Three-phase short circuit (3PSC) is a critical condition that may cause irreversible demagnetization of surface-mounted permanent magnet synchronous motor (SPMSM). In the literature, 3PSC current was often calculated under constant rotor speed condition. However, due to the disturbance of current and electromagnetic torque during 3PSC, the rotor speed usually changes, which influences 3PSC current and electromagnetic torque in turn. This article investigates the 3PSC responses and demagnetization of SPMSM under typical types of loads, such as gravitational lift torque (GLT) load, dry fiction torque (DFT), viscous friction torque (VFT), torque square of speed (TSS), and constant power (CP) load etc. It shows that different load types can lead to different 3PSC responses and demagnetization. Moreover, since the d-axis short circuit (SC) current is more detrimental to demagnetization, this article focuses on the influence of different load characteristics on the d-axis peak SC current. The investigation shows that the d-axis peak current under GLT load can be larger or smaller than that under constant speed load, whereas the 3PSC currents under DFT, VFT, TSS, and CP loads are all smaller than that under constant speed load. In addition, the influence of initial q-axis current, temperature, and machine dimension on the d-axis peak SC current is also investigated. According to the analysis results, some guidelines are given for the demagnetization check when designing an SPMSM. Furthermore, experiments are conducted to validate the theoretical analysis.

Demagnetization is one of the major concerns in PM machines, especially in surface-mounted PM (SPM) machines. Three-phase short circuit (3PSC) is one of critical faults that may cause demagnetization and thus 3PSC current should be reduced. Since key factors influencing 3PSC current, such as PM flux linkage, resistance and inductance, heavily depend on dimensional parameters, 3PSC current can be reduced by properly selecting them. In addition, dimensional parameters also influence armature field paths and thus demagnetization can be further mitigated by proper design of dimensional parameters. This article first investigates the influence of dimensional parameters on 3PSC current and demagnetization in an internal rotor concentrated-winding SPM machine. The results show that although larger airgap length reduces 3PSC current, demagnetization deteriorates. Moreover, 3PSC current increases and demagnetization aggravates when pole-arc to pole-pitch ratio, slot opening width and rotor yoke height increase or tooth tip height decreases. 3PSC current first increases and then decreases when split ratio and tooth width increase, which makes demagnetization first get worse and then eased. Then the influence of PM temperature, rotor speed, external rotor and winding configuration is investigated. Finally, experiments are implemented to validate the theoretical analysis.

This article delves into the post-demagnetization torque ripple of fractional-slot surface-mounted PM wind power generators after short circuit faults. Firstly, demagnetizing magnetomotive force from three-phase short circuit (3PSC) currents is modelled and demagnetization distribution pattern is analyzed. Then, this article investigates the post-demagnetization cogging torque. It is shown that the period of cogging torque becomes 360 N_p / N_s electrical degrees, where N_s and N_p are the slot and pole pair numbers of unit generator, respectively. In addition, the amplitude increases when the overall demagnetization level increases with 3PSC rotor speed. Following this, this article analyzes the post-demagnetization back-EMF and load torque ripple. It is found that the demagnetization adds low-order harmonics in the back-EMF and leads to additional torque ripple component under the same stator current, which also increases with demagnetization level. Although the harmonic torque ripple offsets the cogging torque, it has smaller amplitude and thus the total torque ripple still increases with demagnetization level. Most importantly, it is also found that the demagnetization distribution pattern heavily depends on the 3PSC start rotor position. Specifically, when 3PSC starts at the rotor position kπ /3 ( k = 0, 1, 2…), the demagnetization distribution is the most uneven in different PMs, which results in the largest post-demagnetization load torque ripple and the smallest average torque. Conversely, the opposite condition occurs around the 3PSC start rotor position π /6+ kπ /3. In addition, the influence of 3PSC and single-phase short circuit are compared. Finally, the theoretical analysis is verified by experimental evidence.

This article investigates the influence of start rotor position on three-phase short-circuit (3PSC) current in dual three-phase surface-mounted permanent magnet (DTPSPM) machines. For DTPSPM machines with symmetrical winding configurations, the 3PSC analytical model considering PM flux linkage harmonics is developed. It shows that the variations of 3PSC peak current with start rotor position mainly repeat every multiple of π/3 in an electrical period due to the fifth and seventh harmonics. The start rotor positions causing the largest 3PSC peak currents heavily depend on the harmonic magnitudes and phases. Then, the 3PSC analytical model for DTPSPM machines with asymmetric mutual inductances in one winding group is developed. It shows that the variations of 3PSC peak current with start rotor position mainly repeat every multiple of π. For winding configurations with smaller M_ac , M_bc , and M_ab , start rotor positions causing the largest d -axis peak current are about π/6+ k π, π/2+ k π, and 5π/6+ k π ( k = 1, 2, 3…), respectively. To obtain the largest q -axis peak current, such start rotor positions are about 7π/24+ k π, 5π/8+ k π, and 23π/24+ k π ( k = 1, 2, 3…). In addition, the influence of rotor speed and asymmetric level is discussed. Finally, finite-element simulation is performed and experiments are conducted to validate the theoretical analysis.

Dual three-phase permanent magnet (DTPPM) machines are suitable for applications with high power density and high reliability requirements. However, uneven demagnetization fault (UDF) can occur due to harsh working conditions. This article proposes a non-invasive, online detection method of UDF in DTPPM machines based on the electrical signal difference between two sets of three-phase windings, which can be more conducive to UDF diagnosis with higher sensitivity. Firstly, the influence of dual three-phase winding arrangements on phase permanent magnet (PM) flux linkage is investigated in DTPPM machines under UDF. It shows that the fault characteristic harmonics appear in fractional-slot DTPPM machines and integral-slot DTPPM machines where a single set of three-phase windings is rotationally asymmetric. Further, according to the three-phase relationship of harmonic vectors, it is analyzed whether the fault characteristic harmonics are reflected in stator currents or zero-sequence voltage components (ZSVCs). Notably, taking advantage of the structural feature in DTPPM machines, the proposed method obtains higher sensitivity of UDF detection, as the amplitudes of fault characteristic harmonics in electrical signal difference are larger than that in stator current or ZSVC in a single set of three-phase windings. Finally, experiments are conducted to validate the effectiveness of the proposed method.

This article investigates postdemagnetization unbalanced magnetic force (UMF) in single-three-phase permanent magnet (STPPM) and dual-three-phase PM (DTPPM) machines considering three-phase short circuit (3PSC). First, when eccentricities are not considered, for rotationally symmetrical STPPM machines and DTPPM machines with each winding group being rotationally symmetrical, demagnetization distribution is symmetrical. Therefore, there is still no UMF after demagnetization. For rotationally asymmetric STPPM machines and DTPPM machines with each winding group being rotationally asymmetric, the first-order harmonic is added to the UMF $x$ and $y$ components. Then, the influence of eccentricity on 3PSC current and demagnetization is analyzed, and postdemagnetization UMF for STPPM machines with eccentricities is investigated. It shows that eccentricities almost have no influence on 3PSC currents but gather demagnetization around PMs facing larger airgap length. As a result, under static eccentricity, the original UMF components decrease, while the first-order harmonic is added after demagnetization. Under rotating eccentricity, the original UMF components decrease in asymmetric STPPM machines, whereas they increase in symmetrical ones. The postdemagnetization UMF for DTPPM machines considering eccentricities is similar to that for symmetrical STPPM machines. In addition, the variations of UMF harmonics increase with the eccentricity ratio. Finally, experiments are conducted to validate the theoretical analysis.