High-speed motor rotor structure and design characteristics (stator design, different types of rotor structure design)

High speed motor rotor has the characteristics of small size, high power density, direct connection with high speed load, eliminating the traditional mechanical acceleration device, reducing system noise and improving system transmission efficiency, etc. It has broad application prospects in the fields of high-speed grinding machines, air circulation refrigeration systems, energy storage flywheels, fuel cells, high-speed centrifugal compressors for natural gas transmission and distributed power generation systems as power supply equipment for aircraft or ships, and has become one of the research hotspots in the international electrical field.

The main characteristics of high-speed motor are high rotor speed, high stator winding current and magnetic flux frequency in the core, high power density and loss density. These characteristics determine that the key technology and design methods of high-speed motor are different from those of constant speed motor.

The rotor speed of the high-speed motor is usually higher than 10 000 r/min. When rotating at high speed, the conventional laminated rotor can not withstand the huge centrifugal force, so special high-strength laminated or solid rotor structure needs to be used. For permanent magnet motors, the problem of rotor strength is more prominent, because the sintered permanent magnet material can not withstand the tensile stress generated by the high-speed rotation of the rotor, and protection measures must be taken for the permanent magnet. The high speed friction between the rotor and the air gap causes much greater friction loss on the rotor surface than that of the constant speed motor, which brings great difficulty to the rotor heat dissipation. In order to ensure that the rotor has sufficient strength, the rotor of the high-speed motor is mostly slender, so compared with the constant speed motor, the possibility of the rotor system approaching the critical speed of the high-speed motor is greatly increased. Ordinary motor bearings can not operate reliably at high speed, and high-speed bearing systems must be used.

The high alternating frequency of winding current and magnetic flux in the core of high speed motor will produce large high frequency additional loss in the winding of motor, stator core and rotor. When the stator current frequency is low, the effect of skin effect and proximity effect on the winding loss can be ignored, but at high frequency, the stator winding will produce obvious skin effect and proximity effect, and increase the additional loss of the winding. The magnetic flux frequency in the stator core of high-speed motor is high, the influence of skin effect cannot be ignored, and the conventional calculation method will bring large errors. In order to accurately calculate the stator core loss of high-speed motor, it is necessary to explore the calculation model of iron loss under high frequency conditions. The space harmonics caused by the stator slot and winding non-sinusoidal distribution, as well as the current and time harmonics generated by PWM power supply, will produce large eddy current loss in the rotor. Due to the small size of the rotor and poor cooling conditions, it will bring great difficulties to the rotor heat dissipation. Therefore, the accurate calculation of rotor eddy current loss and the exploration of effective measures to reduce rotor eddy current loss will be discussed. It is of great significance to the reliable operation of high speed motor. At the same time, high-frequency voltage or current also brings challenges to the controller design of high-power high-speed motors.

The volume of the high-speed motor is much smaller than the constant speed motor of the same power, not only the power density and loss density is large, but also the heat dissipation is difficult, if the special heat dissipation measures are not used, the temperature rise of the motor will be too high, thereby shortening the life of the winding, especially for the permanent magnet motor, in the case of the rotor temperature rise is too high, the permanent magnet is prone to irreversible demagnetization. A well-designed cooling system can effectively reduce the temperature rise of the fixed rotor, which is the key to the long-term stable operation of high-power high-speed motors.

To sum up, there are many special key problems in rotor strength, rotor system dynamics, electromagnetic design, cooling system design and temperature rise calculation, high-speed bearing and controller development that are not available in conventional motors. Therefore, the design of high speed motor is a comprehensive design process of multiple iterations of physical fields such as electromagnetic field, rotor strength, rotor dynamics, fluid field and temperature field. At present, the main types of motors used in high-speed fields are induction motors, permanent magnet motors, switched reluctance motors and claw pole motors, and each motor type has a different topology.

This paper analyzes the development status of different types of high-speed motors at home and abroad, and summarizes the limit index of different types of high-speed motors. The structure and design characteristics of high-speed motor are analyzed in detail, including stator design, rotor structure design, rotor system dynamics analysis, bearing selection and cooling system design, etc. At last, the main problems facing the development of high-speed motor are analyzed, and the development trend and prospect of high-speed motor are prospected.