Patent GB-896641-A (GB)
Title: Improvements in and relating to grid controlled current converter equipments
Inventor: BECK ALAN
IPC: B66B1/30
Filed: 1958-04-22
Abstract: 896,641. Control of D.C. motors. ENGLISH ELECTRIC CO. Ltd. April 21, 1959 [April 22, 1958], No. 12813/58. Class 38 (3). [Also in Group XXXV] A current converter 11 is adapted to operate either as a rectifier or as an inverter, and has a control unit 22 for applying positive firing impulses to the negatively-biased control grids and for varying the phase angle of the impulses relatively to the respective anode potentials in accordance with the energization of control winding 22a. As the input to the winding 22a is increased from zero, the grid-phase is advanced continuously from a value producing high inverter voltage to one corresponding to high rectifier voltage, the winding being supplied from a magnetic amplifier 21 having a feedback winding 21b for limiting the convertercurrent. A magnetic amplifier 34 energizes the winding 2Ib when a current-responsive winding 34a overcomes the combined effect of a bias winding 34b and a winding 34c whose excitation is dependent upon the condition of operation of the converter as determined by the magnitude of current in the winding 22a. Thus, the voltage developed across resistor 42 is less than a predetermined value for inverter operation, and greater than this value for rectifier operation. Whilst the converter functions as an inverter, the current in winding 34c increases as the grid-phase is advanced so that the currentlimiting value, at which the feed-back winding 21b responds, also increases with the phaseadvance. When the voltage developed across resistors 42, 43 exceeds the opposing voltage of source 46, a by-pass current flows through rectifier 45 and resistors 43, 44. The voltage applied to winding 34c then remains substantially constant despite further increase in the current flowing in the winding 22a. Consequently, the current-limiting value at which the feed-back winding 216 is energized remains substantially constant with advancement of the grid-phase into and through the rectifier range of values. The system functions to maintain the speed of an electric motor 10 in accordance with the setting of a control-lever 18, the motor being associated with a tachometer generator 19 in the manner described in Specification 892,231. Change from motoring to regenerative braking operation is effected as described in Specification 894,583. When mechanical brakes are applied to the motor, contacts 37 close and contacts 40 open, thus energizing a negative feed-back winding 34d for the amplifier 34 and reducing the excitation due to bias winding 34b. The amplifier has its gain reduced and produces an output signal varying linearly with the converter-current. The motor torque is thus controlled in accordance with the setting of the lever 18, the voltage of the tachometer generator being zero whilst the motor is stationary. The rate at which the phase angle of the firing impulses applied to the control grids of the converter is advanced may be limited automatically. This may be effected by limiting the rate of change of the input to winding 22a, the amplifier 21, or the speed reference voltage derived from potentiometer 17.