Multiphase Stator Winding: New Perspectives, Advanced Topologies, and Futuristic Applications

Multiphase winding configurations have gained significant attention in high-performance variable speed drives and wind energy conversion systems (WECS) owing to their myriad advantages. In the available literature, various multiphase winding layouts have been designed aiming at boosting the machine performance to meet the requirements of the proposed applications. Ultimately, this paper surveyed the state-of-the-art in the available multiphase winding layouts proposed for various innovative applications. Various types of windings were discussed, while investigating their advantages and limitations. This typically considers the winding layouts employed in multiphase induction motors (IMs) and permanent magnet (PM) machines with prime phase and multiple three-phase orders. This study extensively provides innovative winding arrangements that offer better machine characteristics in terms of torque density, efficiency, and fault-tolerance capability. Moreover, the construction of multiphase machines with general $n$ -phase using standard three-phase stator frames has been elaborated. This latter technique obviates the basic necessity of special stator frames with a prime number of phases. Finally, this paper sheds light on the commercial applications that include multiphase winding layouts.

instance, the multiphase IMs modeling has been presented 95 in [35], [36], and [37]. Generally, the number of planes is 96 (n − 1)/2 with one zero sequence and n/2 for an odd and 97 even number of phases n respectively. For example, the funda-98 mental αβ, secondary xy, and zero-sequence 0 + 0 − subspaces 99 are the orthogonal subspaces of a six-phase system. Each 100 subspace yields a significant air gap flux distribution, which 101 mainly depends on the utilized winding topology [38]. The 102 αβ subspace is considered the fundamental torque-producing 103 subspace, while the zero magnetizing flux is produced by the 104 secondary xy and 0 + 0 − subspaces. This assumption has pri-105 marily been utilized to ease machine modeling, even though 106 applying it to any winding arrangement results in significant 107 errors [39]. 108 Furthermore, the feasibility of enhancing the multiphase 109 machine torque production through current harmonic injec-110 tion (CHI) has been extensively investigated for both the IMs 111 [3], [40] and PM machines [41], [42], [43]. For n-phase 112 IMs with concentrated windings, all odd current harmonics 113 with an order less than n can be utilized to interact with 114 their respective MMF harmonics, resulting in extra torque-115 producing components [3]. The same concept is applied 116 to FSCW PM machines; however, the injected odd current 117 harmonics couple with the corresponding air gap PM flux 118 density harmonics [41]. It is worth mentioning that the 119 control of multiphase drives has recently been introduced 120 in [44] and [45]. 121 This paper introduces several winding layouts proposed 122 in the literature for multiphase machines, as shown in 123 FIGURE 1. Since multiphase machines are mainly proposed 124 for high-power applications, improved fundamental wind-125 ing factor and flux distribution and simple winding layout 126 are among the main design objectives of such machines. 127 Double-layer winding layouts are commonly employed with 128 conventional three-phase induction machines thanks to their 129 high-quality flux production. However, increasing the num-130 ber of phases stands as a simple technique to ensure a high-131 quality air gap flux distribution, while a single-layer winding 132 layout is preserved. This point is first explained in detail, 133 and the most suitable winding layouts for different phase 134 orders are also presented. Another important aspect related 135 to the multiphase system is its high fault tolerance capability. 136 Some other innovative winding layouts are introduced to 137 suppress the flux components of the secondary subspaces, 138 which degrade the machine's performance. Eventually, a sim-139 ple technique to rewind standard three-phase stator frames 140 with any n-phase symmetrical prime phase order winding is 141 elaborated.
142 Section II will cover the conventional multiphase wind-143 ing layouts with a further classification by the number of 144 layers: single-layer-based layouts with prime phase orders 145 and double-layer-based layouts with composite phase orders. 146 In Section III, multiphase winding configurations for IMs 147 are reviewed and classified into winding topologies for 148 fault-tolerance enhancement and those employed for build-149 ing prime-phase-based windings with standard three-phase 150   n-phase (δ = 2π/n), and asymmetrical n-phase (δ = π/n) 166 configurations. As an illustrative example, several six-phase 167 winding arrangements, i.e., dual three-phase (D3P), symmet-168 rical six-phase (S6P), and asymmetrical six-phase (A6P), are 169 presented in FIGURE 2 [47]. A nine-phase winding can be 170 either symmetrical or asymmetrical. Generally, an asymmet-171 rical winding layout corresponds to a higher number of phase 172 belts per pole, increasing the order of the lowest harmonic 173 [48]. Furthermore, multisector stator winding configurations, 174 in which the winding sets are placed in several stator sectors, 175 have been proposed in the literature [49], [50], [51], [52].

