GB664408A - Improvements in or relating to control systems for mercury arc rectifiers - Google Patents

Abstract

664,408. Rectifying. NEVELIN ELECTRIC CO., Ltd., and BERTELE, H. C. Oct. 21, 1949 [Oct. 21, 1948], No. 27418/48. Class 38 (ii). A grid control circuit for a mercury arc rectifier of the type comprising a saturable grid transformer includes an auxiliary D.C. supply circuit having an impedance therein which is common to the main D.C. saturating current supply circuit, means for producing in the auxiliary circuit an auxiliary current which flows through the common impedance and means for automatically changing the value of the auxiliary current when the load current exceeds a predetermined value so that the auxiliary current acts in the common impedance to change the saturating current in the grid transformer thereby to change the point in the A.C. cycle at which the grid pulses occur so as to reduce the rectifier load to, or below, the predetermined value. A potentiometer 28 is connected across the output of a mercury arc rectifier 12, the saturating winding of the saturable grid transformer being supplied from an adjustable tapping 33 thereon through a reactor 34 and a resistor 35. The T-bridge comprising three resistors 35, 62, 65 has a number of voltages applied to it including a variable voltage from a rectifier 61, which is fed with A.C. through windings on a saturable reactor 58, and a fixed voltage from a rectifier 64 fed with A.C. from a constant voltage source 63. When the rectifier output current exceeds a safe value the saturating current in the saturable grid transformers is reduced by injecting into the resistance 35 an auxiliary current obtained by comparing in the T-bridge the output of the rectifier 61 with the output of the rectifier 64 ; thus the rectifier output current is reduced. The T-bridge can be dispensed with by the provision of another winding 101 on the saturable reactor 58, Fig. 4. In this embodiment the saturating current for the grid transformers is provided by the voltages derived from the rectifiers 76, 78 which act in opposition across the resistors 79, 80, the saturable reactor 58 being biased from the resistor 79. Again an increase in the rectifier output current beyond a predetermined value causes a reduction in the saturating current in the grid transformer and thus reduces the rectifier output. By the provision of a further system comprising another T- bridge, similar to that shown in Fig. 1, current surges in the output circuit on switching on may be avoided, Fig. 2 (not shown). In the embodiment in which the T-bridge is dispensed with switching surges may be avoided by the provision of a further saturable reactor, in series with the original reactor. A current transformer may be used in conjunction with the input connections to the rectifier 12 to obviate the need for a winding on the saturable reactor 58 to carry the output current. Specification 664,400, [Group XXXVIII], is referred to.