a resistor 249 is impressed in the cathode-grid circuit of two rectifying valves of the arc-like type 221 and 223, which control the supply to the motor field 217, so that full field strength for the motor is provided, regardless of the setting of a potentiometer 245, which is also in the control circuit of the valves 221 and 223. When the circuit to the armature is closed, condenser 191 has been discharged and the potential across the potentiometer 185 is zero. A second auxiliary thermionic valve 179 is therefore conductive and a high voltage obtains across the resistor 87 in the control circuit of the master valve 101 which is rendered conductive to develop a high potential across a resistance 73 in the control circuit of the valves 17 and 23 which are rendered non- conductive. The charging of the condenser 191 is initiated at a rate dependent on the setting of a resistor 203. The potential across the potentiometer 185 gradually increases and the firing-point of the control valves 17 and 23 is gradually advanced. The conductivity of the valve 179 is controlled according to the difference in potentials across the potentiometer 185 and the resistor 29, which carries the armature current. Should the armature draw excessive current, the voltage across the resistor 29 rises sufficiently to delay advance of the firing-point of the control valves. When the condenser 191 is sufficiently charged it no longer delays the firing-point. When the rated motor speed is reached the relay 255 is energized to short circuit the resistance 249 and bring the field excitation under the control of the potentiometer 245. At the end of the acceleration period the valve 179 becomes non- conductive, and thereafter the valve 161 is effective for compensation for armature voltage-drop. A switch 273 is opened to stop the motor and an armature-shunt is closed through the resistor 303 to provide dynamic braking. To reverse the motor, potentiometers 267 and 269 are set for the desired reverse speed and a switch 211 is closed. The potentiometer 185 and the resistor 189 are placed across the condenser 191, the discharge circuit of which is closed. A relay 37 is energized after the relay 33 has been de-energized, thus affording time for the discharge of the condenser 191 and partial dynamic braking of the motor, which is quickly reversed upon the application of reversed potential. The control of the motor is thereafter similar to that obtaining with forward working.