GB674167A - Improvements relating to electric motors and their control means - Google Patents

Abstract

674,167. Control of A.C. motors. REYROLLE & CO., Ltd., A., and HIBBERT, J. W. Feb. 9, 1950. [Feb. 11, 1949; Aug. 9, 1949] Nos. 3846/49 and 20704/49. Class 38(iii). [Also in Group XXXV] A reversible series polyphase commutator motor has means operable when reversing the direction of rotation for shifting the phase relationship between the rotor and stator through an electrical angle equivalent to not less than a complete phase interval, and at least a part of said phase shift is produced by switch means arranged to change the connections of the rotor or stator circuit. In Fig. 1, a three-phase reversible series commutator motor is arranged for speedadjustment by movement of the brushes. The rotor has brushes A<SP>1</SP>, A<SP>2</SP>, A3, connected to transformer secondary windings B<SP>1</SP>, B<SP>2</SP>, B<SP>3</SP>. The transformer primary windings C<SP>1</SP>, C<SP>2</SP>, C<SP>3</SP>, are connected to motor stator windings D<SP>1</SP>, D<SP>2</SP>, D<SP>3</SP>, through a phase-shifting switch E, and to a three-phase supply through a reversing switch G. A shift of the contacts of the switch E from one closed position to another shifts the stator voltage through 120. electrical degrees. Movement of the brushes in either direction from a neutral, no-torque, position closes one or other set of the contacts E, so that, when changing from full-torque operation in one direction it is not necessary to move the brushes through a wide angle in order to obtain full-torque motor operation in the opposite sense. For applications, such as hoisting, where the starting torque in one direction needs to be greater than in the other, it is arranged that, with one set of contacts of the switch E closed, the phase relation between rotor and stator is further from the relationship obtaining in the neutral position than when the other contacts are engaged. Thus, only a small brush-movement from the neutral position may be necessary to develop the starting- torque for hoisting whilst a larger movement may be necessary for lowering. In order to obtain good commutation the reversing switch G is also operated when the brushes pass through the "off" position. The switches E and G may be combined in a single contactor. When the motor-speed is adjusted by an induction regulator the phase-shifting switch is interlocked with the regulator. The brushes may be fixed and reversal effected by changing the connections of the motor stator winding. In this case, operation of switch E may shift the rotor-stator phase relationship through an interval of 240 electrical degrees, such interval including the neutral position. The brushes may be arranged asymetrically in relation to the neutral position so that the starting torque in one direction may be greater than in the other. In order to limit the starting torque impedance may be introduced in circuit, or tappings provided in the supply transformer primary windings, Fig. 4 (not shown). Small changes of phase relationship over a wide range between the rotor and stator may be obtained by alternative polyphase connections to points of the stator winding which is then delta-connected, Fig. 5 (not shown).