Patent GB-642443-A (GB)
Title: Improvements relating to wound rotor induction motors and to processes of dealing with surplus energy generated therein
IPC: B66B1/12
Filed: 1947-12-19
Abstract: 642,443. Control of A.C. motors. ANGLOAMERICAN CORPORATION OF SOUTH AFRICA, Ltd. Dec. 19, 1947, No. 33689. Convention date, Dec. 21, 1946. [Class 38(iii)] [Also in Group XXIX] A wound rotor induction motor has the primary winding of a step-up transformer connected to its rotor circuit, an electrical high-voltage transmission circuit of substantial length includes the secondary winding of the transformer and a resistance load, the resistance load and the transformer being physically isolated and remote from one another, and means are afforded to produce a variable virtual resistance in the rotor circuit. In order to prevent excessive generation of heat in the vicinity of a wound rotor induction motor 14-15 which drives a hoist in a mine, the rotor circuit is coupled by a transformer 26-27 to a resistor 29 at the surface, the major portion of the rotor losses being absorbed in the resistor. Speed control of the motor is affected by regulation of the resistor 29, or variation of the transformation-ratio of the unit 26-27, to adjust the effective resistance of the motor rotor circuit. Movement of a lever 22 in the forward direction closes a reversing switch 21 and connects the motor stator winding 14 to the mains. A master controller 23 is moved at the same time, and, since the first movement of the hoist causes a driven directional switch 49 to occupy the position shown, a relay 40 is energized to close contacts providing a delta-connection of the transformer primary windings 26. The transformer is of the on-load tap-changing type, the secondary winding 27 being star-connected and having inter-tap reactors 38 which are connected to the primary windings of a booster transformer 43, thereby to afford a high output voltage suitable for transmission through the cable 31. Further movement of the controller energizes a contactor-coil 34 which closes contacts 34y thus establishing a low degree of linkage between the rotor circuit and the resistor 29, the virtual resistance of the rotor circuit being correspondingly high. The motor develops the required torque to start the hoist. Acceleration is effeced by moving the controller to close contactors and so vary the tapping-point of the winding 27, until, when the contct 37y is closed alone,. the motor reaches about three-quarters of its full speed. Additional movement of the controller to close contactors removing portions of a local resistor 45 in sequence, and then short-circuiting the rotor winding through contacts 51x, causes the motor to reach full speed. Condensers 44 provide power-factor correction. Reverse-current braking is afforded by positioning the controller at the first reversing notch, a contactor 41 being energized to afford a star-connection of the transformer primary windings 26. The basic transformation-ratio of the unit 26-27 is thereby reduced and an excessive rate of retardation is avoided. Thereafter retardation is continued by progressively reducing the virtual rotor resistance of the motor by notching the controller in the reverse direction. Reverse operation is similar to that described, initial movement of the hoist moving the switch 49 to the reverse-position and enabling the contactor 40 to be closed for delta-connection of the windings 26. Fig. 2 (not shown) illustrates the connection of contacts of the contactor 41 to tappings in the windings 26 thus affording a different basic ratio of transformation with the star-connection of the primary winding. In a modification, the transformer secondary winding 27 is not tapped but is connected to an induction regulator which is adjusted by the driver's lever. The deltastar connection of the transformer primary winding 26 is retained. This modification is illustrated in Fig. 3 (not shown). A hydraulic servo system may be provided which couples the driver's lever to the induction regulator so as to afford a linear relation between movement of the lever and the rate of change of the ratio of transformation. The rate of movement of the induction regulator from its neutral position is restricted whilst return movement is not. See Fig. 4 (not shown). In a further modification the resistor which is placed at the surface of the mine is adjustable, the driver's lever being linked to a controller for regulating such resistor by means of a " Selsyn " system. This is illustrated in Fig. 5 (not shown). The above-mentioned resistor may be of the liquid type, the moving electrodes of which are coupled to the rotor of the receiving unit of the " Selsyn " system through a hydraulic servo-motor, see Fig. 6 (not shown). The values of the resistance load and the transformation ratio may be so determined that the virtual resistance thereby brought into the rotor circuit is equal at all times to the total reactance in the rotor circuit.