Patent GB-877041-A (GB)
Title: Improvements in or relating to electrical rectifier systems
IPC: H02P7/26
Filed: 1959-11-03
Abstract: 877,041. Rectifying; automatic speed-control systems. BROOKHIRST IGRANIC Ltd. Nov. 3, 1959 [Nov. 5, 1958], No. 37250/59. Classes 38 (2) and 38 (4). [Also in Group XXXVII] In a polyphase rectifying system of which some rectifiers are uncontrolled and others are controlled by the application to their grids of alternating voltages phase displaced relative to their anode voltages, a steepfronted pulse is applied to the controlling grid of each controllable valve during the latter part of its normal conducting periods to ensure commutation in the correct sequence. The armature A of a D.C. motor is supplied from a three-phase rectifier bridge 6 connected across supply lines L1, L2, L3 and the field winding SF is supplied through rectifier bridge 14. The speed and direction of the motor is regulated by control network 36. The rectifier bridge 6 consists of three controlled rectifiers T1, T2, T3 and three uncontrolled rectifiers T4, T5, T6. Transformers PT1, PT2, PT3 have primary windings connected across lines L2- L3, L1-L3, L1-L2, respectively, and three secondary windings, of which the upper two heat cathodes of the uncontrolled and controlled rectifiers respectively and the lowest supplies rectifier bridge 14. The field winding SF is connected in parallel with a voltage divider 2R, 3R across output terminals 24, 26 of bridge 14. The voltage divider applies a control voltage DC (Fig. 2B) to the grids of controlled rectifiers. Transformers PT4, PT5, PT6 having primary windings star connected to lines L1, L2, L3, supply A.C. control voltages to the grids of their associated rectifiers which lag the anode voltages by 90 degrees. The A.C. control voltage e.g. CV of rectifier T1, causes the rectifier to fire when curve CV cuts the grid control locus CL. The other rectifiers fire in a 120-degree sequence. To ensure firing in the correct sequence a pulse is applied to the grid of each controlled rectifier later in its conducting period. The peaking circuits connected across the third secondary windings of transformers PT1, PT2, PT3 comprise rectifiers RT1, RT2, RT3, primaries of transformers PT7, PT8, PT9 and in common, parallel connected capacitor C4 and resistor 1R. Thus capacitor-C4 is continually charging and discharging as shown by curve EC4 and permits flow of current pulse P as EC4 increases. IR has such a value that pulse P occurs during the last 15 degrees of the anode voltage wave ET 1 which transmitted from the secondary winding of transformer PT9 causes T1 to fire if voltage CV fails to make it fire. Half-wave rectifier RT4 is provided between positive output terminal 24 and cathodes of T1, T2, T3 to prevent terminal 24 becoming negative relative to the cathodes and thus rendering pulse P ineffective. Centre taps on the lower secondary winding of transformers PT1, PT2 supply power to the control circuit 36 through leads 32-34. In parallel with divider 2R-3R are setting and maximum and minimum potentiometers SC, JS, MS in series, which determine the value of a reference voltage for setting the speed of the motor. Power is supplied to timer TMR which times the closing of TMR1 to allow cathodes of rectifiers to heat up. To run in the forward direction, switch- FOR is depressed energizing contactor F and run control relay RCR. Forward contactor F closes switches F1, F2 and connects resistor SAR in series with armature A across output terminals 2-4 of rectifier bridge 6. Relay RCR closes switches to provide a path between the reference potentiometer and the voltage divider 8R-9R across which the motor develops a D.C. voltage which is compared with the reference voltage across potentiometer SC, the difference being applied to the control electrodes. Current limiting relay CLR is provided to limit current in the armature; a dynamic breaking relay DB to insert a resistive load DBR across armature A; and an anti-plugging relay AP which prevents restarting the motor in either forward or reverse directions until the armature voltage decreases to a safe value. A reverse relay R is provided for reverse motion of the motor. In another embodiment (Fig. 3, not shown) tha peaked pulse is applied to the grid of controlled rectifier in parallel with the A.C. control voltage.