Patent GB-295368-A (GB)
Title: Improvements in electric rotary converters
IPC: H02K47/08
Filed: 1928-01-18
Abstract: 295,368. Allmõnna Svenska Elektriska Aktiebolaget. Aug. 12, 1927, [Convention date]. Rotary converters. - The invention relates to rotary converters and consists in the provision of means for preventing oscillations, and the resultant bad commutation, on the occurrence of sudden variations in the continuous current load. In Fig. 1, which is a vector diagram illustrating the properties of the usual converter, E represents the line voltage constant in magnitude and phase, I the load current which causes an ohmic voltage drop Er in phase therewith and an inductive drop Ex at right angles, and Ea is a voltage drop due to armature reaction at right angles to the voltage Ei induced by the main flux. The angle a between the line 'voltage E and the induced voltage Ei thus varies with the load, but in order to avoid the oscillations referred to, it is a condition that this angle be maintained constant. According to the invention the angle alpha is kept substantially zero, and for this purpose an additional balancing voltage is introduced opposed to and greater than the voltage Ea. This voltage is induced by the provision of a flux at right angles to the main flux which counteracts the alternating current armature reaction, that is, a flux which strengthens the continuous current armature reaction. One method of obtaining the result is to reduce the continuous current voltage and correspondingly increase the continuous current armature reaction by giving the main flux an appropriate wave form, for example, a strong third harmonic which reaches its crest value simultaneously with the fundamental wave may be produced by providing additional excitation on a portion of the pole shoe or pole core. In another method, Fig. 3, the converter 1 has in addition to the normal shunt field winding 3 and commutating winding 4, a winding 5 acting in the same direction as the cross-magnetizing ampere turns of the continuous current, and carrying the load current or a current proportional thereto. The number of turns of the winding may be variable. The winding may be located as shown in Fig. 6, but preferably as shown in Figs. 7, 8 since with the arrangement shown in Fig. 6 the flux in the interpolar space has little balancing effect but, on the other hand, necessitates a larger number of turns of the commutating poles. Since the presence of damping circuits in converters of the normal type neutralizes the balancing effects required, the damping circuits may be located on a mechanically connected machine, for example a starting motor, or the winding 5 may be provided with a low resistance shunt to provide a path for damping currents. In a modification, Fig. 4, the balancing winding 5 is located on an additional machine 6 connected in series with the alternating current side of the converter and mechanically coupled thereto. In cases where the converter is provided with a balancing winding, the winding may be placed unsymmetrically so as to assist or oppose the main flux. According to the Specification as open to inspection under Sect. 91 (3) (a), a mechanically coupled additional machine having a shunt or separately excited neld winding is connected in series with the alternating current side of the converter and acts as a motor. The converter therefore acts partly as a continuous current generator. and the armature reaction so produced is not neutralized by an alternating current armature reaction. The additional machine may be employed for voltage regulation of the converter. A shunt or separately excited cross magnetizing winding may be provided to displace the main flux with respect to the poles; this arrangement may be used in combination with partial balancing so that the angle a has a value of opposite sign to that shown in Fig. 1 and is reduced when the load is increased. This subjectmatter does not appear in the Specification as accepted. Reference has been directed by the Comptroller to Specifications 232,588 and 235,901.