176
For a multiphase machine with an odd and even number 177 of phases n, there are (n + 1)/2 and n/2 connection alterna-178 tives, respectively. These winding connections are differen-179 tiated with respect to the series connection of each pair of 180 phases [45].

181
For instance, an star (no series connection and single 182 neutral point), pentagon (each two consecutive phases are 183 connected in series), and pentacle (the phases are con-184 nected in series with a step of two phases) represent the 185 three connections of a five-phase machine, as shown in 186 FIGURE 3 [53]. For a six-phase machine, star, hexagon, and 187 double delta constitute the available winding connections 188 [54], [55]. For both PM machines [56] and IMs [57], the 189 pentagon connection gives superb performance in faulty con-190 ditions over its star counterpart. From the efficiency per-191 spective, the star-connected machine is the best in healthy 192 cases since it omits the induced zero sequence compo-193 nent in the pentagon connection. Besides, considerable work 194 proposed winding topologies based on combined winding 195 VOLUME 10, 2022 where S and p are the number of slots and pole pairs, respec-218 tively, and n is the phase order. The optimal value of q for 219 both five-and seven-phase IMs is 2. However, q is preferably 220 set to 1 for eleven-phase IM. For example, the number of 221 slots for 2-pole five-and seven-phase IM are 20 and 28, 222 respectively. FIGURE 4 shows the corresponding winding 223 configurations.

224
Furthermore, the design of several multiphase IMs with 225 prime phase order; namely three-, five-, seven-, and eleven-226 phase IMs, is presented in [40]. For instance, the three-227 and eleven-phase winding configurations are shown in 228 FIGURE 5. The developed torque is enhanced based on the 229 harmonic injection, i.e., 3 rd harmonic in the five-phase IM 230 and 3 rd and 5 th harmonics in the seven-phase IM. The same 231 rotor design, phase current, and flux per pole have been used 232 to assess the proposed designs. The rotor design eliminates 233 parasitic torque and cogging issues, ensuring proper opera-234 tion. The rotor parameters computation technique, previously 235 presented in [65] and [66], has been extended to the n-phase 236 case in order to evaluate the impact of increasing the number 237 of phases on the machine's parameters. The design proce-238 dures are similar to that described in [46], but with a single-239 layer stator winding and 30 rotor bars.

240
Consequently, the n-phase machine offers the same output 241 power and average torque production as its equivalent three-242 phase machine with identical characteristics. Moreover, the 243 four designs give the same magnetic flux density distribution, 244 which further highlights that the four machines are equiva-245 lent. Amongst the proposed designs, the effectiveness of the 246 harmonic injection in torque enhancement is significant in the 247 five-phase machine, while being limited in other designs with 248 phase numbers higher than five. As an illustrative example, 249 a five-phase machine is simulated, supplying the stator with 250 a fundamental sinusoidal voltage. The machine is assumed 251 to run steadily at a constant speed of 1757 rpm, i.e., the full 252   In [47], a six-phase IM with different stator connections 280 has been introduced to highlight the effect of the winding con-281 figuration on the machine parameters. Any six-phase winding 282 layout can simply be obtained by rewinding a three-phase 283 24-slot/4-pole machine as a 12-phase machine, as shown 284 in FIGURE 8. Moreover, various stator connections can be 285 obtained, including D3P, S6P, and A6P, as presented in 286 FIGURE 9 [47]. These three winding configurations have 287    employed. This is mainly due to the fact that the stator leakage 300 inductance of the xy subspace of the A6P is quite small. Under 301 D3P, the dc-link voltage utilization is maximized since the 302 zero-sequence impedance is minimized. 304 This section presents multiphase winding layouts for IMs 305 with a further classification into winding layouts for fault-306 tolerance enhancement and winding topologies based on 307 standard three-phase stators. The fault-tolerance capability is a distinct advantage got from 311 the multiphase machines. Based on the available literature, 312   Under optimal current control, the five-phase IM is better 315 than the six-phase one because it can maintain 70% of its 316 rated load [57] compared to 66% by an asymmetrical six-317 phase machine [69]. Under open-loop control, these maxi-

