Patent GB-521663-A (GB)
Title: Improvements in and relating to speed control systems for direct current electric motors
IPC: H02P7/26
Filed: 1938-11-24
Abstract: 521,663. Field control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. Nov. 24, 1938, No. 34210. Convention date, Nov. 24 1937. [Class 38 (iii)] [Also in Group XXXV] Varying E.M.F.-Speed of a D.C. motor 1, Fig. 1, of which the armature 2 and/or field circuit is supplied through a valve rectifier 7, 8, is maintained constant by a D.C. pilot generator 25 which introduces its voltage (dependent on speed), into the circuit in energizing a control electrode 16 of the rectifier and the rate of change of the potential of that electrode is controlled to prevent hunting. Fig. 1 shows a phase shifting circuit 18 energizing the control electrodes, through a transformer 17, the secondary of which is connected to an excitation circuit including in series (1) the-reverse connected pilot generator 25, (2) an adjustable source of reference voltage determining the normal motor speed, which is provided by a potentiometer 28, (3) a resistance 32, and (4) a condenser 31. The effective phase relationship between anode and control electrode voltages is thus varied upon a change in speed to cause the armature current to be adjusted in a compensatory manner, but large changes in speed are damped by the action of the condenser 31 and hunting thereby prevented. A condenser 33 in parallel with the resistance 32 facilitates regulation within narrow limits. As shown in Fig. 2 the output from a source 62 through a resistance 50 in the grid circuit of valves 7, 8 in the field circuit of the motor is controlled by a vacuum type valve 51 the control electrode potential of which is determined by a potentiometer 57 in opposition to the pilot generator 25. In this case changes in the potential across resistance 50 produce current through a condenser 69 and a resistance 70 whereby a voltage is introduced into the grid circuit of valve 51 to delay the change. In a modification the condenser 69 and resistance 70 are connected across a resistance in the armature circuit so as to be responsive to current changes therein. The pilot generator may feed through a bridge circuit comprising impedance elements of non-linear characteristics such as those disclosed in Specification 292,110, [Class 38 (iv)], as indicated at 137 in Fig. 5, whereby its voltage changes are accentuated. Further modifications are described in which the current variations due to the pilot motor are applied to a saturating winding of an inductance in the phase shifting circuit 18 of Fig.. 1. In one form the anti-hunting means comprise a circuit across the armature including a condenser and a resistance in series with the saturating winding such that the charging current in the condenser produces a drop of potential in the resistance to vary the saturating current; in another, in which the field excitation is controlled by the rectifiers 7, 8 the anti-hunting circuit is similarly constituted. The Specification as open to inspection under Sect. 91 comprises a modification in which an A.C. pilot generator supplied a voltage proportional to its speed through a saturated transformer (producing a peaked wave) and rectifier to the anode and cathode of.the valve 57 in Fig. 2. This subject-matter does not appear in the Specification as accepted.