Comparative Studies of Modular PMSMs with Symmetrical or Asymmetrical Six-Phase Windings

Abstract

Nowadays, multiphase permanent-magnet synchronous machines (PMSMs) equipped with fractional-slot concentrated windings (FSCWs) are increasingly attractive for the industrial applications due to their high torque density, high efficiency and high fault-tolerant capacity [1], [2]. Meanwhile, owing to their characteristics of easy manufacturing, convenient transportation and high fault-tolerant capacity, the modular permanent magnet synchronous machines (PMSMs) are also favored by various industrial applications, such as electric vehicle and wind turbine applications [3]. Fortunately, the FSCWs, especially the single-layer FSCWs, are inherently easy to modular manufacture. However, due to the manufacturing tolerance, the additional mechanical gaps between the modules are inevitable which will affect the magnetic field distribution and hence the electromagnetic performances. The influences of the additional mechanical gaps on electromagnetic performances of three-phase modular PMSM have been investigated [4]. Nevertheless, the influences of the additional gaps between the modules in a six-phase modular machine have not been covered. Moreover, the influences of the mechanical gaps on the performances under post-fault operating conditions in a six-phase PMSM have not been investigated in current literature. Therefore, in this paper, the influences of the additional mechanical gaps on the performance under healthy, faulty and post-fault operating conditions of modular PMSM with symmetrical or asymmetrical six-phase windings are investigated. In this paper, firstly, by analyzing the slot star diagram of a conventional 12-slot/14(10) -pole three-phase PMSM with double-layer FSCW, three different six-phase winding layouts can be obtained by dividing the conventional 12-slot/14(10) -pole three-phase winding into two sets of independent three-phase windings as shown in Fig. 1. It can be found that the winding of scheme I is asymmetrical six-phase winding with an electrical angle of 30° between the two sets of three-phase windings and the other two schemes are symmetrical six-phase winding with an electrical angle of 60° between the two sets of three-phase windings. Scheme III will be abandoned because the electromagnetic performances of scheme III are all the same with II while its magnetic isolation capacity is much lower than scheme II. To enhance their magnetic isolation capacity further, the 12 double-layer slots are divided into 24 single-layer slots so that three 24-slot/14(10)-pole six-phase PMSM with unequal teeth can be obtained. And, the modular stators are used to enhance their practicability and fault-tolerant capacity, as shown in Fig. 2. It can be seen that for scheme I, there is only one modular method—one module with one coil. On the other hand, there can be two different modular methods—one modular with one coil and one modular with one-phase (one phase possesses two adjacent coils). The different modular methods will introduce different additional mechanical gaps which cannot be avoided resulting from the manufacture limitations and tolerances, as shown in Fig. 2. The influences of these mechanical gaps on the electromagnetic performances, such as the winding factor, average torque, torque ripple and stator magnetomotive distribution, are fully investigated. Moreover, the influences of these gaps under faulty operating conditions, such as one-phase open-circuit, two-phase open-circuit and one-phase short-circuit failure, and under post-fault operating conditions are also investigated.