GB212910A - Improvements in and relating to asynchronous motors - Google Patents

Abstract

212,910. Henry, E. March 12, 1923, [Convention date]. Armatures and field magnets; alternating. current motors; motors, controlling.-The starting current and torque of a squirrel cage induction motor are controlled by magnetic plates or sleeves which are movable with respect to the rotor core for deflecting a portion of the stator flux, the rotor bars being joined by resistant conductors in which a self-induction is created. on starting by means of the movable magnetic plates. The magnetic discs 9, Fig. 1, may be moved by a fork or the like along the rotor shaft so as to engage the end plates 5 of the rotor when the motor is started. The rotor bars 6 are extended over the plates 5 and connected by resistant conductors 8 to the rings 7, the magnetic shunts 9 being grooved to receive the rings and conductors 8. The rotor may be made of thin laminae entirely, or partly of thick plates, or it may be solid, and the shunts 9 may be partly laminated. Fig. 6 shows a ring form of rotor 4 containing short-circuiting resistances 8 and having a magnetic shunt 9a in the form of segmental arms which enter the ring between its spider arms. The winding is also connected by resistances R, as shown in Fig. 8. The effect of the conductors 8, Fig. 1, and connecting resistances between the rotor bars is shown in Fig. 5. The stator flux normally enters the rotor surface at B, A, C, D and leaves at C, D, F, E, the effect of the magnetic shunts causes it to enter at C, H, L, E, D; D, J. K, F, and leave at A, G, H, C; B, I, J, D. The non- inductive resistances r carry a power current. The rotor bars 6, Fig. 1, may be connected in groups to the rings 7 by radial conductors 8, Fig. 11, having enlarged heads 8<1> and cast integrally with the rings 7, or as in Fig. 12, each conductor 6 may have several radial bars 8. The bars 8 may be replaced by a. flat iron-plate 8, Fig. 14, soldered to the ring 7 and having saw cuts 15; or an uninterrupted disc may be used with thin radial copper insets to increase the radial conductivity without increasing it circumferentially. The contacting surfaces of the magnetic shunts and the rotor may be coned, corrugated, or otherwise shaped so as to increase the contact area. When the inner surfaces of the magnetic shunts are not grooved to receive the end parts of the rotor bars the shunts may run loosely on the shaft or they may be supported on the shaft bearing during normal running. They may, alternatively, have only an axial movement on a fixed part of the motor without rotation, in which case thev are formed of a helically laid band of thin metal. In another form, they have movable projections which are moved out for the starting period. The shunts may be moved by hand, electromagnetically, by a servo-motor or a centrifugal device may be used in which after a preliminary setting against the action of a spring they are withdrawn from the rotor at a limiting speed. They may also be drawn in by the flux of the motor itself at starting.