Patent GB-978211-A (GB)
Title: Improvements in or relating to induction machines
Inventor: WILLIAMS FREDERIC CALLAND
IPC: H02K17/30
Filed: 1960-03-31
Abstract: 978,211. Change-pole induction motors. NATIONAL RESEARCH DEVELOPMENT CORPORATION. March 21, 1961 [March 31, 1960; Dec. 1, 1960], Nos. 11364/60 and 41346/60. Heading H2A. In a variable speed induction motor operating by " stretching " and " shrinking " the pole-pitches a number of discrete speeds are obtained by dividing the polyphase stator winding into several mutually gradedly overlapping coil-sets, and connecting the different phases of the supply in various combinations to the corresponding coils of the sets so that the phase position of the flux in the sets with respect to each other is altered and the average phase difference between currents in adjacent slots is also altered, more or less uniformly because of the grading. In the discontinuous stator shown in Fig. 1 (Prov. 11364/60) the graded coils-set are shown at 14, 15, 16. If the windings of each set are connected as shown in Fig. 2 (Prov. 11364/60) there will be no phase difference between the phase coils of the three sets, but if they connected in a sequentially displaced manner as shown in Fig. 3 (Prov. 11364/60) there will be a phase difference between the corresponding coils of sets 15 and 16, and 15 and 14 respectively. The current in the coils of set 14 is advanced 60 degrees with respect to that in corresponding coils in set 15, while 16 is retarded 60 degrees with respect to 15, so that the total phase displacement between the two ends of the primary winding has been increased 120 degrees, i.e., the poles have been " shrunk." This sequence 16, 15, 14 is regarded as positive, but if the coils of the sets are interconnected in the reversely displaced sequence, the currents in corresponding sets will be in the sequence 14, 15, 16, which is regarded as negative and the poles are " stretched." Moreover, if the coils of set 15 are energized positively, instead of negatively as shown, the phase difference between the currents in the sets will be 120 degrees instead of 60 degrees. The total amount of phasedisplacement between the two ends may be increased adding two further sets of reversibly connected coils 17, 18 into the stator as shown in Fig. 8 (Prov. 11364/60) the coils of set 17 being connected in series with the coils of set 16, and of 18 with 14 so that coil set 15 is undisplaced, sets 14 and 16 are shifted oppositely by angle #, and sets 17, 18 by angle 2#. This doubles the maximum phase displacement between the ends of the stator, and makes five different speeds available, i.e., 0; Œ120 degrees and Œ240 degrees. This may be increased to nine if a star-delta transformer is interposed before set 15. It is advantageous to prevent sudden changes of rotor flux, for the stator core to be bridged over the inactive arc by an unwound section, and for the active section to correspond in pole-number to a section of a complete stator, e.g. if the stator of Fig. 1 extends over 270 degrees it should generate three poles, to be three quarters of a complete four-pole circle. It is described how this stator may provide pole-numbers of 2, 4, 6, 8 and 10 without discontinuity. It is also advantageous to use two opposed pole sections as shown in Fig. 13 (Prov. 11364/60) for balance, and once again, these sections should be proportionate in length and polenumber for all of the available pole-numbers to avoid rotor flux discontinuities. Each of these stators may carry five coil-sets, and may be designed for a full arc of greater than 180 degrees, e.g., as shown in Fig. 22 (Prov. 11364/60), they are designed for an arclength of 240 degrees, but are " chopped off" to 180 degrees to make them form a continuously wound stator. The conductors in the stator slots have a triangular distribution as shown in Fig. 1 (Prov. 41346/60) in which the reference set of coils M (15 in previous Fig. 1) is fed directly from the mains while the other sets are shifted by Œ#, Œ2# respectively by altering the phase connection of the coils in the sets. However, the reference set of coils may be at one end instead of in the centre. The stator winding may be continuous, i.e., have no gap, provided the total phase displacement is 360 degrees or a multiple thereof. To obtain a continuous stator winding, the number of sets of coils must be a multiple of the number of phases. A simple machine in which this multiple is one is shown in Fig. 3 (Prov. 41346/60), in which the coils which have a pitch of three slots are distributed over an 18-slot stator in sets indicated #, 2# and 3# respectively, the references 1R, 2R &c. indicating the connection of the phase windings, this being set out in more detail in Fig. 4 (not shown). With the single connection the pole-number is 6; when the 2# and 3# group are advanced 120 degrees and 240 degrees respectively the pole-number is 8; and when they are retarded by this amount the pole number is 4. A modification having six coil-sets is described giving pole-numbers 8, 10, 12, 14, 16 for phase shifts of 0, Œ60 degrees and Œ120 degrees, Fig. 10 (not shown), shows coils of pitch equal to 1/8 of the group spread, i.e. 3/22 slots and by increasing the coil pitch to 2/3 the group spread (Fig. 11, not shown) harmonics may be reduced by the use of two groups of coils, mutually offset (Fig. 12, not shown). It is described how short-circuited coils may be used to improve the flux waveform. The relationship between the number of conductors per slot in each set to the slot number may be a triangular one as shown above or the triangular form may be truncated by the insertion of additional coils having the maximum number of conductors in each set of coils or the relationship may be a stepped one. It is described how, if only two alternative speeds are required, these may be obtained by simple reversal of alternate sets of coils. Specification 896,627 is referred to.