With the every-increasing demands for miniaturization, the efficient cooling techniques for axial-flux permanent magnet synchronous machines (AF-PMSMs) have attracted great attentions. In this paper, the thermofluid behaviors of a 7-kW 4000-rpm AF-PMSM are numerically investigated based on a coupled model, and the computations are validated by thermal tests performed on the AF-PMSM prototype. To enhance the heat dissipation capacity of the rotor components, it is proposed a self-circulated oil hermetically cooling system, for which the cooling liquid circulates in a hollow shaft. During operation, the oil flow driven by the centrifugal blades forms a closed oil circuit, and can create an effective heat flow path from the hot areas to the cold regions. The oil cooling system is employed in the AF-PMSM to effectively maintain, and evenly distribute, the motor temperature rise. The cooling effectiveness is proved by fluid flow and thermal coupled analyses. To further improve the heat dissipation capacity, the self-circulated oil-cooling structure is modified based on the calculated fluid flow distribution. Finally, the structural parameters are optimized by Taguchi method to minimize both the temperature rise and the rotor weight increment, and the optimization results are numerically and experimentally validated.

This paper presents a modified thermal model-based double-loop circulatory technique for the thermal analyses of permanent magnet synchronous machines. The inner loop of the circulation procedure is employed to guarantee the energy conservation of the air enclosed in the endcaps and modify the temperature-dependent heat convection coefficients, while the outer loop is used to correct the heat conductivity coefficients and the electromagnetic losses that change with temperatures. The thermal model based on finite formulation method is modified to simplify the modification process of the outer loop. The temperature estimation techniques are then applied to analyze the temperature distribution of a permanent magnet motor with a spirally mounted water-cooling system. A novel ventilation structure is established to maintain the temperature rise of the rotor parts. The analyses based on the proposed methodologies are validated by experiments.

To avoid overheating of a totally enclosed high-speed permanent magnet synchronous machine (PMSM) with an amorphous alloy core, this paper proposes a hybrid cooling system with both radial and axial vents to maintain the temperature rise below the rated value. The analytical models of cooling ability and frictional loss generated in the rotor ducts are derived in relation to the cooling structure parameters. The sensitivity of each parameter to the cooling effect is researched, and the parameter scopes are then determined. A coupled fluidic-thermal model based on the cell method is developed to predict numerically the temperature distribution to check the effectiveness of the cooling system. By analyzing the influences of the numbers and sizes of the cooling ducts on the efficient cooling air quantity and temperature, the feasible parameters that yield reasonable temperature distribution can be determined. The theoretical results are confirmed by experimental test results on a 15-kW 30 000-r/min PMSM prototype.

Short-circuit faults are inevitable in split-winding transformers with stabilizing windings, and the resulting transient electromagnetic force may cause detrimental damages to the equipment. This paper focuses on a comprehensive analysis of the characteristics of the split-winding transformer with stabilizing windings under different short-circuit faults. In this regard, a FEM based on field-circuit coupled approach is proposed. And a SFFZ10-88000 kVA split-winding transformer with stabilizing windings is used as a prototype to investigate its transient performances with both full-crossing and half-crossing conditions under different short-circuit faults. The symmetrical component method is presented to compute short-circuit currents to compared with the simulation ones, and a prototype test model is established to verify the correctness of the proposed method. The results reveal that the axial forces exerted on the winding in half-crossing short-circuit faults are generally larger than those in full-crossing short-circuit faults. Moreover, there is a considerable short-circuit force in the stabilizing winding in cases of a single-phase earthed fault and a two-phase earthed fault and there is no current in the stabilizing winding under other short-circuit fault cases. The numerical modeling approach dealt with in this paper is expected to be useful in the design of the split-winding transformer with stabilizing windings.

A 30-kV direct current vacuum circuit breaker with double-break (DB-DC VCB) in series was taken into consideration; the interrupting current and the transient recovery voltage (TRV) under both synchronous and asynchronous interrupting conditions were measured, and the arcing process during the interrupting was captured by a high-speed camera. The continuous transition model (CTM) was used to simulate the postarc dielectric recovery characteristic during the asynchronous interrupting. The experimental and simulation results show that when one of the breaks in the DB-DC VCB is delayed under 5-kA current interrupting due to the dispersion performance of the mechanism, the two breaks will exhibit a significant difference in the arc characteristic and the TRV. The delayed break has higher arc energy, higher concentration of vaporized metal particles (VMPs), and lower postarc dielectric recovery capacity. The different break gap widths during the arcing process led to the arc characteristic difference. When the delay exceeds a critical value (0.5 ms), the VMPs between the break for the delayed break are difficult to dissipate and easy to reignite at the breakdown weak point.

Driving motors are one of the most critical components in modern electrified transportation systems, deciding directly the system's working reliability and controllability. To extend the life-cycle of the driving motors while operating them with enough torque output, the temperature rise maintenance techniques have attracted extensive research attention. This paper presents a novel stator-casing fitting approach by filling grease materials into the assembly gap between the dovetail-groove-fitted stator core and motor shell of PMSMs. The grease-filled dovetail-groove-fitted structure can reduce effectively the thermal resistance between the machine's internal components and the external coolants, and can also relieve the stress-caused magnetic performance regressions of the electrical steel sheets laminating the stator core. In addition, due to both the thermal conduction enhancement and the stress-buffering effect, the deformation occurring in stator cores can also be reduced, which is helpful to guarantee the machine's precision. Therefore, the electromagnetic, thermal, and stress-deformation performances in the PMSM can all be improved by using grease buffers filled between stator cores and motor casings. To validate the electromagnetic-thermal-stress efforts of the buffers on the prototype together with the simulation methodologies, a 44-kW 6000-rpm line-started PMSM is taken as the reference machine, and a multi-physical coupled model of the problem is numerically calculated. The influences of the grease properties on the machine's working performances and temperature rises are investigated through comparative analyses. Finally, the numerical calculation results are compared with experimental ones to validate their effectiveness.