Patent GB-526013-A (GB)
Title: Improvements in systems employing electric valve means for the control of electric motors, particularly applicable to arc-welding control
IPC: B23K9/073
Filed: 1939-03-06
Abstract: 526,013. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. March 6, 1939, No. 7278. Convention date, March 5, 1938. [Class 38 (iii)] [Also in Group XXXV] An electric motor energized from an A.C. source through valve means having a control member which determines an electrical condition of the motor circuit has means responsive to the rate of change of an electrical characteristic of the motor circuit to control the current through the valve to stabilize the control. The direction and speed of rotation of the D.C. motor 1 are controlled by two reversely connected electric valves 67. The control electrodes 10 of these are connected to phase shifting circuits 14, 15 comprising saturable reactors 21 with or without fixed condensers 24; further condensers 25 shunted by resistances 26 serve as self-biassing means to provide a normal negative bias. The saturating windings 22 may be shunted by adjustable resistances 23 to determine maximum motor speeds in each direction. A saturating resistance 62 limits current in the motor armature during standstill. The saturating windings 22 are excited through electric valve means 38 the outputs of which are balanced against each other but variable in opposite senses by the arrangement of the circuits supplying their control electrodes. A full wave rectifier 34 supplies a potential divider 48, 49, 50 to the negative end of which the cathodes 41, 42 are connected through an impedance 47 ; the normal current flow in this impedance is such as to give a bias on the electrode 44 which is connected to a point 52 on the potential divider sufficient to equal the potential on the electrode 43 due to the control influence in its normal condition. Fig. 1 shows an arrangement for maintaining constant arc conditions in a welder, the motor 1 operating in the welding electrode which in this instance is supplied with A.C. A transformer feeds a unidirectional potential in accordance with arc voltage to a resistance 58 connected through an impedance 64 between an adjustable point 51 on the potential divider and the control electrode 43 so that the motor 1 will run in the appropriate direction to correct that arc condition by the variation produced in the saturation of reactors 21 when the potentials on electrodes 43, 44 are unbalanced in opposite senses by the changing of voltage at 58 and the resulting change in current in resistance 47 (which affects electrode 44). The motor speed will be dependent also on the arc voltage. Thus far the arrangement is as described in Specification 516,837, [Group XXXV]. Hunting is prevented by a transformer 63 connected across the motor 1 which applies a transitional potential to the impedance 64 to damp the effect of changes in the arc voltage. Modified damping arrangements are described in which (1) a centre tapped transformer such as 63 supplies through separate rectifiers a centre tapped impedance such as 64 and (2) separate transformers in series with the respective-valves 6, 7 supply potential to resistances in the respective grid circuits of these valves. The time delay action in all these cases is adjustable and in the two latter independently adjustable with respect to each direction of motor movement; condensers may be used in shunt to the adjusting impedances as a further damping means. A control circuit for the grid 43 in the case of a D.C. welder is also described arranged to be independent of arc polarity. Instead the transformer 55 for an adjustable potential divider may be used across the arc electrodes.