Six-Phase Electric Machines

In this chapter, some specific theoretical problems of six-phase electrical machines were analyzed. It was evaluated theoretically which harmonics or rotating magnetomotive force and of what type may emerge in the six-phase symmetric electric current systems. It was proven that the harmonics that are multiples of three as well as all remaining even harmonics of rotating magnetomotive force cannot appear in such systems, contrary to the three-phase current systems.

Further, in order to supply power to six-phase motors, it is recommended to use a step-down voltage transformer that changes the number of phases; in such transformer, at the terminals of the secondary six-phase winding, a guaranteed symmetric six-phase voltage system is obtained, where alternation of voltages is the closest to the sinusoidal law. It was graphically shown, that in the symmetric six-phase system, contrary to the three-phase system, the phase and line voltages are of an equal magnitude. Six possible variants of connecting both positive and negative voltage phase sequences to the terminals of the motor were presented in the tables.

In this chapter, the general aspects of the six-phase windings were also analyzed, their main equations were presented, and symmetric spatial arrangement of the starting and ending points of these windings was determined graphically.

In the last subsection of this chapter, the evaluation of electromagnetic properties of six-phase windings based on two methods is presented. The first method of investigation is partial, and it is based on the calculation of the winding factors for the fundamental and higher-order harmonics. The second method of research is more comprehensive but also more difficult. It is based on harmonic analysis of instantaneous spatial distributions of rotating magnetomotive force generated by six-phase windings. On the basis of the results of this analysis, the electromagnetic efficiency factors of such windings are calculated.

In this chapter, the design of the electrical circuit diagrams for the single-layer symmetric concentrated (q = 1), preformed, concentric, and chain (q = 2 and q = 3) six-phase windings is presented. For the designed windings, the investigation of their electromagnetic properties was accomplished using two methods discussed in the first chapter. The results of the research of electromagnetic parameters of the discussed windings were compared with the corresponding results of analogous three-phase windings, and this comparison clearly indicates a huge advantage of the single-layer six-phase windings against three-phase windings.

In this chapter, the design of electrical circuit diagrams for the two-layer symmetric preformed, full average pitch, and short average pitch concentric (q = 2 and q = 3) six-phase windings is presented. For the designed windings, the investigation of their electromagnetic properties was accomplished using two methods discussed in the first chapter. The results of the research of electromagnetic parameters of the discussed windings were compared with the corresponding results of analogous three-phase windings, and this comparison clearly indicates a notable advantage of the two-layer six-phase windings compared to three-phase windings.

In this chapter, the design of electrical circuit diagrams for the two-layer symmetric fractional-slot (q = 1/2, q = 3/2, and q = 5/2) six-phase windings is presented. For the designed windings, the investigation of their electromagnetic properties was accomplished using two methods discussed in the first chapter. The results of the research of electromagnetic parameters of the discussed windings were compared with the corresponding results of analogous three-phase windings, and this comparison indicates a clear advantage of the two-layer fractional-slot six-phase windings compared to three-phase windings.

In this chapter, the theoretical and experimental investigation was performed using two-pole 2.2 kW three-phase induction motor. Also, the same motor was analyzed after replacing its single-layer preformed winding (q = 4) with the single-layer preformed six-phase winding (q = 2) selected on the basis of conducted design calculations. In both scenarios, the windings of this motor with different numbers of phases induced a two-pole rotating magnetic field with completely identical electromagnetic parameters (k_ef1 = k_ef2 = 0.914). When performing experimental tests of the motors under different loads, the three-phase motor was supplied with the nominal voltage of 230 V, and the six-phase motor was supplied using lowered six-phase symmetric voltage of 130 V. Based on the results of experimental tests, the calculation of operating characteristics using segregated-losses method was completed. These characteristics of the three-phase and six-phase motors were compared graphically.