Analysis of Air-Gap Field Modulation and Magnetic Gearing Effect in Fractional-Slot Concentrated-Winding Permanent-Magnet Synchronous Machines

Abstract

Abstract-In this paper, the torque production of fractional slot concentrated winding (FSCW) permanent magnet synchronous machines (PMSM) are analyzed from the perspective of the air-gap field harmonics modulation accounting for slotting effect. It is found that the average torque of FSCW PMSM is produced by both the principle of conventional PMSM and the magnetic gearing effect. A finite element analysis (FEA)-based equivalent current sheet model and harmonic restoration method is firstly used in FSCW PM machines with different slot-pole number combinations to quantify the respective contribution of the conventional PMSM and the magnetic gearing effect to the average torque. The influence of slot opening on the magnetic gearing effect, cogging torque and torque ripple is analyzed. The results show that the magnetic gearing effect makes a non-ignorable contribution to the average torque when a large slot opening stator is used. The expression of the gear ratio in FSCW PMSMs is derived. The influence of gear ratio on the contribution of the magnetic gearing effect to the total torque is investigated by FEA. The FEA predicted torques are validated by experiments on the prototypes.

Influence of Gear Ratio on the Performance of Fractional Slot Concentrated Winding Permanent Magnet Machines

“…1, where r it the mechanical speed of the rotor. From [20], useful torque can be generated under the following circumstances.…”

“…However, the (Ns-pr)th harmonic can be modulated by the Ns-pole stator teeth, which results in the space harmonic with the order of pr and the speed of r. The modulated harmonic can interact with the prth PM harmonic to produce a steady torque as shown in Fig 1 (b). This phenomenon is called magnetic gearing effect and it has been proven in [20] that it can contribute up to 30% to the total torque in FSCW PMSMs.…”

“…As mentioned above, these two harmonics are critical harmonics in FSCW PMSMs since they always exist in pairs, have larger amplitude than any other harmonics and are highly involved in the torque production, flux weakening operation and losses generation. Their behavior has decisive influence on the related machine performance, and therefore, Gr, as a function of their orders can not only reflect the magnetic gearing effect in FSCW PMSM as shown in [20] but also be used as an index for the performance comparison between FSCW PMSMs with different slot/pole number combinations. The variation of the performance can be shown directly with respect to Gr.…”

“…Recently, the magnetic gearing effect [18]/modulation effect [19] in FSCW PMSMs was revealed [20]. The parameter used to represent the gearing effect in FSCW PMSMs is defined as the gear ratio, which is a function of the slot and pole numbers.…”

“…The parameter used to represent the gearing effect in FSCW PMSMs is defined as the gear ratio, which is a function of the slot and pole numbers. [19] and [20] focus on revealing the magnetic gearing/flux modulation effect as the torque production mechanism in different machine types. In this paper, the influence of gear ratio on different machine performance metrics will be investigated for machines with different slot/pole number combinations and particularly different phases.…”