Patent GB-666191-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: H02P3/14
Filed: 1949-07-01
Abstract: 666,191. Control of D.C. motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. July 1, 1949. [July 3, 1948] No. 17436/49. Class 38 (iii). [Also in Groups XXXV] A method of regeneratively braking a directcurrent motor, energized from an alternating- current source through controllable rectifier means, comprises impressing on said rectifier means during the normal operation of the motor a control voltage which is continuously variable from a rectifier means cut-off value through a range of values corresponding to a range of rectifier means firing angles which begins after a positive maximum of the rectifier anode voltage and before the next zero point of said anode voltage and ends after that zero point but before the next negative maximum of said anode voltage, and reversing the polarity of the connection of the motor to provide for regenera tive braking whilst said motor is running anc at a time when said control voltage has said cut-off value. The armature A of a direct-current motor M is supplied from the secondary winding 3 of a transformer MT through thyratrons 5, 6, the respective grid control circuits of which comprise the secondary winding 27 of a transformer 38. This transformer is energized through a phase-shift circuit 29 and provides an alternating-voltage component lagging the anode voltage of the valves 5, 6 by approximately 120-135 degrees. A constant positive direct-voltage grid-bias component for the thyratrons is represented by the algebraic sum of the voltages across the active portion of a rheostat 33 and across resistors 36, 37, 38. A variable grid voltage component for the thyratrons is derived from resistor 35 connected in the anode circuit of a main control valve 51 the grid voltage of which comprises a reference component derived from speed-control rheostat 39 and an opposing speed-responsive component obtained from'resistor 23 connected across the armature terminals. The grid circuit of the valve 51 also includes rheostat 54 providing a control voltage component proportional to the motor current and compensating for resistance-drop in the armature circuit. The rheostat 54 is connected in the anode circuit of valve 61 the control voltage of which comprises a constant component derived from rheostat 64 and a current-responsive component obtained from rheostat 67 which is energized from transformer 9 through rectifier 70. An amplifying vacuum valve 71, the control voltage of which comprises a component derived from rheostat 74 and a currentresponsive component from rheostat 67, limits the armature current of the motor. When the speed-control slider 41 is in the "off" position, the closing of start switch 95 energizes relay 3 CR which short-circuits the switch. When the slider is moved towards the right, contact 89 engages strip 92 to energize relay ICR. Contactor FRC is then energized, the armature circuit-closed, and the motor accelerates to the speed selected in accordance with the position of the slider 41. If the slider is moved further from the neutral position, the grid potential of valve 51 becomes increasingly negative, the voltage across resistor 35 decreases, and the firing-points of the thyratrons 5, 6, are advanced. The resultant increase of speed produces a larger voltage across resistor 23 and a balanced condition is again reached. When the armaturedrop increases, the conduction of valve 61 increases and to the rheostat 54 a larger voltage is applied which, in the grid circuit of valve 51, is cumulative to the voltage from rheostat 39. An increasing armature current, measured by the voltage across resistor 67, reduces the negative potential of valve 71 to increase the potential across resistor 56. The anode current of valve 51 becomes greater and the firing-points of the thyratrons 5, 6, are consequentially delayed.. The valve 71 conducts only when the motor current exceeds a pre-determined limit, but, when effective, its operation over-rides that of the valve 61. When starting, the charging of condenser 76 slowly decreases the conductivity of valve 71 and gradually advances the firing of the thyratrons so that the magnitude of the early armature current pulses is limited even before the valve 71 responds to the control effected through resistor 67. In order to reverse the motor the slider 41 is moved through the "off" position in which relay ICR is de-energized. The capacitor 76 is discharged through contacts 78, 84, before relay 2CR picks up, the voltage across rheostat 75 dropping to zero when contact 7 opens. At the instant of closure of contactor RCR valves 71 and 51 both conduct full current and the thyratrons 5, 6, are cut off. When relay 2CR closes, resistor 75 is connected across capacitor 76 which is then gradually charged, the motorconnections being simultaneously reversed. During the period of charging of the capacitor 76 the thyratrons again become conductive, the motor returning current to the mains through the transformer MT and being regeneratively braked until the speed approaches zero, whereupon the motor is accelerated in the opposite direction. During reversing operation the current-limiting control effected by valve 71 remains in force, Fig. 2, provides a graphical illustration of the motor-performance during the re-generating period. In that figure the transformer voltage is indicated by Aa, the alternating grid voltage component of one of the thyratrons at different moments is represented by Ag1-Ag7 and the direct grid voltage component of the same thyratron at corresponding moments by Ec1-Ec7. The corresponding voltages generated in the motor armature are shown by Eg1-Eg7. If the motor is running at full speed and the contactors RCR, FCR, are reversed so that the voltage generated in the armature changes its relative polarity, and the conditions at the closing- instant of the main contactor are represented by Ec1, Ag1, Eg1, then the thyratron will not fire (assuming that the critical grid voltage coincides with the cathode potential). At later instants the tube fires at points X2-X7. The slider 41 may be moved only slightly beyond the "off" position to allow reversal of contactors RCR, FCR, and to bring the motor to standstill. Mechanism may be provided allowing limited movement of the slider beyond the neutral position in a selected direction, other movement being free, Fig. 5 (not shown). Reverse running after re-generative braking may be prevented by a zero-speed switch, Fig. 6 (not shown). In a modification, Fig. 7 (not shown), reversing performance is controlled by push-button or travel limit switches, and the motor may drive a reciprocating part such as the table of a planer. In this embodiment, a dynamic braking resistor becomes effective only when the motor is to be stopped.