346
A recent combined star/pentagon five-phase winding con-347 figuration has been presented in [20]. The proposed winding 348 layout is a split-phase dual five-phase winding that can be 349 obtained from conventional five-phase windings, as shown 350 in FIGURE 11. FIGURE 12 shows that the five phase 351 can be connected either in star, pentagon, or combined 352 star/pentagon. The proposed combined star/pentagon wind-353 ing layout consolidates the merits of the star and pentagon in 354 a specific manner, in which the number of turns per coil of 355 the second five-phase group N c2 is 1.1756 times the number 356 of turns per coil of the first group N c1 . Each winding corre-357 sponds to a unity winding factor since the number of slots per 358 pole per phase is one, q = 1.

359
The suggested winding not only provides a better flux 360 distribution than a traditional single-layer winding, but it 361 also cancels out the third-order harmonic flux component 362 created by induced third sequence currents. As a result, 363 machine losses are reduced, improving total machine effi-364 ciency. In addition, the proposed winding gives a similar per-365 formance to the star connection in the healthy case. Whereas 366 the proposed winding is more advantageous than its star and 367 pentagon counterparts in faulty cases since it can support 368 higher loads under open-line fault and offers higher stability 369 limit and lower derating factor. For example, experimental 370

391
Recent literature introduced the idea of delivering the same 392 performance as a higher-order multiphase machine while 393 offering a lower number of machine terminals, which is done 394 using either static winding transformations [78] or specific 395 stator winding connections [20], [76], [58]. 396 An innovative single-layer winding arrangement has been 397 presented for high-power medium-voltage IM [28]. The IM 398 is intrinsically an asymmetrical nine-phase machine with 399 a spatial phase shift of 20 • between the three three-phase 400 winding groups. The nine phases are connected in a specific 401 manner to provide six terminals that are tied to three-phase 402 inverters, as shown in FIGURE 14. FIGURE 15 shows the 403 proposed nine-phase six-terminal (9P6T) winding configu-404 ration with a unity winding factor. From FIGURE 14(b), 405 it is clear that the currents of the three three-phase wind-406 ing sets are different in magnitude. Thus, the number of 407 turns of the third winding group, i.e., abc 3 , is decreased by 408   1.88 to preserve the same ampere-turn from the three winding 409 groups.

410
The proposed 9P6T winding layout is more advantageous 411 than the traditional asymmetrical six-phase (A6P) configura-412 tion owing to its better flux distribution, higher torque den-413 sity, improved torque/current ratio, and simplified winding In [79], a novel pseudo six-phase (P6P) winding layout has 424 been elaborated employing quadruple three-phase winding 425 groups to enhance the performance of a 36-slot/4-pole IM. 426 The proposed P6P winding configuration is similar to the 427 9P6T in flux distribution with a slight difference that the third 428 winding group abc 3 is subrogated by two three-phase wind-429 ing groups, i.e., abc 3 and abc 4 , as depicted in FIGURE 18. 430 The coils of abc 3 and abc 4 are wound together in the same 431 slots to preserve the advantages of the single-layer winding 432 configuration. Since each two three-phase winding groups 433 are connected in series, e.g., abc 1 and abc 3 , they shared the 434 same current magnitude. Therefore, the number of turns of 435 abc 3 and abc 4 is reduced by 0.532 to preserve the same MMF 436 production.

