GB602377A - A.c. induction motor speed control - Google Patents

Abstract

602,377. Automatic control systems for induction motors. HENDY IRON WORKS, J. Sept. 26, 1945, No. 24978. Convention date, Aug. 1, 1944. [Class 38 (iv)] [Also in Group XXXVII] Speed, or speed and direction of rotation, of an induction motor is controlled by reactors in the stator circuit which are variably saturated by D.C. current resulting from the difference between an adjustable reference voltage and a voltage generated in proportion to motor speed. In Fig. 7 the D.C. controlled saturable reactors 5a, 5b, and 5c are connected between the supply terminals 12a, 12b and 12c and the primary winding 2a, 2b and 2c of a 3-phase squirrel cage induction motor. Each reactor is of the type having the D.C. controlling coil (3a, 3b or 3c) on a centre limb and the A.C. coils (4a, 4b or 4c) on two outer limbs wound oppositely so as to induce no A.C. in the D.C. coil, and each can be varied, by energizing the control coil, from a very high inductance to a very low inductance, thereby varying the voltage across the motor windings. The variable control current for the coils 3a, 3b, 3c, which are connected in series between the terminals 6 and 7, is derived from the transformer 10 through the grid-controlled thyratrons 81 and 91. Alternating potentials of variable phase are applied to the grids 42 and 43 of the thyratrons by the transformer 36 through the transformer 38 and the phase shifting circuit made up of the D.C. controlled saturable reactor 33 and resistance 37. The motor is automatically controlled to run at a speed determined by a slider contact 30. A speed-proportional voltage is generated by inductor alternator 40 driven by the motor shaft 13, and rectified by rectifier 22 and applied across the condenser C1 and resistance R1 to make the grid of amplifier 23 negative relative to the cathode. Also in the grid circuit is a variable part of the voltage-dividing resistance R4 across which, and condenser C2, is applied the supply voltage for the amplifier 23 from transformer 26 and rectifier 25. The output of amplifier 23 is amplified by the amplifier 24 supplied by transformer 32 and rectifier 31 and in series with the D.C. control coil 35 of the saturable reactor 33. When the sliding contact 30 on the resistor R4 is moved to the right to make the grid of amplifier 23 positive, coil 35 is energized, the thyratrons 81 and 91 supply coils 3a, 3b and 3c and the inductance is reduced to allow the motor to start. Should the motor speed exceed that determined by the position of slider 30, the output of amplifier 23, the current through coil 35 and the output of the thyratrons is reduced and the inductance in the motor circuit is increased to reduce the motor speed. In Fig. 8 (not shown), two reactors 5a and 105a are connected between winding 2a and terminal 12a. Winding 2b is connected to terminal 12b through reactor 5b and to terminal 12c through another reactor 105b while winding 2c is connected to terminal 12c through reactor 5c and to terminal 12b through another reactor 105c ; thus the motor will rotate in one direction or the other according to whether reactors 5a, 5b and 5c have a very high and reactors 105a, 105b and 105c a low inductance or vice versa. The alternator 40 supplies one set of apparatus as in Fig. 7 for controlling reactors 5a, 5b and 5c and another set for controlling reactors 105a, 105b and 105c ; the regulating resistors for each are combined into a reversing and regulating switch which, over a range of positions for adjusting the one set of reactors, keeps the other set at maximum inductance. In a modification, batteries supply thyrations 81 and 91 and amplifier 24, and the latter directly applied grid potentials to the thyratrons ; the grid of amplifier 24 is connected to the anode of an amplifier 23 and the grid of the latter is connected to the positive end of resistance R1, the contact 30 being connected to the other end and slidable on a resistance in parallel with part of R4. With valve 23 non-conducting and valve 24 conducting, the thyratrons supply the control coils and the motor starts; if the motor speed becomes excessive, the positive potential of the grid of valve 23 makes this valve conduct, thereby blocking valve 24 and biassing the grids of the thyratrons sufficiently to stop their output temporarily and so reduce the motor speed. In a simple form of such " on and off " control of the thyratrons, the grids are supplied, through a transformer fed through a resistancecondenser phase-shifting combination ; on excess speed a governor switch on the motor short-circuits the condenser, causing sufficient phase-shift to stop the thyratrons output.