Patent GB-738367-A (GB)
Title: Improvements in and relating to devices for measuring currents in polyphase rectifying systems
IPC: G01R19/20
Filed: 1953-12-30
Abstract: 738,367. Automatic speed control systems. GENERAL ELECTRIC CO. Dec. 30, 1953 [Dec. 30, 1952], No. 36236/53. Class 38 (4). [Also in Groups XXXVI and XXXVII] Apparatus for measuring current drawn from a 3-phase power supply system and rectified to energize a load, comprises a current transformer having a 3-legged core carrying on two of its legs four primary and four secondary windings, the first primary being connected in the first phase and wound on the first leg, the second primary being connected in the second phase and wound on the second leg, the third and fourth primaries being series connected with opposite senses in the third phase and wound respectively on the first and second legs,. the first and second secondaries being wound on the first and second legs, the third and fourth secondaries being series connected and wound respectively on the first and second legs, and there being provided three auxiliary rectifiers in circuit respectively with the first, the second, and the third and fourth secondaries to energize a common load resistor across which a voltage drop is developed proportional to the total load current in each primary of the current transformer. In Fig. 1 the armature of a D.C. motor 1 is energized from the common cathodes. of thyratron rectifiers. 2, 3, 4 whose anodes are connected to terminals 8a, 8b, 8c of the starconnected secondary of a main transformer energized from the 3-phase supply. The motor current is controlled by a voltage on'the grid of each thyratron comprising an alternating component lagging the anode voltage by 90 degrees from the star connected secondary 12d, 12e, 12f of a grid voltage transformer whose primary is delta-connected to the terminals 8a, 8b, 8c of the main transformer, combined with a variable direct component which varies the phase of the firing instant of each thyratron, and thus the average current in each. A single phase transformer 16 energizes a voltage doubler circuit comprising oppositely poled rectifiers 19, 20 and capacitors 17, 18 in series across the output which is centre-tapped to the common return 21 of the thyratron cathodes, and energizes potentiometer 13 across capacitor 18, whose slider voltage biases the grid of a triode 14 whose anode is energized from the positive side of capacitor 17 through resistor 15 and whose cathode is returned to the junction of a ripple filtering condenser 25 and resistor 24 across the motor armature. A further direct voltage from rectifier 23 energized by single-phase transformer 16d is added to the difference between that across capacitor 17 and resistor 15 to bias the thyratron grids whereby the latter are biased to cut-off when potentiometer 13 is set at one extreme position so that the motor is inoperative, while on movement of the slider towards the opposite extreme the net bias progressively becomes more positive whereby the conduction of the thyratrons increases and the motor runs at an increasing speed limited by the counter-E.M.F. across resistor 24 increasing the current in triode 14 and thereby driving the thyratron bias more negative to retard their instant of firing in each positive half-cycle whereby the motor speed is checked at a value corresponding to the slider setting. The slider of a potentiometer 26 across capacitor 18 biases triode 50 in opposition to a voltage proportional to the thyratron current developed across resistor 31 in the triode grid return by the common cathode current of three half-wave rectifiers 28, 29, 30, whose anodes are respectively energized by secondary windings 35, 36 and 40 in series, and 39 of a 3-phase current transformer 27 (Fig. 2), whose primaries 33, 34 in series with 37, and 38, are respectively connected in the anode circuits of thyratrons 2, 3, 4. The transformer has a 3-leg core of which one outer leg 32a carries primary windings 33, 34 and secondary windings 35, 36 while the other outer leg 32b carries primary windings 37, 38 and secondary windings 39, 40. The primary windings are all equal while secondaries 36, 40 have half the turns of secondaries 35, 39, and the fields of windings 33, 34 and 37, 38 are opposed so that there. is no direct component of flux in the transformer. Secondaries 35, 39 are shunted with loading resistances 42, 43 while secondaries 36, 40 are oppositely poled, and it is shown (Fig. 3, not shown) that the voltage drop across resistance 31 during the conducting period of each thyratron is an accurate measure of the current therein during such period, and is applied to the grid .of triode 50 whose cathode bias derived from variable potentiometer 26 across capacitor 18 determines the current limit in the thyratrons. Triode 50 is non-conducting so long as no thyratron passes current greater than the limit, but when the voltage across resistor 31 exceeds the cathode bias, anode current flows through resistor 15 together with anode current of triode 14 to increase the direct bias on the thyratron grids whereby the firing instants of the latter in each positive half-cycle are retarded and the currents in each reduced below the pre-set limit value.