4.1. The Effect of Single Ventilation Path on the Fluid Field of High-Voltage Line-Start Solid Rotor Permanent Magnet Synchronous Motor (HVLSSR-PMSM)
The air-cooled hybrid ventilation systems of HVLSSR-PMSM with single ventilation path is shown in
Figure 5. The single ventilation path is composed of two cooling loops: the out cooling loop and the inner cooling loop, as shown in
Figure 5. In the out loop, the air enters into the cooler from the out loop air inlet. Then the air that pushed by the centrifugal fan goes into the aluminum tubes, by which the hot air in the inner loop is cooled. Finally, the warm air flows into the outside from out loop air outlet. Whereas, for the inner cooling loop, the cooling air in the inner loop flows into the motor from the inlet forced by the axial fan, and all the air passes the air gap, the stator radial ducts, and the axial ducts located at stator back yoke. The heat generated within the motor components can be taken away by the cooling air, and then the temperature of the cooling air will arise; finally, the heat of high temperature cooling air will be transferred to the air in the out cooling loop.
In inner loop, the warm air in the outlet flows through the cooler and then flows back into the motor inlet, and the cycle repeats.
According to the ventilation system structure of HVLSSR-PMSM with single ventilation path, the inner loop wind resistance equivalent model of the motor including the axial fan and the cooler is established, and then the air-flow equivalent network of the motor is solved. The boundary conditions of the inlet and outlet of the motor are determined.
The boundary conditions of the calculation model of HVLSSR-PMSM with single ventilation path are as follows:
- (1)
The inlet boundary conditions for a given mass flow inlet of the motor, whose mass flow is 0.075 kg/s.
- (2)
The boundary conditions of the given pressure of the outlet of the motor, of which the ambient pressure is 1 atm.
Based on the theories of the computational fluid dynamics, the finite volume method is adopted to calculate the whole region 3D fluid-thermal coupled fields, both the fluid and the temperature distributions inside the HVLSSR-PMSM are obtained by using established 3D fluid thermal coupled model. According to the experimental environment, the ambient temperature is set as 16 °C. Considering the structure of prototype, the cross-section A-B-C-D section in
Figure 4 is emphasized, and the fluid distribution in HVLSSR-PMSM with single ventilation path is shown in
Figure 6.
The fluid velocity in
Figure 6 is described as the composed vector of the axial velocity, circumferential velocity, and the radial velocity. As seen from
Figure 5, all the cooling air enters the air gap, so that the wind velocity in the air gap is increased. The internal flow maximum velocity of the motor is 17.2 m/s, which appears in the air gap corresponding to No. 4 stator radial ventilation duct.
In order to facilitate analysis, we assumed that the direction of fluid flowing from the stator teeth to the stator back yoke belongs to the out-wind zone and those with fluid moving from the stator back yoke to the stator teeth as the in-wind zone.
By viewing cross-section A-B-C-D in
Figure 6, No. 1, No. 2, No. 3, and No. 4 stator radial ventilation ducts pertain to the out-wind zone, and No. 5 and No. 6 stator radial ventilation ducts are within the in-wind zone.
The exchange of the flow and the heat between air gap and stator yoke back axial ventilation duct is realized by stator radial ventilation ducts. Meanwhile, the stator radial ventilation ducts are also the main paths of the stator windings heat dissipation. The velocity and the temperature of the fluid inside the stator radial ventilation ducts have a great influence on the temperature distribution of the stator windings. To estimate their thermal effects, the radial flowing velocity of fluid of the stator radial ventilation ducts are investigated. The average velocity of air in all circumference stator radial ventilation ducts at the same radial positions is analyzed, and the changing trend of the radial flowing velocity of fluid of the stator radial ventilation ducts are shown in
Figure 7.
Figure 7a shows the air flowing into the stator radial ventilation ducts from the air gap. The fluid velocity is larger and it changes obvious along radial direction of the machine because of the smaller ventilation area of the stator teeth. The maximum flow velocity locates at the top of the stator tooth. As the air goes further to the stator tooth root, the fluid velocity is almost linearly decreasing due to the increasing area of the ventilation ducts. While the cooling air goes through the stator yoke, the flowing velocity reduces rapidly.
The maximum velocity of flow in No. 1 stator radial ventilation duct is 1.81 m/s. The velocity of flow decline to 1.3 m/s at the stator tooth root, and the velocity of flow at the yoke back is the minimum, which is 0.13 m/s. The trends of fluid in No. 2, No. 3, and No. 4 stator radial ventilation ducts are basically the same as that in No. 1 stator radial ventilation duct, but obviously different in the velocity values. The maximum velocity of flow in No. 2, No. 3, and No. 4 stator radial ventilation duct are 1.58 m/s, 1.31 m/s, and 0.72 m/s, respectively.
From
Figure 7b, in the out-wind zone, the velocity of flow at the yoke back inlet is the lowest in No. 6 stator radial ventilation duct, which is 0.08 m/s, and the velocity of flow rise to 2.39 m/s at the tooth root. The velocity of flow in stator tooth region along radial direction increases dramatically, and the maximum velocity reaches 4.31 m/s. The velocity of flow distribution of air in No. 5 stator radial ventilation duct is similar to that air in No. 6 stator radial ventilation duct, in which the lowest velocity of flow in the yoke back is 0.015 m/s, and the velocity of flow in tooth root region is 0.39 m/s. The maximum velocity of flow in No. 5 stator radial ventilation duct is 0.76, which is located near the wedge.
4.2. The Effect of Single Ventilation Path on the Temperature Field of HVLSSR-PMSM
The variation of the velocity of flow in the stator radial ventilation duct will directly affect the temperature distribution of each component in the motor. By solving the 3D fluid thermal coupled calculation model, the 3D temperature distribution of the components in the motor with single ventilation path are obtained, as shown in
Figure 8.
In
Figure 8, the maximum temperature difference of HVLSSR-PMSM with single ventilation path is 100 °C. The lowest temperature in the motor is 180 °C, which locates at the end windings that near the fan end. The temperature of each component gradually increases from the fan end to the shaft end in the axial direction. The highest temperature in the stator core is 270 °C, and the highest temperature in the stator winding is 280 °C, but it is 230 °C for rotor core and permanent magnet.
In this paper, the permanent magnet used for the machine is NdFeB (N35EH160), which has a maximum temperature of 200 °C. The magnetic properties and demagnetization curves are illustrated in
Figure 9. The highest temperature of the permanent magnet in HVLSSR-PMSM with single ventilation path is 230 °C, which has exceeded the maximum temperature for utilization. The highest temperature could affect machine performance and even led to the thermal demagnetization for permanent magnet.
By testing the prototype of HVLSSR-PMSM with single ventilation path, the temperature rises of stator and rotor in the motor are higher than the insulation material. Due to the large internal wind resistance, the wind pressure provided by the axial flow fan cannot supply sufficient cooling air flowing velocity. Meanwhile, the air amount inside the motor is less and the cooling channel design is unreasonable, so that the air cooling system cannot effectively reduce the temperature rise in motor.
Therefore, the air-cooled ventilation systems with single ventilation path is optimized to a dual ventilation path.