Thermal Magnetic Analysis And Efficiency Optimization Design Of Three-Phase Squirrel-Cage Asynchronous Motor

Improving motor efficiency is an inevitable trend of our country's motor development and at the same time motor life and operating reliability must be guaranteed. Motor operating temperature has a direct and obvious effect to motor life and operating reliability. To improve motor efficiency and studying motor temperature field has an important engineering significance.According to electromagnetic field theory, a field-circuit coupled model of three-phase squirrel-cage asynchronous motor was set up and transient electromagnetic field of the motor was computed. Motor's operating characteristics were obtained and total magnetic field and current density change of rotor bar were analyzed in starting progress. It is found that magnetic lines distribute in stator and rotor evenly and current density of rotor bar also distributes evenly when the motor steadily operates. Magnetic lines are mainly focused in the thin layer close to rotor surface. Current density of rotor bar gradually becomes lager along the radial direction and the skin effect is obvious when starting.A model for three-dimensional transient temperature field of induction motor was set up and its temperature field is computed under rated load, temperature field with different loads is also computed; and some influence factors were analyzed. It is shown that the temperature of stator and rotor increases rapidly in the beginning, and then the rising trend slows down until reaching the stable state. In the whole process, the rotor temperature is higher than the stator temperature all the time, and the maximum temperature point is around the axial center of the rotor bar. The axial temperature difference of the rotor core is larger than that of the stator core while the radial temperature difference of the stator core is larger than that of the rotor core. The temperature of air gap decreases linearly with the increase of thickness, and the thermal insulation effect of air gap is so strong that it makes the temperature difference of stator and rotor obvious. The motor temperature grows with the increase of load. There is no linear relation between load and temperature. The higher the wings of shell, the lower motor temperature is, whereas the descend tendency slows down; Using cast copper instead of cast aluminum as rotor bars and end rings can reduce the temperature of rotor effectively.Motor structure was improved for increasing its efficiency and the impact of these measurements to motor operating characteristics was analyzed. Motor efficiency was optimized by Genetic Algorithm and Pattern Search Algorithm in the final. It is shown that increasing wire diameter can reduce stator copper loss and increase efficiency effectively within a limited coil space factor. Reducing end adjustment of stator windings and gap thickness can increase motor efficiency, reducing gap thickness can increase power factor also but will increase locked-rotor current ratio. Increasing core length properly can raise motor efficiency and power factor, but it will reduce locked-rotor torque ratio and breakdown torque ratio. Motor efficiency rises further through the optimization of Genetic Algorithm and Pattern Search Algorithm.