Patent GB-918441-A (GB)
Title: Improvements in and relating to electric remote control devices
IPC: H02P1/28
Filed: 1960-11-29
Abstract: 918,441. Control of A.C. motors. BROOKE, W., and HOWE, H. Nov. 29, 1960, No. 40932/60. Addition to 864,714. Class 38 (3). [Also in Group XXXIX] In the controlled saturable reactor described in the parent Specification in which internal feed-back is produced by a rectifying circuit connected at W1, W2 between the parallel main winding groups PA1, PA2 and PB1 PB2 associated with respective cores A and B, additional feed-back excitation which is externally derived is supplied to the rectifying circuit in parallel with, and to augment the internal feedback. As shown in Fig. 1, the rectifying circuit comprises a full-wave bridge-rectifier T having its D.C. terminals connected to W1 and W2 and its A.C. terminals energized from the secondary winding of a potential transformer E. The transformer primary winding Ep is connected in series with the A.C. windings CRa of a saturable control reactor CR to the output circuit L1a, L2 of the main reactor C. When a load WA is to be energized, a switch Q is closed so as to connect the main reactor excitation windings NA1, NA2, NB1, NB2 to the supply lines L1, L2 over a rectifier F and transformer P. The switch also connects the excitation windings CRe of the control reactor CR to a rectifier Fa connected in parallel with the main reactor at terminals Z1, Z2. Consequently the impedance of the control reactor winding CRa is inversely related to the main reactor voltage drop. Since the degree of additional feed-back excitation depends on the load voltage distribution between the control reactor and transformer E, the high-order voltage drop across the main reactor due to a high load current tends to be reduced by increase of the additional feed-back excitation. In a modification the excitation windings CRe are connected to transformer windings SA, SB on the main reactor cores, as shown in broken lines, instead of to terminals Z1, Z2. Also the control reactor A.C. windings CRA may optionally be connected in the secondary circuit of transformer E. As modified circuit employing push-button control is shown in Fig. 2. In this arrangement the excitation windings NA1, NA2, NB1, NB2 are initially energized from a transformer P connected in parallel with the main reactor when a start button K is depressed and the excitation is taken over by a transformer C connected across the load, the excitation circuit then comprising a full-wave rectifier H and a by-pass rectifier U. Operation is as previously described, the rectifying circuit between W1 and W2 in this example comprising two rectifiers T1, T3 and a centre-tapped secondary winding Es of potential transformer E. The load WA is effectively isolated from the supply by depressing a stopbutton J, which must be mechanically reset, to isolate the excitation circuits of the two reactors. In Fig. 4, the load comprises a threephase motor M, each phase being connected to a line L1, L2, L3 by a respective main saturable reactor C, CA, CB. When the start push-button K is depressed, the main reactor excitation is initially derived from star-connected transformer windings SA1, SB1 on the cores of each reactor. As the motor speeds up, the main reactor excitation is taken over by a transformer G in the load circuit. To permit the use of a stop-button biased to the closed position, a further saturable reactor CRB is provided which is saturated by the main reactor excitation current passing through its control winding Ye. This enables the output of star-connected transformer windings SA2, SB2 on the main reactors to pass over a rectifier FB to the excitation winding of control reactor CRA which is connected between the load circuit and a transformer E. In operation, a sufficiently high voltage drop across the main reactors due to a high load current provides sufficient output through reactor CRB to saturate control reactor CRA, and auxiliary feedback is then effective from transformer E to reduce the voltage between line and load. The secondary circuit of transformer E comprises centre-tapped secondary windings Es and rectifiers T1, T3 which together provide the rectifying circuits of the main reactors. A modified three-phase circuit for motor control may be used, Fig. 3 (not shown), which is essentially the same as Fig. 2 as applied to a three-phase system except that the main reactor voltage drop is obtained from transformer windings on the reactor cores. The further saturable reactor CRB is not provided in the Fig. 3 embodiment. Specification 900,196 also is referred to.