Patent GB-302904-A (GB)
IPC: H02K47/24
Filed: 1928-12-14
Abstract: 302,904. British Thomson-Houston Co., Ltd., (Assignees of Merrill, F. W.). Dec. 23, 1927, [Convention date]. Rotary transformers. - A frequency changer comprises a motor winding and a generator winding of higher pole number on the primary member, which is preferably the stator, and a secondary member provided with a starting winding and formed with polar salients corresponding to the lower pole number, whereby the motorruns synchronously as a reaction motor, the salients being arranged for a flux distribution with respect to the generating winding such that a current is produced therein of a frequency higher than that supplied to the motor winding. The high frequencies obtainable may be classified in three groups-(1) the odd multiples of line frequency, (II) the even multiples of line frequency, and (III) the fractional multiples of line frequency. Fig. 1 shows a machine of group (I) having a frequency conversion factor of 3. The stator 10 carries a two-pole energizing winding 11, and a six-pole generator winding 12, and the rotor 15 has a commutator winding 16 shortcircuited as in normal repulsion motors. If a polyphase motoring winding is employed on the stator, the rotor winding may be of the squirrelcage type. As shown the rotor has two polar salients formed by cutting short some of the rotor teeth. The rotor rotates synchronously with the rotating field set up in the stator motoring winding, and the pole flux is so spaced with respect to the winding 12 that a voltage is produced therein of three times the frequency supplied to the winding 11. If a frequency factor of five is required, the stator winding 12 may be wound with ten coils, but since the construction would be uneconomical as a result of narrowing the rotor salients, the arrangement shown in Fig. 3 may be adopted. In the construction the stator has ten generating coils 12<1> and the rotor 15 has two salients per pole so spaced that the voltages induced in the stator generating coils are additive. For a frequency factor of 7, the rotor may have three salients per pole; for a frequency factor of 9, four salients per pole, &c. Fig. 5 shows a machine of the group (II) having a frequency factor of 4. The stator has a two-pole motor winding 11 and an eight-pole generating winding 12". The rotor is unsymmetrical in that it is formed with two salients of N-polarity and one of S-polarity, but may be suitably balanced. For a conversion factor of 6, the stator generating winding would have twelve coils, and the rotor would have two salients on one side and three on the other. Fig. 7 shows diagrammetically a machine of group (III) having a frequency factor of 2¢. The motor winding 11 is wound for four poles and the generating winding 12 for ten poles. The rotor 15 has two salients of the same polarity spaced unsymmetrically, and a third salient of opposite polarity located midway in the larger angle separating the two like salients. The spacing is such that the two like salients are under generating coils of like polarity, whereas the odd salient is under a generating coil of opposite polarity. A rule is deduced that the number of salients per pair of fundamental (i.e. motoring) poles equals the frequency factor minus one. In order to damp harmonic fluxes, the commutator may be automatically short-circuited as the machine approaches synchronous speed, and the flow of harmonic currents in the motoring winding may be reduced by distributing the winding so as to produce a flux wave shape as nearly sinusoidal as possible. Tooth ripple in the high frequency voltage is avoided by skewing the rotor teeth. The slot areas allotted to the stator windings should be nearly equal so as to obtain a low current density and low resistance in the high frequency windings. According to the Specification as open to inspection under Sect. 91 (3) (a), a separate synchronous driving motor may be employed if with the unsymmetrical arrangement shown in Fig. 7 the machine fails to operate satisfactorily as a motor. This subject-matter does not appear in the Specification as accepted.