Comparison pole-slot combinations: of : 8-pole 9-slot, 8-pole 18-slot, 10-pole 12-slot, and 10-pole 24-slot

Content Summary:

This paper primarily compares the performance of four types of interior permanent magnet synchronous motors (IPMSM) with different pole-slot combinations: 8-pole 9-slot, 8-pole 18-slot, 10-pole 12-slot, and 10-pole 24-slot under the same constraints. The conclusion drawn is that the 10-pole 24-slot motor has the smallest torque ripple and the highest torque density.

Constraints:

• Same rotor dimensions
• Same number of turns per phase
• Same amount of permanent magnet material
• Same level of saturation in the stator teeth and yoke

Four Motor Models

Figure 1: Motor models

No-load Back Electromotive Force (EMF) Analysis

Figure 2: Comparison of no-load back EMF

Table 1: Harmonic comparison of no-load back EMF

From Figure 2 and Table 1, we can observe the following:
(a) When the number of poles is the same, the back EMF distortion rate is lower when the winding span y = 2 compared to y = 1.
(b) Motors with 10 poles have a lower back EMF distortion rate than those with 8 poles.

For (a), this is because the fractional-slot concentrated winding has a larger distribution factor, and doubling the number of stator slots can reduce the harmonic distribution factor.
For (b), when the winding span is 5/6 pole pitch (represented by stator slot number Z/Q), it can simultaneously weaken the 5th and 7th harmonics. The 5/6 pole pitch of a 10-pole 12-slot motor is an integer, which results in a lower distortion rate of the no-load back EMF.

Cogging Torque Comparison

Figure 3: Cogging torque waveform

Table 2: Cogging torque comparison of the four pole-slot combinations

From Figure 3 and Table 2, we can see that the cogging torque of the 8-pole 9-slot motor is the smallest. The size of cogging torque is related to the M value, where M = LCM/GCD (GCD is the greatest common divisor of the pole and slot numbers, and LCM is the least common multiple of the pole and slot numbers).

Output Torque Comparison

Figure 4: Output torque waveform

Table 3: Comparison of output torque

From Figure 4 and Table 3, it is evident that when the number of poles is the same, the output torque of motors with winding span y = 2 is larger than those with y = 1. Also, with an increase in the number of poles, the thickness of the yoke decreases, reducing the material usage of the motor. The power density of the 10-pole 24-slot motor is higher than that of the 8-pole 18-slot motor.

Conclusion

When the pole-slot ratio is a multiple of 5/6, the 5th and 7th harmonic components of the back EMF are reduced, and the waveform of the back EMF becomes closer to a sine wave. On this basis, doubling the number of slots can reduce the harmonic amplitude in the air gap flux density under no-load conditions, improving the output torque of the motor and reducing torque ripple.