Patent GB-664625-A (GB)
Title: Improvements in or relating to control systems for direct current motors
IPC: B65H23/198
Filed: 1949-05-02
Abstract: 664,625. Control of D.C. Motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. May 2, 1949. [May 5, 1948] No. 11677/49. Class 38 (iii). [Also in Group XXXV] A control system for a direct-current motor having an armature and a field winding comprises a first controllable electronic rectifier means connecting said armature to an alternating-current supply and having a control circuit, a second controllable electronic rectifier means connecting said field winding to said supply and having a control circuit, an electronic master tube having a grid circuit and a plate circuit, said plate circuit being connected with the control circuits of said first and second rectifier means for jointly controlling said rectifier means and grid voltage supply means connected with the grid circuit of said master tube and having circuit means associated with said motor to provide a voltage indicative of the motor speed for controlling the said master tube. A direct-current motor M drives a reel 3 and is energised from transformer 9 through thyratrons 11, 12, 13. The main field winding 7 of the motor is energised from transformer secondary winding 23 through thyratron 27, full-wave rectification for the field winding being obtained by means of diode valve 32. The grid bias voltage of valves 11, 12, 13, comprises three main control components, the first being that derived from transformers 43, 44, 45, which produce an alternating component for each valve lagging the associated anode voltage by substantially ninety degrees. The second component is derived from resistors 48, 49 which form part of a voltage divider connected across constant voltage source 54. The third grid voltage component for the armature rectifying valves is derived from load resistor 47 connected in the anode circuit of a master amplifying valve 35 the anode voltage of which is derived across resistor-sections 48, 49, 51, 52. The grid voltage of valve 35 comprises a bias component derived from speedcontrol rheostat 66, which is connected across resistor-section 53 shunted by voltageregulating tube 80, and an opposing speedmeasuring component represented by a selected portion of the output of a tachometer generator 4. Any deviation of the speed of the motor M from the valve set by rheostat 66 causes the master valve 35 to alter the voltage across resistor 47 and to advance or retard the firing- point of the rectifiers 11, 12, 13, in such manner as to correct the deviation. The load resistor 47 is also connected in the grid circuit of valve 27 which also includes resistor-section 48, providing a constant direct grid-voltage component, and rheostat 83 which affords an alternating component. When the master valve 35 is conditioned to vary the grid potential of the valves 11, 12, 13, that of the valve 27 is modified in the opposite sense, and it may be arranged that decrease of field strength is allowed only when the armature voltage is at a maximum, the field excitation being adjusted to its highest value before the armature voltage is reduced. The joint control of the armature and field rectifiers by the master valve is also effective during changes of speed adjusted by rheostat 66. Current-limiting means comprise resistors 87, 88, across which a rectified voltage proportional to the motor armature current is developed. When the armature current tends to become excessive a tube 89 breaks down, so changing the grid voltage of the valve 35 that the motor current is reduced. A capacitor 90 has a stabilizing effect in the grid circuit of the valve 35. When voltage is applied to transformer 24, relay 4CR closes its contacts and depression of start button 74 energises relay 1CR, so energising the armature of the motor through relay FCR. Operation of stop button 75 de-energises relays 1CR, FCR, and the motor is dynamically braked through resistor 19, relay 3CR preventing relay FCR from being re-closed as long as the motor speed is above a safe value. A contact 17 of the relay FCR short-circuits a capacitor 92, which is connected between the control grid and anode of the valve 35, when the relay is de-energised, so that the valve is fully conductive under these circumstances, the armature rectifiers being blocked and field rectifier fully opened. When the relay FCR is energised the capacitor 92 is charged so delaying the rise of motor armature current. After the capacitor 92 is fully charged, the current-limiting circuit energised from current transformer 22 becomes effective until normal operating conditions are reached and the tachometer generator is effective to regulate the speed. The capacitor 92 also acts as a stabilizer for the valve 35 during normal operation. When inching button 76 is actuated relays 2CR, 1CR, are respectively energised and de-energised. Resistor 55 is placed in the anode circuit of valve 35, so reducing the voltage-drop across load resistor 47 to a value affording low speed operation. Specification 664,610 is referred to.