Patent GB-190314542-A (GB)
Title: Improvements in Controlling Devices for Electric Motors
Inventor: LINDQUIST DAVID LEONARD
Filed: 1903-06-30
Granted: 1903-12-03
Abstract: 14,542. Lindquist, D. L. June 30. Electromagnets; motors, controlling.-Relates to an automatic system for varying the resistance in the rotor circuit of an induction motor as the speed rises and the induced electromotive force falls off on starting the motor, the motor being controlled by changes of voltage in its secondary coil. In the arrangement shown in Fig. 1, in which a three-phase motor is supplied from three-phase conductors a, b, c, the stator m is connected to switch contacts 4, 5, 6 placed behind other contacts 1, 2, 3 which are connected to the coilsj, k, l of three electromagnets of the construction shown in Fig. 2. The rotor o is connected to three resistances P, Q, R and to second coils s, t, u of the same magnets. Fixed contacts w, x, y under the magnet armatures C, D, E are connected to intermediate points of the resistances P, Q, R. In the operation of the switch f, after the transformer e has been connected to the mains a, b, c, the secondary currents from the transformer e are first supplied to the coils j, k, l to raise the armatures C, D, E, and are afterwards supplied also to the stator m. The large electromotive force thus generated in the stationary rotor causes the coils s, t, u to hold up the armatures C, D, E after the coils j, k, l have been cut out by the further movement of the switch f. As the rotor speed increases, the induced electromotive force falls off ; the coil u of least magnetizing strength finally releases its armature E, thus cutting out part of the resistances P, Q, R. The other coils s, t afterwards act similarly, so that all the resistance is cut out when the rotor is running at its proper speed. The zero points of the coils j, k, l are connected to that of three induction coils n which are connected to the stator as shown in Fig. 1 to act as a safety device to allow current to flow back through the zero points so long as one branch of the coils j, k, l is operative. The resistances P, Q, R and the coils s, t, u are similarly connected. Resistances v are placed in the leads of the coils s, t, u to ensure sufficient resistance in their circuits. A three-phase star-connected transformer may be used to feed the stator and the coils j, k, l, the zero points of these coils being then connected to that of the transformer secondary. Similar branches of the coils s, t, u are in this case connected in series. In another arrangement, a two-phase transformer and a twophase stator are employed, and the magnet coils are wound for a two-phase current. The middle points of the stator and of the coils j, k, l are connected respectively to the contacts 5, 2, and the middle points of the coils s, t, u to the zero point of the resistances P, Q, R. In a further arrangement, the two-phase stator L, Fig. 6, is supplied with current from a single-phase transformer J, the necessary phase difference in the two stator coils being obtained by the use of a resistance M. The magnet coils are wound for a single-phase current, and are connected as shown to the rotor N, the resistances P, Q, R, and the switch f. When the motor in any of the above arrangements is running at its proper speed, there is no appreciable current flowing through the electromagnet coils.