Patent GB-670533-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: H02P3/14
Filed: 1949-08-05
Abstract: 670,533. Control of D.C. motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. Aug. 5, 1949. [Aug.. 10, 1948] No. 20415/49. Class 38 (iii). [Also in Group XXXV] An electric motor has an armature and two separately excited field windings of mutually opposing field polarities, rectifier means connect the armature to a power supply, two controllable field rectifiers each comprising a plurality of electronic tubes provide. full wave rectification and connect the power supply to the respective field windings, the. electronic tubes having a common cathode lead and respective grid circuits connected with said lead and having voltage source means for providing variable grid control voltages, and field control means provide a variable . control voltage, the grid circuits being connected to the control means to respond in mutually inverse relation to the variable control voltage whereby, dependent upon the direction of the departure of said control voltage from a given value, one or other of the field rectifiers is controlled to excite its associated field winding in accordance with said deparature. The armature 13 of a direct-current motor is supplied through thryatrons 16, 17, having a common cathode lead 18. The motor has opposing field windings' 14, 15, which are respectively excited through rectifying valves 31, 32, and 33, 34, all of which have a common cathode lead 35. The armature rectifiers 16, 17, are controlled in response to a motor'currentmeasuring voltage from rectifier 25 to provide normally-constant armature current, and the field rectifiers 31-34 are controlled to maintain the motor speed at a value set by rheostat 40 the voltage of which is compared with one derived from tachometer generator 28. The valves 16, 17 have a variable grid voltage component derived from resistor 51 which is in the anode circuit of amplifier valve 63. A voltage proportional to the motor current is developed across resistor 26, and, when this voltage exceeds a given value, tube 66 breaks down to render valve 63 conductive, so that a change of armature current above a set value is opposed by appropriate grid control of the armature rectifiers. When contact 12 of relay CR2 is closed a capacitor 52 is charged. When after energization of transformer 1 the relay CR2 opens contact 12 the capacitor discharges through resistor 51 so biasing the valves 16, 17, that current peaks are limited during the initial period of acceleration. Resistors 75 and 88 provide variable, unidirectional grid voltage components for the rectifiers 31, 32, and 33, 34, respectively. These two resistors are connected in the respective anode circuits of a twin triode valve 83 which is regulated by a similar valve 101. The slider of the rheostat 40 may be positioned so that the field windings 14, 15, carry equal current and the motor develops no torque. When the slider is moved away from this position the current passed by one of the parts of valve 101 is increased, that of the other part being decreased. The valve 83 is similarly controlled, and, in turn, the rectifiers 31, 43, and 33, 34, so that one of the field windings is predominantly excited and the motor rotates in a direction dependent upon the direction of movement of the slider and at a speed in accordance with the magnitude of such movement. The tachometer generator 28 develops a voltage across rheostat 30 which opposes that of the rheostat 40 in the grid circuits of the valve 101, so that the motor speed is held substantially at the pre-set value. A change-over in field excitation may take place as a result of overspeeding or by adjusting the rheostat 40 towards the zero-speed setting, or beyond to a reversed torque position. In these circumstances regenerative operation is obtained. When a large change of speed is called for by appropriate adjustment of the rheostat 40, the voltage derived from rectifier 70 exceeds a minimum value at which relay CR3 responds to close contact 68 and connect resistors 26, 27, in parallel. This allows a larger armature current to be passed. The rectifier 70 may be omitted or replaced by other means.