Patent GB-644133-A (GB)
Title: Improvements in or relating to control systems for alternating current motors
IPC: H02P25/32
Filed: 1947-08-08
Abstract: 644,133. Automatic speed control systems. ASKANIA REGULATOR CO. Aug. 8, 1947, No. 22006. Convention date, Dec. 23, 1944. [Class 38 (iv)] [Also in Group XXXVII] A system for controlling the speed and direction of rotation of an A.C. motor 10 comprises two saturable reactors 15 and 16 which each have A.C. windings, 67, 68, 69, and 71, 72, 73 respectively, connected in the supplylines to the motor stator terminals. Normally, these windings have sufficient impedance to prevent the application of an effective voltage to the motor, but D.C. reactor control windings 70 and 74 may be variably and selectively energized to reduce the impedance of one of the sets of A.C. reactor windings and so determine the magnitude and phase-sequence of the current-supply thereto. The control windings are adapted to be energized in accordance with magnitude and polarity of a main signal voltage derived from a battery 17. This voltage is normally opposed by a negative feed-back voltage derived from a motor-driven tachometer generator 12, and the resultant voltage is amplified and applied to one of the saturable reactor D.C. windings. The magnitude of the direct saturating current is dependent upon the value of the main signal voltage, the polarity of which determines which winding, 70 or 74, is to be energized. If a contact 19 of a batterysupplied potentiometer 18 is moved from a midposition, wherein terminals A and B of a voltage-divider 25-25 are at equal potential, to a position wherein the terminal A has the higher potential, a thermionic pentode valve 30 is rendered more conductive. This valve is normally biassed, however, so that a positive change of the grid potential produces only a slight change of anode current, a negative change leading to a large decrease. A similar valve 33 is negatively biassed and its anode current substantially reduced. The potential of a terminal C is slightly reduced and that of a terminal D rises, the difference of potential being applied across a voltage-divider 57-58. The grid potential of a valve 34 is increased, its anode current rises slightly, and the potential of a terminal G is slightly lowered to maintain the grid potential of a valve 35 below its cut-off value. The decrease in grid-potential of the valve 31 substantially reduces its anode current, and the potential of the terminal F rises appreciably. The valve 32 is rendered conductive to energize the winding 70. If the main signal voltage is adjusted in an opposite sense, reverse operation obtains wherein the valve 32 remains at cut-off and the valve 35 has an anode current substantially proportional to the strength of the signal. The voltages developed across the resistors 24 and 25 due to the positioning of the contact of the potentiometer 18 is modified by the pilot signal voltage derived from the generator 12 in such a manner that the resultant voltages controlling the valves 30 and 33 are sufficient to maintain the motor speed at fixed values determined by the settings of the potentiometer. Any changes of motor-speed due to changes of load are registered by a change of pilot signal voltage which corrects the deviation of speed from the desired value. The mains voltage may be much higher than the rated voltage of the motor, and, if the potentiometer 18 is adjusted to provide higher speed operation, the sudden increase in the resultant control voltage will provide a heavy torque producing rapid acceleration. When the potentiometer is adjusted to afford a lower speed, the pilot control signal becomes greater than the main signal voltage and the resultant control signal voltage is reversed so producing reversal of motor torque for a period and rapid deceleration. When the element 18 is adjusted to provide reversal of the motor the main and pilot control signal voltages become additive for a period. This produces a heavy reversed torque and a rapid change to the desired new condition. In Figs. 6 and 7 (not shown) the operating conditions obtaining during change of speed and motor reversal are graphically illustrated. The control of wound-rotor and squirrel cage induction motors is referred to. It is stated that the operation of a squirrel cage motor on the normally-unstable portion of its speed-torque characteristic is obtainable. Figs. 3 and 4 (not shown) illustrate the construction of a suitable saturable reactor, so that no alternating-current flux is linked with the direct-current coil.