Patent GB-711402-A (GB)
Title: Improvements in or relating to ward-leonard controls
Inventor: FONSECA EVELYN CECIL MUSCHAMP
IPC: H02P7/34
Filed: 1952-03-25
Abstract: 711,402. Control of D.C. motors; controllers. BRISTOL AEROPLANE CO., Ltd. March 17, 1953 [March 25, 1952], No. 7683/52. Class 38(3) Relates to Ward Leonard systems in whicl trimming resistances are provided in the excitation circuit of the generator in order to allow for differ ences in the electrica characteristics of the gen erator and motor resulting from manufacturing toler ances. As shown, a tapped potentiometer 14 is used for controlling the generator field winding 15, a trimming resistance R1 being connected at the low-potential end of the potentiometer. Adjustment of the resistance R1 therefore has its maximum effect on the potential applied across the field winding 15 when contact 16 is on the tapping 17 corresponding to the first voltage increment, the effectiveness of the trimming resistance being reduced as the movable contact is shifted towards the increased-field positions. Additional trimming resistances R2, R3, are used in a known manner for adjusting the speed of the motor with the potentiometer 14 at its maximum-speed setting. In an arrangement for reversing the motor, a pair of potentiometers are connected in parallel and are variable together step-by-step in opposite senses, the ends of the generator field winding being connected respectively to the selecting contacts of the potentiometers, Fig. 2 (not shown). The generator excitation is increased by shifting the selecting contacts of the potentiometers from a central position, the direction of rotation of the motor being dependent upon the sense of such movement. Each selecting contact is arranged to short-circuit a resistance portion of its potentiometer when in the central position, trimming means being provided to adjust the value of one or both of such resistance portions. In another arrangement, a continuously variable rheostat is placed in series with the generator field winding, and a variable trimming resistance is connected between the selecting contact of the rheostat and its high-potential end, Fig. 3 (not shown). The ends of the rheostat may be interconnected so that the maximum-resistance position is at the centre, a reversing switch cooperating with the selecting contact of the rheostat. In this case, a pair of trimming resistances are each connected between a selecting contact and one end of the resistance winding, Fig. 4 (not shown). In another modification involving the use of a series rheostat connected so that the maximum-resistace position is at the centre, the rheostat is tapped and the trimming means is adapted equally to adjust the value of the resistance of those portions of the rheostat winding constituting the first step on either side of the centre tapping, Fig. 5 (not shown). A construction for equally and simultaneously adjusting the trimming resistances comprises formers supporting the resistance windings in side-by-side relation, the selecting contacts each being in the form of a curved shoe which is shifted by a screw-and-nut device, Figs. 6, 7 (not shown). Specification 578,118 is referred to.