Patent GB-249579-A (GB)
Title: Improvements in and relating to alternating current machines
IPC: H02K47/04
Filed: 1924-11-21
Abstract: 249,579. Creedy, F. Nov. 21, 1924. Alternating-current commutator and induction machines; exciting; rotary converters and transformers; windings; compensating windings.-A non-salient pole machine of non- internally - cascaded type adapted for use as a motor-generator or converter, a double-current, generator for alternating or continuous currents, a commutator and synchronous motor, or as a commutator machine for phase or speed regulation, has windings on its stator which produce (a) a generator field, (b) a motor field, and (c) a field neutralizing the commutator currents of the rotor, and on its rotor it has a commutator winding and a second winding or additional circuits in the commutator winding in each turn of which the motor field of the stator induces e.m.f.'s while the generator field induces none. The stator fields (a) and (b) differ in pole-number by four or more and these fields. together with the neutralizing ampere-turns (c), are designed without the use of transformers. Figs. 2 to 5 relate to a 2-pole induction motor and a 6-pole continuous-current generator converter machine in which the 6-pole continuous field is produced by the winding shown in thin lines in Fig. 2, each coil having as many evenly spaced open loops as there are pole-pairs so that no e.m.f. may be induced by the alternating field. The neutralizing winding, shown in thick lines, has a similar form to the generator field winding and preferably over-compensates the rotor ampere-turns to give a commutating-pole effect. The stator 2-pole alternating-field winding, Fig. 3, is a single-phase winding with pitch equal to one third of the circumference to eliminate third harmonics or multiples thereof, and with a starting winding, shown in dotted lines, in the vacant slots. The rotor motoring winding for the 2-pole field consists of shortcircuited coils, Fig. 4, each of pitch equal to one third of the circumference. The rotor continuous-current generating winding is of twocircuit type, having three coils equidistant around the circumference in series between the brushes, so that the alternating e.m.f.'s induced by the 2-pole field in each group of three coils between the brushes cancel out. The two rotor windings may be combined into one, Fig. 5, with equalizers A, B, C connecting points on the commutator one third of the circumference apart. If the machine is to run on a 6-pole alternating field and generate continuous-current in the 2-pole field, the winding of Fig. 3 is used for the continuous-current excitation, the starting winding being omitted, and the 6-pole winding of Fig. 2 is reduced to single bars in each slot so connected as to form eight short-circuited sets of six bars. Mutual induction between these sets and the continuous-current 2-pole winding is avoided by giving the 2-pole coils a pitch equal to one third of the circumference. An alternative to the scheme of Fig. 5 consists in omitting the equalizers and tapping the winding symmetrically to three slip-rings, and in this case the motor element may be operated synchronously by supplying continuous current to one slip-ring and to the two others joined electrically. Fig. 7 shows a form in which the continuous and alternating-current windings are combined on the rotor and stator and in which a separate exciter is used. The stator windings comprise (a) a parallel star system P, Q of known type excited by continuous current at the star points and giving a 10-pole field with a 4-pole neutralizing distribution and (b) a 4-pole shunt winding S. The rotor winding is short-circuited as regards the odd number of pole-pairs and produces an e.m.f. at the commutator on the other pole number. It has twenty coils spaced apart by an even number of pitches of the 4-pole flux and each coil consists of two bars, one at at the top and the other at the bottom of a slot united by clips A, B. These clips are further joined by connections A C, B C so that the winding is, in fact, an ordinary winding with each bar split into two bars in parallel and placed diametrically opposite on the armature. The exciter is drum-wound and has four poles, two of which are series-excited from the main commutator while the others are shunt-excited from its own brushes and energize the main shunt coils S. To ensure that no coil shall lie simultaneously under shunt and series excited poles in the exciter, a pair of diametrically opposed poles is removed in a 6-pole or other multipolar machine. An alternative form of 4-pole exciter is described in which two brushes are short-circuited and the shunt and series coils are connected between these and the remaining two brushes respectively placed under shunt and series-excited poles. An alternative rotor winding is shown in Fig. 8, having twenty-four slots and giving two and eight poles or any multiples thereof. Six groups of fours bars, such as a, b, c, d, are connected in closed series and united by clips, as in Fig. 7, at the commutator end, the pairs of bars, such as a, c and b, d being diametrically opposite. Such windings may be adapted to give any pair of pole numbers which, when divided by their G.C.M. gives results respectively odd and even. The Specification gives other examples of parallel-star groups fed with continuous current as illustrated in Specification 175,306. Harmonics, other than of the third order referred to above, may be eliminated in the coils undergoing commutation by short-circuited coils placed on the stator in coincidence with the commutated coils and having a pitch equal to an even number of the alternating pole pitches. Alternatively, the stator winding may have all poles of one sign in each phase connected in parallel. The machine may give alternating instead of continuous voltage at the brushes by replacing the continuous-current stator winding by an appropriate alternating one and fitting brushes of the desired phase number on the commutator. Thus, the form shown in Figs. 2, 3 and 5 could supply alternating current from a series-generator element obtained by using the winding shown in thin lines in Fig. 2 as a series field winding, the neutralizing winding acting as before. Alternatively, the thin and thick lined windings may be used as a 2-phase system. Such a machine may be used in series with the sliprings of an induction motor to feed low-frequency currents back to the mains. A machine of this kind may be used for speed or power-factor regulation.