437
The proposed P6P winding layout seems equivalent to the 438 9P6T from the torque and efficiency perspectives, as depicted 439 in FIGURE 19. However, it outperforms the 9P6T because 440 it allows various neutral arrangements; namely, an isolated 441 or connected neutral, as a conventional six-phase machine 442 and thus better DC-link voltage utilization. Moreover, the 443 zero-sequence current component, which affects the perfor-444 mance in faulty cases, is avoided. Unlike the conventional 445 asymmetrical six-phase winding configuration, the proposed 446 P6P enhances the winding factor by 5% and thus the torque 447 density. Besides, the copper fill factor is increased, and the 448 insulation process is simplified. Eventually, the stator current 449 waveform is enhanced due to the rise in secondary subspace 450 impedance. 451 VOLUME 10, 2022     Thus, the proposed winding layout gives superior perfor-473 mance of the six-phase IM over the conventional concentrated 474 one owing to its smoother harmonic spectra. Since the motor 475 laminations were designed for the DW, the performance of the 476 machine with pseudo winding can be improved by redesign-477 ing the motor laminations. This is proved from the similar 478 ratio of the output power to the used copper volume by both 479 the pseudo and distributed winding configurations. Due to 480 the modular structure of the proposed pseudo concentrated 481 winding, it is preferred over the DW under faulty conditions 482 because the damaged coils can be easily repaired. Due to the fact that multiphase machines with multiple three-486 phase winding groups can be obtained from conventional 487 three-phase stators and can utilize commercial three-phase 488 inverters, they are considered optimal candidates for aca-489 demic research and industrial applications. On the other hand, 490   For the proposed windings, the MMF spectra appear to be 517 even better than the standard three-phase case, while they 518 yield 2% and 3% increases in the fundamental component 519 for the five-and seven-phase windings, respectively. This 520 is proved from FIGURE 22. Furthermore, the five-and 521 seven-phase windings provide a torque improvement of 2.3% 522 and 2.8% above the normal three-phase scenario, respec-523 tively, for a sinusoidal input supply. An additional 10% gain 524 is obtained when a third harmonic current injection is used, 525 comparable to traditional five-and seven-phase windings 526 winding layouts [40], resulting in a torque gain of 13.4% and 527 13.5% over the initial three-phase winding for both windings. 528 Although the novel winding layouts based on standard three-529 phase stator frames create a high-quality flux distribution, 530 they are more expensive and require a more complex winding 531 structure. Due to the differences in leaking stator inductances 532 across phases, they also produce minor secondary sequence 533 current components. This latter issue, on the other hand, can 534 be easily mitigated with current control.

535
Several multiphase winding configurations for IMs, sur-536 veyed thus far, have been revisited in this study. A com-537 parative insight into different characteristics of each layout, 538 including number of phases, winding arrangement, winding 539 connection, winding factor, losses, torque ripple, and har-540 monics, has been provided in Table 1.

542
This section elaborates multiphase winding arrangements for 543 PM machines with a further classification into winding lay-544 outs with minimum space harmonics, special/ unconventional 545 winding topologies, and winding configurations based on 546 stator shifting concept. First, the effect of the winding connection on the performance 550 of five-phase PM machines has been investigated in [82]. 551 Five-phase machines have several winding connections, such 552 as star, pentagon, and pentacle, as shown in FIGURE 3. 553 According to the employed winding connection, the voltage 554 of the windings varies. The performance of the five-phase PM 555 machine when equipped with these winding connections has 556 been compared, highlighting the torque-speed and efficiency 557 characteristics. The maximum torque is obtained with the star 558 connection. While the base speed is extended, and highest 559 efficiency is obtained with the pentacle connection. More-560 over, the dynamic performance has been presented based on 561 the winding changeover topology.

