Research On Temperature Field Of High Power Traction Motors In High Speed Railways

High-speed railway, as the integration of multiple cutting edged techniques in contemporary era, has been developing at an alarming rate from all over the world for its characteristics of speediness, high efficiency and environment protection. High-speed Electric Motor train Unit (EMU) plays a core role in the rail transportation system which has great influence on the safety of passengers as well as the electrical equipment. With the rapid development of China’s high-speed railway, the output power of traction motor increases significantly. Due to the limited space of the bogie as well as the maximum output voltage of converter switching device, the input voltage of traction motor cannot be further boosted. Moreover, the operating experience indicates that when the electrical insulation system is strengthened by one level, the output power of motor will correspondingly increase by10%to15%. So it’s advisable to boost the output power of every single traction motor by increasing the winding current. However, the large current will inevitably cause huge loss in the motor which results in high temperature rise of each mechanical part (sometimes even reach up to200℃and above). In addition, in order to make full use of magnetic materials, the motor often designed at the point with heavy electromagnetic load. So the loss of the motor increases dramatically which leads to extra heat generated in the motor. Therefore, it has become a hot issue to limit the temperature rise by improving motor cooling system.The field-circuit coupled model of inverter-fed traction motor was established based on electromagnetic field theory. The output characteristics of the traction inverter using the two level, three-level and current tracking control were analysed. The two-dimensional transient electromagnetic field of the motor was calculation.lt was demonstrated that output voltage did not contain low harmonic spectrum using two-level control, while under three-level control model, the high harmonics was significantly reduced. Current tracking control provided motor current a good dynamic response. Electromagnetic field was mainly concentrated in a thin layer near the surface of the rotor when the motor started. The flux density was relatively high in the stator and rotor tooth. When the motor operated stably, the magnetic field lines were evenly distributed within the stator and rotor core. Due to big starting current, the rotor winding current density along the radial direction became larger.The global temperature field model of inverter-fed traction motor was set up based on the heat transfer theory. Static three-dimensional temperature field and thermal stress distribution of the motor were calculated at rated load. The influence factors of temperature field were analysed. It was demonstrated that the highest temperature was located at the rotor bars when the motor temperature was stable. There was less different of temperature between rotor core and bars. The overall temperature of the stator region was below that of the rotor region. Stator slot temperature was higher than that of stator core. The temperature of stator surface was the lowest. The maximum heat flux was located in the stator core yoke while the flux density of stator teeth was relatively low. The maximum thermal stress was located in the contact surface between the end of the winding and the stator slot. Thermal coefficient has a negative correlation relationship with the maximum temperature of the stator.The convection coefficient of the stator core surface was the most important factor which impacted the maximum temperature of the stator. Convection coefficient of the ventilation vents played the key role in the maximum heat flux of the stator.The influence of traction motor temperature distribution on the motor insulation was studied. Insulation material of inverter-fed taction motor winding-polyimide film was set as specimen frequency traction motor. The failure time of the insulating material was tested under the continuous square pulse. The influence of combined thermal and electrical stresses on the failure time was studied. It demonstrated exponential relationship between the aging temperature and insulation life. Also there was inverse power relationship between frequency and life. At last, multifactor life models under combined thermal and electrical stresses were proposed using the multi-stress approach. All the determination coefficients were above0.98which verified its accuracy.