Patent GB-719866-A (GB)
Title: Improvements in and relating to the control of electric motors
Inventor: WOODLIFF GEOFFREY EDMUND
IPC: H02P3/22
Filed: 1952-01-14
Abstract: 719,866. Control of A.C. motors; controllers. BRITISH THOMSON-HOUSTON CO., Ltd. Oct. 21, 1952 [Jan. 14, 1952], No. 1040/52. Class 38(3) An induction motor is braked by the application of direct-current excitation to its stator winding, and a current or voltage measuring device, which is supplied in accordance with the direct-current excitation, operates when a pre-set value of such excitation is reached to prevent the rotor resistance of the motor either from being further reduced, to limit the rate at which the resistance can be further reduced, or to increase the resistance until 'the excitation falls below the pre-set value. As shown, a liquid rotor resistance is controlled by a lever a operated by piston b which is slidable on a hollow sleeve c within a cylinder d. Oil under pressure is admitted to the cylinder through conduit e. The driver's lever, by means of connecting rod j, moves the sleeve c, which has discharge ports g, relatively to the piston so that the liquid controller follows the movement of the driver's lever until the ports g are uncovered in the position of correspondence. An electromagnetic valve h restricts the supply of oil to the cylinder when a contact k is operated by a relay responsive to the dynamic braking excitation reaching a pre-set value so that further reduction of rotor resistance is prevented. The relay responsive to the dynamic braking excitation may operate a mechanical locking device for preventing further reduction of rotor resistance, increase of resistance, however, not being restricted. When the rotor resistance is of the metallic type a master controller may regulate contactors for reducing the resistance in steps. If the excitation current reaches its pre-set limit whilst the rotor resistance is being reduced, a relay operates to prevent further reduction of resistance but does not affect those rotor contactors already closed, Fig. 3 (not shown). The rotor resistance may be controlled by an electrical servo device as described in Specifications 588,209 and 719,854. The servomotor controlling the rotor resistance is supplied by an amplidyne or magnetic amplifier having a first field winding energized in accordance with the difference of potentials derived from a control potentiometer and a tachometer generator driven by the induction motor. A second field winding opposes the first and is supplied with a measure of the dynamic braking excitation through a limit circuit comprising a reference potentiometer and a metal rectifier so that, when the dynamic braking excitation voltage exceeds the setting of the reference potentiometer, the second field winding is energized to reduce the supply to the servomotor. The servomotor also drives a master controller for selecting forward operation or braking, and during normal operation the speed of the induction motor is made to correspond to the position of the control potentiometer. When, however, the dynamic braking excitation exceeds a predetermined limit, the servo motor reduces the rotor resistance at a lower rate or re-introduces rotor resistance, the dynamic braking excitation being limited to a value determined by the adjustment of the reference potentiometer, Fig. 4 (not shown). Specification 666,561 also is referred to.