Patent GB-340124-A (GB)
Title: Improvements in and relating to speed and power factor control of induction machines
IPC: H02P27/04
Filed: 1929-12-14
Abstract: 340,124. British Thomson-Houston Co., Ltd., (Assignees of Hull, J. I., and Robinson, P. W.). Dec. 14, 1928, [Convention date]. Alternating - current commutator and induction machines. -In a system for controlling an induction machine for speed, load and power factor by means of a commutator machine cascaded with the secondary side of the main machine and having two stator exciting windings which are connected to the two adjustable brush sets of a frequency changing exciter, the stator winding which supplies the speed-controlling excitation (and at speeds removed from synchronism a part also of the power factor excitation) is conpected to both brush sets and the secondary winding, which supplies the power factor excitation, is connected to one brush set only. By these connections, adjustment of the brushes affects only the phase and not the magnitude of the exciting voltage impressed on the second winding. The invention is shown in connection with an induction machine 12, Fig. 1, connected to a 25 cycle network 11 and coupled to a synchronous machine 13 coupled to a 60 cycle network 10. The regulating commutator machine 19 is motor-driven at 25 from the network 11 and its two exciting windings 35, 36 for speed and power factor control respectively are supplied through the brush sets A, B of a frequency changer 27 on the main machine shaft from the network 11 via. a transformer 34. The winding 36 is connected to the brushes A and closed through a star resistance 38 and the winding 35, by means of leads 29, 32, is connected across both brush sets' through a resistance 37. The resistances are regulated by a motor-operated cross-bar 42 in such a manner that the resistance 38 is reduced to a minimum before the other is altered. Thus, the voltage of the winding 35 is used for most of the power-factor correction at speeds remote from synchronism. The bar 42 is moved by a reversible series motor 39 running on a continuous-current source 47 and controlled either automatically by a meter 51 (shown as a wattmeter) in the mains 11 or by hand by means of switches 52, 53. For automatic control, the switch 54 is placed as shown and the meter arm 57 closes one or other of the relay circuits 56, 49 or 55, 50 to energize one or the other of the motor field coils 44, 45 from the source 47 to rotate the motor in the required direction. For hand control, the switch 54 is moved to the right and one or other of the hand switches 52, 53 is closed. The brush rows A, B are rocked by means of a reversible pilot motor 58 running on the source 47 and controlled either automatically or by hand by relays 49<1>, 50<1> and a meter 51<1> orhand switches 53', 52<1> in a similar way to themotor 39. Figs. 2 and 3 show the brush-operating gear and Fig. 5 the typical positions of one brush of each set during the control. The yoke 64 of the brushes A is pinion-driven from the motor 58 and the yoke 65 of the brushes B is operated. by a cam 66 on the gear shaft 60 and a cooperating roller 68 on the yoke which is weighted at 69 to maintain the roller on the cam. The cam shape and gear ratio are such that when the cam rotates continuously, bringing the points 1, 2, 3 successively in contact with the roller, the relative brush positions corresponding to these contacts are A1, B1, A2' B2 and A3, B3, Fig. 5. Thus, a constant voltage OA1, OA2, OA3 of varying phase is applied to the power-factor correcting winding 36 from the brushes A, while the voltage supplied from both sets of brushes A B to the winding 35 increases from zero, when the brush pairs are on the same commutator segments, through the values A1 B1, A2 B2 to a maximum A3 B3. With no voltage between the brushes A B and the machine 12 at or near synchronous speed, if the resistance 38 is at its minimum value the winding 36 furnishes the requisite power factor correcting excitation. When the machine 12 runs above or below synchronism and with a higher step frequency, the additional requisite power factor correcting voltage in quadrature is provided by the increased current in the winding 35 caused by the reduction in the resistance 37. In an alternative arrangement, the winding 35 is connected between the brushes B and a third set of brushes instead of the brushes A, these extra brushes' being carried on a third yoke which is either stationary or rotatable. The machine 12 may be started by a two-way switch 22 adapted to connect in a starting resistance 24' and then connect the slip-rings 20 directly to the brushes of the regulating machine 19. Specification 221,806 is referred to.