In the calculation of the temperature rise of the motor, the most important thing is to calculate the temperature rise of the winding (armature winding, field winding, etc.) and the core. These components are themselves hot and the heat source is distributed, so that their temperatures are generally non-uniform, and thus there is a maximum temperature or maximum temperature rise and an average temperature or average temperature rise. Although the degree of heat generation of the motor should be measured by the highest temperature (the hottest temperature), since the hottest temperature is quite difficult and inaccurate to calculate, usually only the average temperature rise is calculated in the thermal calculation. There is a certain relationship between the average temperature rise and the highest temperature rise. The average temperature rise can also be used to measure the heating of the motor.
In order to have some conceptual understanding of the heating of the motor, the temperature distribution of the armature winding and the core is outlined below. The temperature of the armature windings is unevenly distributed in the axial direction, but is generally considered to be uniform in the radial direction. In a deeper motor, such as a large steam turbine generator, the temperature difference of the copper wire along the groove height reaches about 10 °C. Below, Ms. participates in several typical cases where the winding temperature is distributed along the axial direction.
1Axis distribution of winding temperature when using a symmetrical radial ventilation system
When the air volume through each radial air passage is substantially the same, the highest temperature of copper and iron rises in the middle of the motor. A portion of the heat consumed by the copper in the middle of the armature winding is transmitted to the cooling air through the core or the air passage, and the other portion is conducted along the copper wire to both ends and is transmitted from the winding end to the cooling air. Therefore, the heat dissipation at the end is significant in the cooling of the winding in a motor having a short effective length.
In the structural design, in order to reduce the highest temperature rise in the middle of the motor, the length of the core lamination stack in the middle is usually arranged shorter than the two ends, which is equivalent to adding ventilation to the middle of the motor and increasing the heat dissipating surface.
2 axial distribution of winding temperature when using axial ventilation system or shaft-radial ventilation system
In general, in asymmetrical ventilation systems at both ends, the location at which the highest temperature occurs is often in the direction of the hot air outlet.
3 axial distribution of the armature winding temperature of the enclosed motor
In a surface-cooled closed motor, the heat loss in the winding is mostly dissipated through the outer circumference of the core and the base. If the end heat dissipation condition is not good, the heat loss at the end portion is also partially dissipated through the core. At this time, the temperature of the winding (copper wire) is high at both ends and low in the middle. As for the temperature distribution in the core, since the thermal conductivity of the silicon steel sheet laminated group in the radial direction is many times larger than that in the axial direction, it can be approximated that the temperature of the lamination core is uniformly distributed in the radial direction, and the temperature is not uniform. The uniform distribution is mainly in the axial direction.
If the air flow through the radial air passages on both sides is different, the temperature distribution of the core in the axial direction will also be asymmetrical. In fact, the temperature variation of the core in the radial direction is not always small and negligible. For example, in a turbine generator and a closed (outer surface cooling) induction motor, the radial temperature difference between the stator cores can reach 20~. Around 30 °C. In the case of a closed induction motor, the maximum temperature of the core often occurs near the crown of the stator.