Patent GB-613049-A (GB)
Title: Improvements in and relating to motor control systems
IPC: H02P7/298
Filed: 1945-12-12
Abstract: 613,049. Automatic control systems for D.C. motors; rectifying. BRITISH THOMSONHOUSTON CO., Ltd. Dec. 12, 1945, No. 33654. Convention date, Dec. 18, 1944. [Classes 38 (ii) and 38 (iv)] [Also in Group XXXVII] A control system for an electric motor comprises electronic valve means for maintaining the armature current of the motor substantially constant at a predetermined value, and further electronic valve means, independent of the first means, for automatically varying the motor field current in such a way as to oppose changes in the predetermined value of a quantity representative of the speed, torque or horse-. power of the motor. The armature of a directcurrent motor is supplied from a transformer secondary winding 3a through two valves 6 and 7, the motor field winding 1b being supplied for forward rotation through valves 10 and 11 and for reverse rotation through valves 12 and 13. These valves are preferably of the thyratron type. The armature thyratrons are controlled by a phase-shift network comprising a saturable choke 17, the direct-current coil of which is connected in series with a valve 34 across a constant potential source. The grid potential of the valve 34 depends upon a signal voltage derived from the anode current of the armature thyratrons which is compared with an adjustable reference voltage derived from a potentiometer 42 so as to maintain the armature current at a rated value. The thyratrons 10 and 11 are controlled by a phaseshift network comprising a saturable choke 19, and the thyratrons 12 and 13 are similarly controlled through a saturable choke 21. A potentiometer 4 provides a source of adjustable reference voltage for pre-setting the speed and direction of motor rotation. A motor characteristic which is to be maintained constant is determined by a signal voltage, as shown, a tachometer generator 59 provides a signal voltage proportional to. motor speed. The valves 10 and 11 or 12 and 13 are energized according to the magnitude of the reference voltage, switching valves 44 or 45 being selectively energized according to the position of the potentiometer 4 to energize one of the valves 46 or 47 which are connected in series with the saturating windings of the chokes 19 and 21, the energized valve being controlled according to the difference between the signal and reference potentials to maintain the speed of the motor substantially constant. Valves 60, 61 and 62 are provided in order to allow the armature current to exceed the rated value for short periods. Valves 67 and 68 afford means to limit that current to a predetermined safe value. If the speed signal voltage increases to a value exceeding that required to turn the field thyratrons full on, one of the valves 60 or 61 is selectively rendered conducting to increase the current in the saturating winding of the choke 17. An increase of the armature signal voltage above a safe value renders one of the diode valves 67 or 68 conductive to block the valve producing the forcing action. When the motor is driven above its rated speed, the field excitation is reduced to limit the armature current to a safe value by means comprising a magnetron valve 75 which is excited by a coil 76 connected across the armature terminals If the armature voltage is increased above a predetermined value, the voltage drop across the valve 75 is increased to render diode valves 77 and 78 conductive and negatively bias whichever of the valves 46 or 47 is active to reduce the field excitation and consequently the armature voltage. If, when operating forwardly, the potentiometer 4 is suddenly adjusted to a lower speed or reverse position, the valve 47 is turned full on and the valve 46 cut off to reverse the motor field winding polarity and cause the motor to regenerate and return power to the source, the valve 60 being rendered conductive to enable the armature current to exceed the rated value. In a modification, the grids of the valves 60 and 61 are respectively connected to the anodes of the valves 44 and 45, thus allowing the valve 62 to be omitted. In a further modification, the valve 34 is omitted, the armature rectifiers being controlled solely through the valves 60 and 61. If the motor is coupled to an overhauling load, the reversed counter voltage of the motor may become sufficient to cause power to flow back to the source.