Transient characteristics in the starting process play an important role in the design and operation of high-power-density and high-voltage induction motor. A field-circuit coupling model is proposed to analyze the transient fluid flow and temperature rise of a YJKK 500-4 2500 kW HPDHV induction motor in the starting process. The wind resistance network model is built to investigate the transient fluid flow and is used to obtain the heat dissipation boundary condition of the transient temperature calculation. The transient electric current is the key to the heat source of the temperature distribution and is calculated by the dynamic mathematical model. According to the obtained heat dissipation boundary condition and heat source, the 3-D fluid-solid coupling model is solved to obtain the transient temperature distribution. Moreover, the highest temperature rise in the starting process is greatly affected by the load. The simulation results show that the smaller fluid flow and the starting current make the winding temperature rise rapidly before the rotating speed of the motor reaches the rated value. When the load is much heavier, the starting time becomes longer and the winding temperature in the starting process will rise rapidly. The experimental results indicate that the proposed model is validated. INDEX TERMS Induction motor, temperature rise, fluid flow, starting characteristic.

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This article addresses the rotating effect on the pressure loss and heat transfer coefficient (HTC) of a hollow‐shaft rotor cooling system for an automotive traction motor. Firstly, a numerical model based on computational fluid dynamics (CFD) was established to fully understand the mechanism of rotational flow patterns. Experiments were then performed to measure and compare heat exchange and pressure loss between rotating and stationary conditions with the assistance of an analytical thermal model. Finally, trends of pressure drop and HTC at various rotational rates were explained in detail. Based on CFD simulations and experimental prototype testing, two opposite effects: boosting or diminishing on pressure drop and heat transfer due to rotation have been identified. Moreover, the rotation can significantly improve the overall HTC and pressure loss of an axial turbulent flow. As a result, a significant drop in terms of rotor temperature is identified for an automotive traction motor when such a rotor cooling system is implemented.

Summary A novel thermal network method is introduced to investigate the heat transfer characteristic of submersible motor in the paper. According to the special structure of motor and the related theories, the thermal network model is established. The equivalent thermal resistance in the model is calculated with improved empirical formula. And to improve the accuracy of the calculation, considering the influence of the manufacturing process on the temperature rise of the motor, the thermal‐contact resistance is introduced for the equivalent thermal resistance of different materials. The influence of oil flow in the inside and outside of the motor to the motor temperature rise is also fully considered, so Gauss‐Seidel method is used to realize temperature field and fluid field coupling. Each node temperature is solved by energy balance equation, and the second law of thermodynamics is introduced to analyze motor heat characteristic, so that it can improve motor performance by analyzing the entropy generation and the exergy destruction rate of motor. Finally, compared with the prototype temperature test, it shows that the calculated results are close to the test data. This method provides a certain basis for the motor heat transfer characteristic analysis.

The electromagnetic characteristics of the permanent magnet linear generator (PMLG), which is the key components of the free-piston engine (FPE), are studied in this paper. First of all, the velocity, displacement, and output voltage of the flat-type linear generator used in the experimental platform in the half-motion cycle are simulated by the model of 3D electromagnetic field and verified by experiment, and the relation curves of the output voltage to running velocity are found. Then, the PMLG is re-designed as a tubular type with two different structures of the permanent magnet on the steel backing of the mover: rectangular and the I-shaped. Third, the new PMLGs are modeled, and the models are used to do a simulation to investigate the influence of the structure of permanent magnet on the electromagnetic characteristics under no-load and load conditions. It is found that the fundamental wave amplitude of the flux density in the air gap of Ishaped PMLG is 0.93 T, greater than the rectangular one (0.87 T), and the proportion of the fundamental wave is higher. The waveform of the no-load induced electromotive force of the I-shaped PMLG is much better than that of the rectangular one, and the voltage total harmonic distortion (VTHD) is only 3.05%. The detent force analysis shows that the permanent magnet shape has a great influence on the detent force, and the peak value of detent force between I-shaped and rectangular PMLGs is significantly decreased from 25.4 to 7.75N. Moreover, the relation of the generator output power-velocity and efficiency-velocity under different loads is found. When the load is constant, with the velocity increases, the difference of output power between the I-shaped and rectangular structures becomes bigger. The optimum range of running velocity of the generator with the higher efficiency under different loads is presented. In conclusion, the I-shaped structure of the permanent magnet is a better choice for FPE.INDEX TERMS Free-piston engine, optimum range of running velocity, permanent magnet linear generator, structures of permanent magnet structure, voltage total harmonic distortion.