Patent GB-632831-A (GB)
Title: Improvements relating to the dynamic braking of induction motors
IPC: H02P3/24
Filed: 1947-07-23
Abstract: 632,831. Control of A.C. motors ; controllers. CARR, L. H. A., and METROPOLITAN - VICKERS ELECTRICAL CO., Ltd. July 23,1947, No. 19784. [Class 38 (iii)] During braking of an induction motor one winding of the motor is excited with directcurrent whilst the other winding is connected with variable load resistance, and the said resistance is arranged for adjustment on the one hand by a manually operable control and on the other hand by an automatic control in proportion to the speed of the motor, whereby for any particular setting of the manually operable control a predetermined braking torque is obtained, which torque is maintained substantially constant over a wide range of speed of the motor by the operation of the automatic control. A driver's control lever is linked through a member 1 with a slotted lever 2 which is pivoted at 3. A main link 4 has one end slidable in the slot of the member 2 and the other end coupled with the adjusting member 6 of a liquid resistor. A second link 7 adjusts the position of the end of the member 4 in the slot and is moved by means of a pilot motor 12 against the action of a spring 13. As shown in Fig. 2 the motor 12 is of the direct-current separately-excited type and is supplied from a generator 14 which is mechanically coupled to a main induction motor 15. The output of the generator 14 is opposed by that of a fixed voltage source such as an exciter 16 which supplies direct-current to the stator of the induction motor for braking, the arrangement being such that the input to the motor 12 at full speed of the generator 14 is zero and increases as the speed falls in proportion to the change of speed. The end of the link 4 which is slidable in the slot in the lever 2 is moved therein in proportion to the speed of the induction motor and approaches the pivot 3 as the speed falls. It may be arranged that, with the driver's control lever a maximum distance away from the neutral position, and with the induction motor operating at full speed, so that the input to the motor 12 is zero and the link 7 is in its uppermost position, the load resistance is adjusted to give maximum braking torque, a stop 20 preventing reduction of the resistance below this value. If the speed of the induction motor falls, the motor 12 moves the link 4 towards the pivot 3, reduces the resistance, and so maintains maximum braking torque. In a similar manner the automatic control operates to maintain the torque constant when the manually-operated lever is in an intermediate position. The automatic control may be afforded by a mechanical governor driven by the induction motor, an alternating-current motor which is supplied at a frequency proportional to the speed of the induction motor and exerts a torque proportional to the speed of this motor, or a direct-current motor which is directly supplied from a generator driven by the main induction motor. The manually-operated control lever may be adjustable to provide forward or reverse, motoring or braking, operation. The control motor 12 may be rendered inactive during motoring, operation, or, if automatic control is desired during such operation, the control motor will be directly supplied from the generator 14. The control motor may be supplied from a rectifier connected in the circuit of the induction motor between the rotor and a control resistance. A solenoid may replace the control motor. In some cases the automatic or the manually-operated control may be arranged to vary the direct-current excitation during braking, to increase it as the load resistance is reduced.