Research Of Electromagnetic And Thermal Coupling For Compact Induction Motors With High Efficiency And Voltage In Starting Process

With the improvement of the technologies on motor field, the motors withhigh efficiency and high power density become the development trend. TheYJKK series motors are new products in China’s motor industry. The center highof YJKK series motors is two levels lower than YKK series motors. The reducedmotor volume makes the cooling area smaller. The increased power density andvoltage levels make the starting characteristics and temperature rise of this seriesmotors particularly concerned. When the motor start-up with a heavy load, thestarting current would make the winding temperature rises rapidly under the longstarting time. The motor may be failure to start if the load is heavier. Stator androtor resistance and starting current in the starting process are important forestimating the temperature rise of windings. Motor starting characteristic curves(current-speed, and torque-speed) are the key indicators for designers to adjust orimprove the design scheme and for users to select reasonable starting equipmentto avoid the starting failure.It is difficult to measure the transient current, torque and impedance forhigh-voltage motors because of the facility limitation. The requirement toaccurately determine the starting characteristics and impedances remains strongwith the development of high-voltage motors, so a practical and feasiblecalculation method is necessary. The advantages and disadvantages of currentcalculation methods of induction motor starting characteristics, impedance andtemperature rise are analyzed and summarized, and the relationship betweenmotor electromagnetic and thermal transition process are deeply studied. Acalculation method of motor dynamic parameters, starting characteristics andtemperature rise conveniented for engineering application is proposed. The saturation and skin effect during motor starting process are studied inthis paper. The motor saturation and flux leakage are calculated by magneticnetwork method according to the distribution of magnetic flux in the motor in thedynamic process.The stator yoke of high voltage motor is longer and saturation isserious when the number of the motor poles is less. The magnetic slot wedges areoften used in high voltage motors in order to improve the flux density distribution.So the yoke saturation and the influence of magnetic slot wedge must be takeninto account when the magnetic network model is established. In order toimprove the accuracy of the calculation, two magnetic network models oftooth-facing-slot and tooth-facing-tooth are established. The leakage flux iscalculated according to the magnetic circuit laws. The multi-layer method isproposed to analyze motor skin effect, and the calculation process of skincoefficients is presented.Motor starting characteristics must be analyzed by dynamic theories becausethe motor start-up is a transient process. In order to reduce the amount ofcalculation, the state equations of induction motor at two-phase coordinate arederived according to the theory of coordinate and vector transformation.Combined with the motion equations of rotation system, the calculation methodof starting current and electromagnetic torque are given, the formulas are simpleand easy to calculate.The motor windage network is built and solved to obtain the flowdistribution of cooling gas in the motor. The heat generation and dissipation arecalculated combined with motor starting characteristics, and the change curve ofwinding average temperature in the starting process is obtained. The locked-rotorsafety running time is calculated according to the limited value of windingtemperature rise. The influence of different start load on the motor temperature isanalyzed.The coupling method is used to calculate the motor impedance, current andtemperature rise because of their interaction during the starting process, whichimprove the accuracy of the motor starting process calculation. The accuracy ofthe calculation method is verified by experiment. The research results provide areference for the new series motors design and safe operation, which has highpractical value.