Patent GB-190812704-A (GB)
Title: Improved Method of, and Apparatus for, Controlling the Voltage of Dynamo Electric Machinery, especially applicable to Variable Speed Generators.
Inventor: STANSFIELD WILLIAM
Filed: 1908-06-13
Granted: 1909-09-13
Abstract: 12,704. Stansfield, W., and Hatt, J. J. June 13. [Cognate application No. 20,764, A.D. 1908, dated Oct. 2.] Lighting, systems of.-To obtain a constant voltage from variable-speed train-lighting generators, a main and an auxiliary generator are coupled in series across the mains, and the field of the main generator is controlled by the auxiliary generator, the voltage of which at low speed serves to build up the field of the main generator, and as the speed increases becomes zero, after which it changes and weakens the field of the main generator, increasing in magnitude with further increase of speed. The field of the auxiliary generator is controlled either by a single winding connected across a Wheatstone-bridge network of resistances, or by two opposing windings having different resistance characteristics or having different resistances in circuit. Fig. 3 shows the two armatures connected across the mains M, N, the field coils Ca, Ea of the main machine A being connected across the mains M, N and the auxiliary armature respectively. The auxiliary machine B is provided with two opposing windings Db, Fb connected across the mains M, N, and the resistances in the circuits of the windings Db, Fb are so chosen that either by virtue of their capacities or the materials of which they are made, the current in the field winding Db, tending to excite the machine B to give a voltage opposed to that of the machine A, predominates as the voltage across the main increases. Resistances with positive or negative temperature-coefficients or with different crosssectional areas, or adapted to alter with the pressure upon them, may be connected in series or in parallel with those windings. For example, a resistance R having a high positive temperaturecoefficient may be placed in series with the winding Fb. An arrangement is also illustrated in which the pressure exerted by a spring and lever upon a resistance in the circuit of the field winding Fb is controlled by means of a solenoid connected across the mains M, N. Fig. 6 shows the machine B provided with a single winding Ob connected to the corners q, q1 of a Wheatstonebridge net of resistances v1, v2, v3, v4, the remaining corners being connected to the mains. The pairs of resistances v1, v2 and v3, v4 vary differently for variations of current through them, and the polarity of the field of the machine B depends upon the potential across the mains. A further arrangement consists in using the pressure-resistances in the branches of the Wheatstone-bridge, and controlling the pressure upon them by the use of a solenoid connected to the mains. Both the machines A, B 'may be provided with additional field windings connected in series with one of the mains, and closed windings may be fitted on the poles after the manner of amortisseur coils. The field winding Db may be connected at one end to an intermediate point in a resistance connected across the armature of the machine B, its other end being connected to the main M ; or, it may be connected across the terminals of the machine A. The armature conductors of the machine A may be carried across the armature core of the machine B so as to be acted upon by the field of the latter. Some or all of the field windings of the machines A, B may be placed upon exciters, the currents from which are used to excite the field magnets of the machines.