Patent GB-285455-A (GB)
Title: Improvements in and relating to speed control systems for alternating current induction motors
IPC: H02K17/38
Filed: 1928-02-07
Abstract: 285,455. British Thomson-Houston Co., Ltd., (Assignees of Hull, J. 1.). Feb. 17, 1927, [Convention date]. Alternating-current induction machines.-Consists in arrangements for the speed control of an induction motor by means of a, cascaded polyphase commutator regulating machine having independent stator exciting windings into which are injected a voltage component independent of slip frequency for balancing the resistance drop in the exciting winding and a second component for balancing the inductive drop, the exciting means including a generator driven at a speed proportional to the slip of the main induction motor. Fig. 1 shows a form for controlling the speed of an induction motor 10 in which a slip-responsive exciter 25 supplies excitation to a synchronous exciting generator 15 which also supplies the resistance drop component. The regulating machine 12 and generator 15 are driven by a synchronous motor 14 floating on the line 11, the machines 14 and 15 having the same pole-number. The frequency converter 16 on the shaft of, and having the same pole-number as, the machine 10 is supplied at 17 by the exciter 15 and excites the regulating machine at 13 with current at the slip frequency of the machine 10. The synchronous exciter 15 is excited from a continuouscurrent source 23 through a rheostat 24, which allows current reversal, by means of a compensating coil 21 and a coil 20 which generates a voltage to balance the ohmic drop in the field coils 13. Another coil 22, in quadrature, with the winding 20, balances the inductive drop in the circuit 13. This coil 22 is supplied by a continuous-current generator 25 which is driven by a synchronous or induction motor 27 on the secondary side 28 of the main machine 10. The speed of the set 27, 25 is thus approximately or directly proportional to the slip of the main machine. Fig. 2 shows the use of an adjustable auto-transformer 30 between the secondary of the main machine 10 and the fields 13 of the regulating machine 12 for giving step-by-step speed regulation, more particularly for speeds away from synchronism, in addition to the smooth regulation given by the arrangement shown in Fig. 1. The synchronous generator 15 has two sets of coils 32, 33 and 20, 21 in quadrature and in series respectively with resistances 34, 24 on the auxiliary continuous-current supply 23. Another resistance 35 is in series with the rheostat 24. The auto-transformer and resistance contactors are linked, as shown. Resistances 36 are provided to prevent short-circuiting, while one set of the transformer contactors is closed before the adjacent set is opened. With the Y-point switch 37 of the field coils closed and the other switches as shown, the auto-transformer is out of use and the control for speeds near synchronism is effected as in Fig. 1. To regulate further away from synchronism, the switch 37 is opened and the contactors at step 1 closed. The corresponding transformer turns then inject the inductive drop-balancing component of the excitation at 13 by means of the coil 22, and the coils 32, 33 are closed through the resistances 34 to give the ohmic and armature reaction components corresponding to this step. Similar actions occur for the other speed steps 2 - - 6, the arrangement being such that the added resistances 35 compensate the decreased values of resistances 34. The excitation circuit controlled by the rheostat 24 may be regulated automatically in accordance with speed or load by known devices. To add to the smoothness of control a small-capacity secondary winding 38 is provided for the first two speed steps to prevent transmission into the field system 13 of any part of the ohmic drop of the current in the main motor secondary when the main machine is regulated to exact synchronism under load : in this condition the transformer frequency is zero and no inductive action into the windings 38 can occur. The synchronous exciter 15 may be replaced by two machines connected in series and respectively supplying the ohmic and inductive volt components. In the arrangement shown in Fig. 3, the frequency converter 16 is supplied from the line 11 through a transformer 40 having the secondary windings 41, 43 wound in quadrature and giving voltages through adjustable resistances 42, 44 for respectively supplying excitation to overcome the resistance drop in the fields 13 of the regulating machine 12 .and adjusting the power-factor of the main machine 10. The inductive drop in the winding 13 is balanced by the stator coil 46 of a commutator machine 45 connected through a limiting resistance 47 to the frequency converter 16 and driven by a motor 27 floating on the secondary side of the main machine. The machines 45, 27 have a common pole-number, so that the machine 45 is driven at a synchronous speed corresponding to the slip frequency of the main machine 10, while the frequency at the converter brushes 19 is exactly equal to this slip frequency. Specification 140,476 is referred to.