Patent GB-662249-A (GB)
Title: Improvements in and relating to control gear for electric motors
Inventor: EVANS OWEN THOMAS
IPC: H02P3/06
Filed: 1949-05-13
Abstract: 662,249. Control of A.C. motors. METROPOLITAN-VICKERS: ELECTRICAL CO., Ltd. April 4, 1950. [May 13, 1949] No. 12860/49. Class 38 (iii). Control gear for an electric motor (or motors) comprises a control member movable in opposite directions from a neutral position to effect forward and reverse running and means responsive to speed and direction of motor rotation prepare a dynamic braking circuit which, whilst the motor is running in the forward (reverse) direction, is completed by movement of the control member into the reverse (forward) running position whether or not the forward (reverse) running connections have been made, the arrangement being such that movement of the control member into the forward (reverse) position connects the motor for running in that direction, together with means controllable by the operator whereby movement of the control member into a braking position may be caused to effect reverse current braking instead of dynamic braking. A control system for a winder motor of the induction type is shown, speed variation being effected by adjustment of rotor resistance, and dynamic braking by supplying one motor winding with direct-current and connecting the other motor winding to a load resistor. The motor may be started in the forward direction by moving a control lever to close a switch MCF and energise contactor F. Interlock contacts Fa and Fb are opened to prevent energisation of braking contactor B and reverse contactor R. The motor is accelerated by movement of the control lever from the neutral position and, at a given speed, contacts SF are closed, these contacts forming part of a switch which is sensitive to the speed and direction of rotation of the motor. This switch may be of the type comprising a motor-driven oil-filled drum co-operating with vanes which are coupled to an external biased contact arm adapted to engage two stationary contacts. If switch S is in the position shown a relay 1 is energised to open contact 1b and close contact 1c. To bring the motor to rest, the control lever is moved from the forward to the reverse running position to opening switch MCF and closing switch MCR. Contactor F is de-energised to close contacts Fa, Fb, so that contactor B is energised to afford dynamic braking operation and contact Ba opens to prevent energisation of contactors F or R. After the motor has slowed down sufficiently to allow contacts SF to open, winding may be continued by re-closing switch MCF. Similar braking conditions obtain if the operating lever is moved from the reverse to the forward running position with the motor operating in reverse. Under these conditions contacts SR will have closed to energise relay 2 so opening contact 2b and closing contact 2c. Opening switch MCR de-energises contactor R and closing switch MCF energises contactor B. When the motor is operating forwardly, for example, and has slowed down sufficiently to open contact SF, relay 1 becomes de-energised to opening contact 1c to open the circuit of contactor B. Contact 1b is closed to energise contactor R for reverse-current braking. This change-over from dynamic to reverse-current braking may be prevented by providing shunt circuits across contacts SF, SR. When the control lever is in the "off" position, the mechanical brakes removed, and the motor is rotated in the forward direction under the action of the load, contact SF will close to energise relay 1 so that dynamic braking may be applied by moving the, control lever to the reverse running position. Reversecurrent braking, without intervening dynamic braking, may be applied by moving switch S to its alternative position. The control lever may be associated with additional contacts so that if a fault occurs during dynamic braking operation the control member must be moved. to the "off" position before reverse-current braking can be effected. A list switch TS may be provided. Specifications 472,192 and 512, 784 are referred to.