GB651379A - Improvements relating to the dynamic braking of induction motors - Google Patents

Abstract

651,379. Control of A.C. motors. MULLIGAN, J. W., GRAY, A. H., WILSON, J. S., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Oct. 28, 1948, No. 28038. [Class 38(iii)] When an induction motor 1 is braked, two phases of its stator winding are supplied with direct-current from an exciter 3 and its rotor winding is connected to a variable resistor 5. A desired relation between the braking torque and the resistance is obtained by varying the exciting current in correspondence with the rotor current. A reduction of the resistor 5, for example, increases the current passed from current transformer 6 through rectifier 7 to the control winding of a transductor 8, and the resultant reduction of the reactance of the alternating-current winding of the transductor increases the output of rectifier 11 to a field winding of the exciter 3. The output of rectifier 11 may be applied directly to the motor, or,it,may be used to regulate a grid-controlled rectifier through which the exciting-current is supplied. The transductor may be controlled in accordance with the difference between currents respectively proportional to the rotor current and the exciting-current, the two representative currents being applied in opposition to one control winding of the transductor, or to two opposing windings. The current proportional to the exciting- current may be derived from a shunt in the output circuit of the exciter 3, or one in the field winding circuit of that machine. Means may be provided to prevent the excitation of the induction motor being reduced below a pre-determined value.