Patent GB-191100498-A (GB)
Title: Improvements in Controlling Apparatus for Electric Motors.
IPC: H02P1/26
Filed: 1911-01-07
Granted: 1911-08-10
Abstract: 498. Otis Elevator Co., [Otis Elevator Co.]. Jan. 7. Systems for alternating- current motors only. - An alternating-current motor is started up automatically, upon closure of the main switch, by means of series coils in the rotor or stator circuits, which first connect the rotor windings up with full resistance in circuit, and the n shortcircuit the resistance. Auxiliary resistances are placed in the rotor circuit when the stator circuit is first made, these resistances being cut out as soon as the starting-resistances are inserted. The terminal points 35, 40, 41 of the starting-resistances 10, Fig. 1, are connected respectively to the bridge 8 of the contactor switch 4, and fixed contacts 13, 14. The cores of the switches 4, 5 tend to fall under gravity on to their contacts, but are pulled up by the large starting-current in their coils, which are in series with one leg of the rotor circuit. Auxiliary resistances 20, 21, which can be short - circuited at the contacts 17, 33 and 18, 32, respectively, are in the rotor circuit at starting. After the core of the switch 5 has pulled up and joined the contacts 30, 31, thereby short-circuiting the resistance 19, a coil 15 is energized sufficiently from the stator leads 1, 2, to lift its core and join the contacts 18, 32 and 17, 33. The auxiliary resistances are thereby cut out, leaving the motor started with full rotor resistance inserted. As the speed increases, the induced current diminishes, until the weaker of the resistance switches 4 drops its core, and cuts out a part of the starting-resistance. The resulting further increase of speed reduces the induced current until the switch 5 also drops its core and short-circuits all the rotor resistance, leaving the motor running at full speed. Simultaneously with the fall of the core of the switch 5, the resistance 19 is inserted in the circuit of the coil 15 and reduces the current in this coil to a hold-on value only. The switches 4, 5 may be arranged in one leg of the stator instead of the rotor. In another method, a single solenoid 44, Fig. 3, pulls up its core with the surge of current at starting, and drops it to cut out the resistance 50 inserted when the core was pulled up. In this case, the auxiliary resistances 20, 21, are, as before, under the control of a coil 15, and the short-circuiting contacts 45, 46 for the resistance 19 are closed by the core of the solenoid 44. Where two switches 4, 5, Fig. 1, are used, a pair of short-circuiting contacts may be arranged to be closed by each, and in this case each pair of contacts are arranged in series. This ensures that all the resistance is in the rotor circuit before the auxiliary resistances are cut out. In another method, three contactor switches cut out the resistance in three steps, each switch having two actuating coils. Homologous coils in each switch are in parallel with each other and in series with one leg of the stator circuit. The resistance 19 is shorted by a circuit that includes contacts closed by the upward movement of the resistance switches.