Patent GB-926101-A (GB)
Title: Improvements in or relating to rotary electric machines
Inventor: RAWCLIFFE GORDON HINDLE
IPC: H02K17/14
Filed: 1958-07-18
Abstract: 926,101. Winding layouts for dynamo-electric machines. NATIONAL RESEARCH DEVELOPMENT CORPORATION. July 17, 1959 [July 18, 1958 ; Dec. 2, 1958 ; May 29, 1959], Nos. 23070/58, 38851/58 and 18410/59. Class 35. The symmetrical pole-amplitude modulation method of pole-changing disclosed in Specification 900,600 was limited to machines providing pole-numbers neither of which was a multiple of three. In order to eliminate one of the pole-numbers produced by modulation it is necessary for the modulation waves to be applied to the individual phase-windings at equal intervals round the machine stator, i.e. at points spaced 120 degrees mechanically round the core. If the modulation waves relate to the phases in the same sequence as the phase sequence of the winding, then the lower-pole number is eliminated to leave the higher. If in the reverse sequence, then the higher polenumber is eliminated to leave the lower. In a machine whose unmodulated pole-number is a multiple of three, these 120 degrees points all fall on the same phase so that the three modulation waves cannot be correctly spaced and elimination of the unwanted pole-number is impossible. Alternatively, if one of the modulated pole numbers is a multiple of three, the modulation waves can only apply in such a phase sequence as to eliminate that polenumber, i.e. the 120 degrees points coincide with phase-bands in that order round the machine. Thus a machine according to Specification 900,600 cannot operate at a polenumber which is a multiple of three in the unmodulated or modulated condition. The present invention overcomes this limitation by providing a means for positioning the origins of modulation waves at points around the winding periphery not occupied by phase-bands of the corresponding phase-windings. According to the present invention the modulation wave is effected as the resultant of two component modulation waves, so spaced as to commence at points occupied by phase-bands of the phase to be modulated. The origins of the two component waves are spaced by one pole-pitch and lie on either side of the desired point, and act as if they were applied to a proportion of the conductors in each phase-band, such that the sum of these two waves has its origin lying at the required 120 degrees point even though there is no conductor of the relevant phase at this point. The coils of each phase are considered as though divided into parts comprising <SP>1</SP>/ 3  and <SP>2</SP>/ 3  of the total number of turns, and each part is modulated with the required wave originating at points displaced on either side from the required origin by two phase bands and one phase band, respectively. This procedure of using two unequal component modulation waves is called asymmetrical pole-amplitude modulation, and it is a method of establishing a modulation wave at a required angular position, for the elimination of a selected pole-number, regardless of the physical position of the phase bands to which the wave relates. The principle of the invention as applied to a machine of the second sort mentioned above, i.e. the required modulated pole-number is a multiple of three, is explained with reference to Fig. 6 which shows a four-pole machine modulated to give six poles. In any machine, one phase taken at random, may be modulated in the way described in Specification 900,600, i.e. by the reversal of half of the winding. This phase may be called phase " A." Where the unmodulated pole-number is a multiple of three, or the phase-bands at 120 degrees intervals are not in the required order, phases B and C cannot be modulated in this simple way but only by the use of asymmetrical modulation. The flux wave produced by phase B is shown at 6(a). The desired modulation wave of amplitude T has its orgin at Y at slot 13, but this slot contains only coils of phase C, which, if modulated at this point would eliminate the six polenumber to give two-pole operation, which is not desired. The nearest phase B bands to point Y commence at slots 7 and 16, one pole-pitch apart. The partial modulation waves 6(c) and 6(d) accordingly start at these two points, and their relative magnitudes are inversely proportional to their distances from the desired point Y. Thus, modulation wave 6(c) commencing <SP>1</SP>/ 3  pole-pitch behind point Y is of <SP>2</SP>/ 3  amplitude T, while wave 6(d) commencing <SP>2</SP>/ 3  pole-pitch before point Y is of <SP>1</SP>/ 3  amplitude T. The combined waves, 6(e) have a sinusoidal approximation passing through the point Y as desired. This compound wave is applied to the four phase bands of phase B as follows, top slot conductors only being considered :-band in slots 7, 8, 9 is reversed and reduced to <SP>1</SP>/ 3  magnitude, band in slots 16, 17, 18 is unaffected, band in slots 25, 26, 27 is reduced to <SP>1</SP>/ 3  magnitude, and band in slots 34, 35, 36 is simply reversed. The number of the turns of phase A must, of course, be reduced on modulation, in the same proportion as the other two phases. A suitable series/parallel arrangement of the windings in which a section of each phase winding is disconnected on modulation is described (Figs. 3, 4, 5, not shown). The arrangement of the component modulation waves for machines whose unmodulated pole-number is a multiple of six, i.e. in which the 120 degrees points all fall on the same phase, say, phase A, is illustrated for a 6-pole machine in Fig. 12 in which each phase-band comprises two slots. In this case a stepped total waveform 12 (b) is desired with its origin for phase B at point Y at slot 13. This gives 8-poles on modulation. Once again two component waves, 12(c) and 12(d) are employed, spaced by one pole-pitch from each other. In this case the waves are stepped, as shown. The larger of these waves 12(c) is advanced <SP>1</SP>/ 3  pole pitch from Y and is of <SP>2</SP>/ 3  magnitude. The smaller, 12(d) is retarded <SP>2</SP>/ 3  pole-pitch from Y and is of <SP>1</SP>/ 3  magnitude. Their sum, 12(e), is an asymmetrical wave of <SP>5</SP>/ 6  magnitude having its equivalent sine-wave with origin at Y. In order to form this composite modulation wave, which is composed of steps of one or more sixths of the total amplitude, it is necessary that 16 of the 36 slots be wound in two sections, containing two-thirds and one-third of the turns respectively. A series/parallel connection change-over with the omission of certain coils is described with respect to Figs. 8, 9, 10, 11 (not shown). Phase A is also reduced, to match the other phases, by elimination of one of the slots in certain of the phase-bands so that as modulated, the phasebands are +1, +2, +1,-1,-2,-1 slots. A modification of this is described, Fig. 13, not shown, in which the component modulation waves are square, i.e. not stepped. This removes the necessity of providing coil sections of one-sixth number of turns and simplifies the winding. Synchronous machines.-For synchronous machines the D.C. excited rotor must also change poles. To avoid the generation of flux harmonics inherent in the conventional method of pole-changing which involves the suppression of poles, the arrangement shown in Fig. 14c is used. This shows the conversion from 10 poles to 8 poles by the reversal of half (i.e. poles 6, 7, 8, 9, 10) and the simultaneous reduction in amplitude of those poles 1, 10 and 5, 6 which lie between the reversed and uncharged portions. Various circuits, using coil omission or coil opposition, and using rotor windings brought out to several slip-rings, are described (Figs. 15, 16, 17, 18, not shown).