Patent GB-741999-A (GB)
Title: Improvements in control arrangements for alternating current induction motors
Inventor: EVANS OWEN THOMAS
IPC: H02P25/26
Filed: 1952-10-23
Abstract: 741,999. Control of A.C. motors; controllers. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Sept. 30, 1953 [Oct. 23, 1952], No. 26642/52. Class 38(3) An induction motor M, driving a winder or the like, is adapted to be connected to a three phase supply S1, S2, S3, through forward contactor F or reverse contactor R, dynamic braking being provided by closing a contactor B to connect the stator to a D.C. source, and opening contactors F, R. The sliprings of the motor are connected to resistors R1, R2, R3, which are arranged to be short-circuited in a number of steps by means of double-poled contactors regulated by the progressive movement of a master controller MC having an operating lever DL. Selector contacts SC1, SC2, SC3, SC4, are actuated by the control lever which moves in a slotted gate so that forward or reverse operation is provided by shifting the lever towards the positions " F" or "R," and dynamic braking is established by movement towards the positions " DB." The circuits of the coils OC11-OC15 of the second to sixth resistance contactors include normally open interlock contacts 10d-14d operated by the preceding contactor of the sequence so that these contactors can only be closed in a predetermined order. The interlock contacts are shunted by hold-in contacts 11c- 15c of the associated contactors. The circuits of the coils OC16-OC-20 - of the seventh to the eleventh resistance contactors include, in addition to the interlock and hold-in contacts 15d-19d and 16c-20c, contacts of first and second current-limit relays CL1, CL2; responsive to the rotor current, the operating coils being connected through rectifier RX to a current transformer CT. The drum controller may thus be actuated to remove the first six steps of rotor resistance without restriction, but thereafter the resistance steps are removed either at timed intervals determined by delay contacts 15e-19e, or under the control of the current limit relay CL1 which has contacts 1a-1e. If the rotor resistance has been short-circuited up to the seventh or succeeding notch during dynamic breaking operation and the rotor current rises above a safe limit, the relay CL2 opens contacts 2a-2e and the resistance contactors are deenergized in reverse order, determined by interlocks 20f-17f, until the current is reduced to a value giving stable operation. The time delay contacts 15e-19e ensure that the rotor resistance is removed at a rate corresponding to the worst conditions of loading, and, under more favourable conditions, the current limit relay CL1 provides that the resistance can be removed more rapidly to maintain maximum braking torque. Contacts B3-B7 may be provided so that automatic removal of rotor resistance under control of the relay CL1 takes place during braking operation only, the relay CL2 being effective, however, during both motoring and braking operation. Automatic control involving the relay CL1, during both motoring and braking operation may be obtained by omitting the contacts-B3-B7. Different controlled values of current for motoring and braking are afforded by changing the setting of relays CL1. CL2. such as by adjusting resistors RM1, RM2. The current limit relays may be energized by means of a transductor controlled in accordance with the rotor current