562
In [76], a generic winding arrangement for an n-phase 563 PM machine with double-layer FSCW has been elaborated 564 to omit low space harmonics. The reduction in low space 565 harmonics yields lower eddy current losses in the rotor core. 566 The proposed methodology has been applied to a 12-slot/ 567 10-pole PM with a combined star-delta stator winding and a 568 20-slot/18-pole machine with a combined star/pentagon sta-569 tor winding. As an illustrative example, FIGURE 26 depicts 570 the winding layout and connection of the 20-slot/18-pole 571 machine. In that case, one five-phase winding set is con-572 nected as a pentagon, while the other one lies between the 573 pentagon connection terminals and the inverter. The proposed 574 technique is simple and employs dual n-phase windings to 575       MMF sub-harmonics and some high-order ones since the 593 number of slots per phase is two which yields only odd 594 order harmonics in the single-phase winding function. Unlike 595 three-phase configuration, the proposed nine-phase winding 596 layout cancels out various odd order harmonics, e.g., 1 st , 5 th , 597 and 13 th harmonics. However, the fundamental 7 th harmonic 598 is barely increased owing to the higher winding distribution 599 factor. The proposed MMF harmonic suppression technique 600 offers several advantages, including low stator and rotor 601 iron losses, low torque ripple, and high reluctance torque. 602 Moreover, it can be applied to FSCW slot/pole combinations 603 except when the slot number is 1.5 times the pole number.

604
In addition, a technique for improving the torque perfor-605 mances of a dual three-phase PMSM has been presented 606 in [85] based on CHI. Unlike third harmonic injection, the 607 proposed method enhances the torque capability by the fifth 608 and seventh harmonics injection without hardware reconfig-609 uration, extra power switching bridge, and current sensors. 610 FIGURE 28 shows the configuration of the dual three-phase 611 PMSM. This paper implements the fifth and seventh current 612 harmonics injection based on the vector space decomposition 613 (VSD) control. Accordingly, the fifth and seventh harmon-614 ics are injected into the stator currents in the xy subspace. 615 On the other hand, the increase in the voltage harmon-616 ics may exceed the output voltage limit and thus decrease 617 the DC bus utilization, a notable limitation of this study. 618 Eventually, an 8.6% improvement in the developed torque 619 is obtained, albeit with a slight rise in the torque ripple 620 harmonic. 621 VOLUME 10, 2022 In [87], a five-phase PM machine with a single set of half-667 coiled winding and an eccentric rotor has been discussed, 668 offering a better fault-tolerance capability and simpler con-669 struction. FIGURE 24 presents the five-phase half-coiled 670 winding configuration. The half-coiled winding is the wind-671 ing with coils distributed under a half pole and provides 672 simultaneous odd and even harmonics needed by the bear-673 ingless motors. Accordingly, the proposed winding layout 674 eliminates the multiples of the 5 th harmonic. Moreover, the 675 forward-rotating MMF harmonics, e.g., the 1 st and 2 nd MMF 676 harmonics, are responsible for the torque production and sus-677 pension force. While, the two backward-rotating ones, e.g., 678 the 3 rd and 4 th MMF harmonics, produce the pulsating torque 679 and extra suspension force. It can be noted that the magnitude 680 of each space harmonic is highly affected by the employed 681 coil pitch. Therefore, the coil pitch is adjusted to minimize 682 the 3 rd and 4 th harmonics. Finally, the machine inductance, 683 which is crucial for suspension force, is derived based on the 684 modified winding function. The proposed modified winding 685 function can be expanded to IMs and reluctance machines.

686
Furthermore, a novel six-phase 78-slot/24-pole surface-687 mounted permanent magnet (SPM) machine with an uncon-688 ventional stator winding layout has been introduced in [88]. 689 Two identical three-phase winding groups are distributed in 690 the two stator halves with 180 mechanical degrees, δ = 0, 691 as shown in FIGURE 30(a). Therefore, the phase resistance 692 is reduced, and thus the copper losses. Another unconven-693 tional six-phase 78-slot/12-pole has been presented in [89], 694 at which the machine windings are distributed along the 695 stator circumference, δ = 27.7 • as shown in FIGURE 30(b). 696 The 24-pole machine offers 28.3% lower copper loss when 697 compared to the 12-pole one because it considerably reduces 698 the phase resistance. However, the MMF total harmonic dis-699 tortion for the 12-pole machine is lower than the 24-pole one 700 with 30.8% and 36.2%, respectively. The main benefits of 701 these unconventional winding configurations -besides low 702 cogging torque -are better torque quality, lower end-winding 703 resistance, simple rotor manufacturing, and low torque ripple 704 with no rotor skewing.

