Patent GB-621860-A (GB)
Title: Improvements relating to alternating current circuits and their application to electric control systems
IPC: H02P25/26
Filed: 1946-04-04
Abstract: 621,860. Frequency measurements. ENGLISH ELECTRIC CO., Ltd., and WOODFORD, C. G. A. April 4, 1946, No. 10438. [Class 37] [Also in Group XXXV] A network connected to a three-phase circuit 1, 2, 3, as shown in Fig. 1, has three arms comprising respectively a resistance 5b in series with a capacitance 5a, a resistance 6b in series with an inductance 6a, and a resistance 4, the arms forming a delta connection and the circuit contacts being adjusted so that at a predetermined frequency the voltages across resistances 5b and 6b are equal and in antiphase so that zero voltage appears across terminals 8, 9, connected to the two resistance-reactance junction points. A rise in frequency causes an increase in the voltage drop across resistance 5b and a decrease in that across resistance 6b, the voltages remaining substantially in antiphase, so that a resultant voltage in phase with the current in resistance 5b appears across terminals 8, 9, and its magnitude is proportional to the frequency deviation. Conversely, on a decrease in frequency the voltage is in phase with the current in resistance 6b. Resistance 4 may be omitted from the net if phase unbalance is permissible. The frequency at which the resultant voltage becomes zero is adjustable by potentiometer 7. In a modification (Fig. 2, not shown), the three arms are star connected. In the application shown in Figs. 3, 4, a prime mover 13 drives an alternator 12 supplying a variable speed induction motor 10 driving a second alternator 11 supplying a frequency-sensitive network 15 of the kind described. The output from terminals 8, 9 is applied to the commoned grids of two thermionic amplifying valves 26, 27, the anodes of which are supplied in antiphase from the secondary of a transformer 29, whose primary is energized in phase with the current in resistance 5b. The amplifier output from terminals 19 and 20 connected respectively to the centre tap of the transformer secondary and to the cathode return of the valves is fed to winding 25b of a two-phase reversible A.C. motor driving a three-phase regulator 14 connected in the rotor circuit of motor 10. The winding 25a of the two-phase motor is continuously energized from the transformer primary supply through a phase-shift condenser 35. Changes in speed of the prime mover 13 and consequently of the motor 10 cause deviations in the output frequency of the alternator 11 from the predetermined value which in turn cause a voltage of appropriate magnitude and phase to be applied to the input of the thermionic amplifier. A pulsating current dependent in phase on the direction of the frequency deviation, then flows in one of the valves, one half of the transformer secondary, and in the motor winding 25b which is tuned to resonate at the frequency of the pulsations by condenser 36. The motor rotates in the appropriate direction to regulate the speed of motor 10 until the alternator speed and frequency are stabilised at the required value. In modifications (not shown), the field of a universal motor used to regulate the speed of motor 10 may be energized by the amplified output of the frequency-sensitive network, while the armature is supplied with current in phase with the current in one of the resistors 5b or 6b. A phase-sensitive relay may control the direction of rotation of a reversible A.C. or D.C. regulator motor, or the latter may be an A.C. commutator motor with rocking brushes adjustable by the speed control motor, which may also be used to vary resistance in the field of a D.C. regulator motor, or the fuel regulator of a prime mover.