GB900600A - Improvements in or relating to induction motors - Google Patents

Abstract

900,600. Winding layouts for dynamo-electric machines. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Aug. 21, 1958 [Aug. 28, 1957; May 28, 1958], Nos. 27112/57 and 17022/58. Class 35. Pole-amplitude modulation.-A close-ratio change in the effective pole-number of induction motors is produced by imposing a modulation on the basic pole-number, the spacing of the windings being so arranged that of the two resultant " beat-frequency " pole-numbers only the desired modified pole-number is operational. As shown, for example, in Fig. 1, in which the flux-waves are shown in rectangular form for simplicity, modulation of an 8-pole machine 1 (a) is achieved according to the invention by reversing one half of each phase winding, thus imposing a 2-pole modulation 1(b) on the original poles. The resultant 1(c) comprises a mixture of 6-poles (i.e. 8-2 poles) and 10-poles (i.e. 8+2 poles). In order that the machine may run at 6-pole or 10-pole speed it is necessary to suppress the unwanted pole-number. This is achieved by arranging suitable spacing of the phase windings round the circumference of the machine, and it is shown that the spacing which produces ideal operation at one pole-number also serves to completely suppress the other pole-number. It is described how, in each phase, the pole windings may conveniently be connected in groups 1, 2, 3, 4 and 5, 6, 7, 8 in parallel in the unmodulated condition, while to produce the modulated condition they are simply connected in series with the centre-tap disconnected, thus reversing the energization of one group, e.g. poles 5, 6, 7, 8, as shown in Fig. 1(b). Ideally, the modulation wave would be sinusoidal instead of the square wave shown in Fig. 1(b), and various methods are described by means of which a closer approximation to a sinewave may be obtained. In the simplest arrangement, the original poles 4 and 8 may be suppressed altogether on modulation to produce the modulated waveform shown in Fig. 2 (c). This is achieved by connecting only the six coils 1-3 and 5-7 in the series-modulated condition. The eliminatable coils 4 and 8 may be connected in series with the remaining parallel-connected coils in the unmodulated condition for ease of disconnection, but for phase-windings having an even number of slots per pole per phase all the coils may be used for both connections. This is done by winding each pole winding in two halves in separate pairs of slots so that in the parallel unmodulated condition all the coils of poles 4 and 8 act additively to produce active poles but in the series-modulated connection, one section of each of these pole-windings is reversed with the poles 5, 6 and 7, and so counteracts the remaining section. The poles 4 and 8 are thus suppressed by opposition instead of disconnection. A closer approach to sinusoidal modulation may be achieved by the use of the modulation wave shown in Fig. 3 (b) involving the reduction in amplitude of coils 1, 4. 5, 8. This is most easily done by omitting one coil from each of these coil groups on modulation, but coil omission can be avoided by the use of an odd number of slots per pole per phase by connecting unequal numbers of coils in mutual opposition. An improved amplitude distribution of the modulated M.M.F. can alternatively be obtained by the use of a sinusoidally varying unmodulated pole-strengths, as shown in Fig. 4 (a). This is produced by winding poles 1, 4, 5, 8 to produce only half the full pole flux. Modulation is then produced by simple reversal of half the winding as shown in Fig. 1 (b). Clearly, the unmodulated conditions may be series, instead of parallel, connected and vice versa, and star/delta transformations may also be employed. Elimination of unwanted modulated polenumber.-The effect of modulating a basic winding having P pole-pairs with a modulation wave having M pole-pairs is to produce one new pole-number having (PM) pairs and another having (P+M) pairs. To eliminate one of these new pole-numbers, the modulation waves corresponding to the three phases must be mutually spaced by 120 degrees mechanical round the whole machine axis, so that the origins, for example, of the three modulation waves, are equally spaced round the machine. The elimination of a pole-number depends on the coincidence of equivalent points of the phase windings with equivalent points on the equallyspaced modulation waves. In fact, the phase windings must have equivalent points spaced at 120 degrees round the machine. The relative rotation of the phase-sequence and the modulation sequence determines whether the higher or lower pole-number is eliminated. Thus, if the modulation waves at 120 degrees spacing relate to phases A, B, C in sequence the lower pole number is eliminated to leave the higher, but if the modulation waves relate to the phases in A, C, B order, then the higher pole-number is eliminated, leaving the lower. It is explained (with reference to Fig. 13, not shown) how when the equally-spaced origins (or other identical points) of the three modulation waves correspond to equivalent parts, e.g. the origins, of three different phases, then a modulated polenumber having three or a multiple of three pole pairs is eliminated, and the three phase windings may each be connected for modulation in an exactly identical manner, e.g. by one of the series/parallel transformations described above. As examples, if the unmodulated pole-number is (3n+1 pole-pairs) i.e. 8 poles, the 120 degrees points coincide with (e.g. the origins of) phases A, B, C, then (3n) pole-pairs (i.e. 6 poles) are eliminated and (3n+2) pole-pairs (i.e. 10 poles) remain. Likewise, for 10 poles (3n+2), the sequence is A, C, B and 12 poles (3n+3) is eliminated leaving a modulated pole-number of 8 poles (3n+1). Examples of operational winding layouts are given.