GB787652A - Improvements in and relating to dynamic braking control systems for electric motors - Google Patents

Abstract

787,652. Control of A.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. Jan. 4,1956 [Jan. 11, 1955], No. 875/55. Class 38 (3). An induction motor has its stator winding supplied with direct-current for braking an overhauling load and the rotor resistance is automatically reduced as the braking operation proceeds, means being provided for automatically decreasing the rate of reduction of the resistance when such operation would cause reduction of the flux in the machine below a predetermined value. The rotor resistance may be of the liquid type operated by an hydraulic servo device having two different rates of travel, the change-over from the higher to the lower rate being initiated by an electrically-operated hydraulic valve. The valve is actuated in dependence upon a saturable detector having a current-voltage characteristic similar to that of the device used in conjunction with the system of Specification 666,561 and being connected across one phase of the rotor windings in series with the coil of a relay for operating the hydraulic valve. Alternatively, the output of the saturable device is applied to a control coil through a potentiometer, Figs. 1, 2 (not shown). In another arrangement, the rotor resistance is controlled by a number of contactors and a pre-determined degree of energization of the relay coil prevents further reduction of the resistance despite progression of the master controller, Fig. 3 (not shown). As shown, an electric servomotor SM controls the rotor resistance and is supplied from amplidyne A, the field winding F of which is energized in accordance with the difference between the voltage generated by a tachometer generator T, driven by the motor under control, and a reference voltage V. When a voltage derived from a saturable device 2 and potentiometer P exceeds a reference voltage at RR, field winding CF is excited and the servo operation is modified to prevent undesired reduction of the rotor resistance. The servo device may be of the hydraulic type controlled in response to speed deviation, the detector device controlling a valve to check the reduction of rotor resistance. In another arrangement, the rotor resistance is both increased and decreased to maintain the flux in the induction motor constant when its speed varies. The output from a detector 2 supplies one field winding of the amplidyne which has another opposing winding excited from a source of constant direct-current. Variations in the current output of the detector control the amplidyne and servomotor in both senses.