Rotor core auxiliary groove below permanent magnet
Although the motor shown in Figure 1 employs a surface-inserted bread-shaped permanent magnet, the radial field magnetomotive force in the air gap of the motor is still not sinusoidal. Moreover, the stator slot opening causes the air gap length to be unevenly distributed in the circumferential direction, thereby exacerbating the non-sinusoidality of the air gap magnetic density. These factors can cause the cogging torque of the motor and the torque ripple during the load operation. For the surface-inserted permanent magnet motor, the auxiliary groove is formed on the rotor core below the permanent magnet to change the equivalent air gap length, thereby changing the air gap magnetic density distribution, and thus it is expected to reduce the torque ripple.
3.1 rectangular auxiliary slot
As shown in FIG. 2, two rectangular grooves symmetric about the center line are formed under each magnetic pole of the rotor core of the motor, and the edges of the rectangular grooves are aligned with the edges of the permanent magnets. The width of the groove is set to l1 and the depth is h1. The torque performance of the motor can be changed with the size of the rectangular groove as shown in Fig. 3. It can be seen from the figure that as the rectangular groove depth h1 is appropriately increased, the motor torque ripple tends to decrease.
At the same time, when the groove depth is constant, the magnitude of the torque pulsation decreases first and then increases as the groove width increases, and the average torque obviously decreases as the groove width increases. It can be seen from the figure that the torque ripple is optimally 6.2% when l1=7mm, h1=4mm, but the average torque is reduced to 49.9Nm. Figure 4 shows the no-load air gap radial magnetic flux waveform of the reference prototype without the auxiliary slot and the motor with the above-mentioned optimal auxiliary slot. It can be seen that a suitable rectangular auxiliary groove is advantageous for reducing the air gap magnetic density harmonic component. Of course, opening the auxiliary groove will cause the equivalent air gap length to become larger, which inevitably causes the average torque to drop.
When four rectangular auxiliary slots symmetric about the center line as shown in FIG. 5 are used, five parameters of l1, h1, x1, l2, and h2 are optimized and analyzed. It can be seen from Fig. 6 that when the size of the groove is constant, the torque ripple of the motor becomes larger as the distance x1 between the two rectangular auxiliary grooves increases. And it can be seen that the performance of the motor is greatly affected by the auxiliary groove near the edge of the magnetic pole. The optimal result in the simulation is that when l1=7mm, h1=4mm, x1=0.5mm, l2=1mm, h2=2mm, the average motor torque is 49.6Nm, and the torque ripple is 5.5%. In contrast to the case where only a symmetrical single rectangular auxiliary groove is added, the reasonable addition of the inner auxiliary groove can further attenuate the torque ripple, but at the same time the average torque also decreases. A simple optimization method is to optimize the inner tank when the outer tank is optimized.
On the basis of the symmetrical four grooves, a pair of auxiliary grooves are opened on the inner side to form a symmetrical six rectangular auxiliary groove structure. Optimize analysis of slot position x2 and size l3, h3. For the sake of simplicity, l1 = 7 mm, h1 = 4 mm, x1 = 0.5 mm, l2 = 1 mm, and h2 = 2 mm are fixed in advance. The finite element calculation results show that the re-opening of the inner groove does not weaken the torque ripple. On the contrary, as the distance of the inner groove is increased, the performance of the motor also decreases. Therefore, the third pair of rectangular auxiliary grooves does not have much significance.
3.2 semicircular auxiliary slot
In order to study the effect of the semi-circular auxiliary groove on the torque of the surface-inserted permanent magnet motor, two semi-circular auxiliary grooves with respect to the center line are opened on the rotor core below the magnetic steel, as shown in Fig. 7, the position and The size can be constrained and optimized with l1, r1, and the result is shown in Figure 8. The torque ripple is a minimum of 4.9%, but the average torque drop is 49.3 Nm. It can be seen that the torque ripple first decreases and then increases as the groove radius becomes larger.
When the outer semicircular auxiliary groove is optimal, a pair of semicircular auxiliary grooves are opened on the inner side. Constraint optimization of the internal auxiliary slots by parameters x1 and r2. However, the finite element calculation shows that the opening of the internal auxiliary groove does not play a role in weakening the torque ripple, so it is not shown.
