Patent GB-664379-A (GB)
IPC: H02P7/298
Abstract: 664,379. Control of D.C. motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. Aug. 5, 1949. [Aug. 10, 1948] No. 20414/49. Class 38 (iii). [Also in Group XXXV] A control system for a direct-current motor energized from an alternating-current source comprises electronic armature rectifier means and electronic field rectifier means connecting the armature and field windings respectively to the source, said rectifier means having a common cathode lead and respective grid circuits, control means connected between said common cathode lead and said respective grid circuits and having a source of variable voltage for controlling said two rectifier means, and a constant grid bias supply means connected in said respective grid circuits and having a bias adjustment for making said field rectifier means conductive prior to conductance of said armature rectifier means when said variable voltage is caused to attain a value required to operate said armature rectifier means to commence operation of the motor. An electric motor has an armature 6 and separate field windings 7, 8, the armature being supplied through thyratrons 11, 12, and the field windings through thyratrons 25, 26, and 27, 28, respectively. A common cathode lead 29 is provided for all the armature and field rectifiers. The control circuit for the armature rectifier includes three sources of component grid voltage for each valve 11 and 12. A first component grid voltage is supplied from transformer 35 and consists of an alternating voltage which is dephased with regard to the relative anode voltage. A constant direct grid voltage component for the armature rectifier is developed across resistor 46 and is derived through transformer 48, rectifier 49, filter 51, 52, 53, resistors 55, 57, and voltage regulating tube 58. A third source of variable grid voltage for valves 11, 12, is represented jointly by resistors 42, 44, which are connected in the anode circuits of a twin-triode valve 61. Both sections of the valve 61 are separately controlled by means of another twin-triode valve 68 which is regulated in accordance with the difference between a voltage derived from a rheostat 15, supplied from a motor-driven tachometer generator 13, and one obtained from a rheostat 84, supplied with constant voltage from transformer 77 and rectifier 78 and shunted by voltage regulating valve 82. The grid circuits for the two sets of field rectifier valves include' each a phase-shift transformer 89 or 93 and resistor 46. In addition, the grid circuit for valves 25, 26, comprises resistor 42, the resistor 44 being connected in the control circuit of valves 27, 28. When tap point M is coincident with point L on the rheostat 84 both sections of valve 68 are equally conductive, the sections of the valve 61 being likewise equally conductive so that all the thyratrons are biased to cut-off. If the slider M is moved upwardly unequal magnitudes of current in the respective sections of the valves 68, 61, results and valves 25, 26, are rendered conductive to energise field winding 7 before the valves 11, 12, commence to pass current to the armature. The valves 27, 28, have additional negative bias and the field winding 8 is not energized. Any deviation between the values of the reference voltage derived from the rheostate 84 and the speed-measuring voltage across rheostat 15 changes the firing- angle of the valves 11, 12, in a sense to maintain the motor speed at the desired value, the field excitation remaining substantially unchanged. Movement of the slider M in the opposite direction energises field winding 8 for reverse operation. When the motor is operating at speed and the slider M is moved towards the zero position, the field winding for reverse rotation is energised and regenerative operation may result. With overhauling loads, regeneration will be obtained under appropriate conditions, the change-over in field excitation taking place as referred to above. When the motor armature current exceeds a given value, a valve 39 breaks down and a voltage is developed across resistor 38 which biases the valves 11, 12, to limit the armature current.