705
Moreover, an unconventional asymmetrical nine-phase 706 SPM machine had been developed in [90] to mitigate the 707 drawbacks of the conventional asymmetrical nine-phase 708 machines. The detailed machine mathematical model has 709   cogging torque when compared to its conventional nine-718 phase counterparts.

719
A recent winding reconfiguration technique from asym-720 metrical to symmetrical, and vice versa, by rearranging the 721 inverter leads in the machine's terminal box has been elabo-722 rated in [91]. First, the number of phases determines whether 723 the rearrangement is possible or not. The winding reconfigu-724 ration is only viable for a composite odd number of phases, 725 i.e., number of phases n ≥ 9. Then, similar rules can be 726 followed if an asymmetrical winding layout is converted to a 727 symmetrical one or vice versa. Eventually, the proposed tech-728 nique can be efficiently used for both IMs and PM machines. 729 VOLUME 10, 2022 FIGURE 32. Nine-phase winding reconfiguration from asymmetrical to symmetrical. Eventually, two combined winding layouts for bearingless 740 motors; namely, the multiphase (MP) and dual-purpose no-741 voltage (DPNV) configurations, have been thoroughly com-742 pared in [92]. Both the MP and DPNV winding layouts can 743 provide decoupled motor and suspension force operations, 744 as shown in FIGURE 33. The former has two balanced groups 745 of currents with different phase shifts, i.e., one set to gener-746 ate torque and the other one to produce force. Whereas the 747 latter employs two torque and suspension inverters which are 748 connected to provide no back electromotive force (EMF) at 749 the suspension terminals. For a fair comparison, analogous 750 force and torque models, space vector equivalent circuits, 751   The concept of harmonic suppression in FSCW machines 762 by stator shifting has been the topic of a significant body 763 of literature [93], [94]. Basically, the number of slots is 764 doubled for the same pole number. Hence, the number of 765 coils is doubled, while the electrical frequency is main-766 tained. The generated windings are overlapped fractional 767 slot windings with two-slot coil span. Accordingly, various 768 slot/pole combinations have been addressed in electrified 769 transportation systems, such as 24-slot/10-pole [95], [96], 770 24-slot/14-pole [94], [97], 18-slot/10-pole [98], and 18-slot/ 771 8-pole [99], [100]. These winding configurations yield a 772 substantial decrease in the dominant slot harmonic and thus 773 the induced rotor losses [93], [101]. As an illustrative exam-774 ple, the 18-slot/10-pole and 24-slot/10-pole winding schemes 775 are given in FIGURE 34 and FIGURE 35, respectively. 776 Amongst available winding layouts, the asymmetrical nine-777 phase scheme is only viable for the 18-slot/10-pole machine. 778 Based on the MMF spectra, the slot harmonic is cancelled, 779 e.g., the 7 th harmonic in the 24-slot/10-pole machine and 4 th 780 harmonic in the 18-slot/10-pole machine [70]. It is worth 781 mentioning that both single-and double-layer designs are 782 viable; however, the latter is better in terms of the efficiency 783 and torque density.

784
In [102], dual three-phase SPM machines with vari-785 ous slot/pole combinations, where the spatial phase angle 786 between the two winding sets is 30 • , have been investigated 787 based on the stator shifting concept under healthy and three-788 phase fault conditions. This paper presented a comprehen-789 sive comparison of 24-slot/10-pole and 24-slot/14-pole SPM 790 machines with a coil span of two-slot pitch, as shown in 791 FIGURE 36. From a torque perspective, the 14-pole machine 792 gives slightly higher torque production when compared to 793 the 10-pole machine under healthy conditions. Both winding 794   Table 1, the summary of multiphase winding 807 layouts for PM machines is revealed in Table 2.    Eventually, multiphase machines are recently being 831 introduced as potential candidates for oil and gas pump 832 applications [106]