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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.

The safety and reliable operation of power grid is directly related to the ability of power transformer to withstand short-circuit, therefore, it is a problem to be solved to improve the ability of large power transformer windings to withstand short-circuit. Taking a three-phase five-limb power transformer as an example, the transient electromagnetic field, short-circuit electrodynamics force of windings and mechanical strength of coils are analyzed in depth. Firstly, the three-dimensional finite element model of the prototype is established, and the magnetic flux density distribution of the three-dimensional transient electromagnetic field of transformer under short-circuit operation and the axial and radial static force magnitude of the winding are calculated by using the field-circuit coupling method, and the distribution law can be obtained. At the same time, the mechanical strength of power transformer winding in its height direction is discussed, and the modal vibration mathematical model of transformer low-voltage winding in Z-axis direction is established. The displacement change and resonance frequency of the winding wire cake in the axial direction caused by short-circuit are calculated, and the short-circuit electrodynamics force of the winding is also checked. The research in this paper provides a theoretical basis for strengthening the design of short-circuit withstanding capacity of windings, and has a certain theoretical and engineering application value.

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.

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