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K. Wang M.J. Jin J.X. Shen H. Hao

Study on rotor structure with different magnet assembly in high-speed sensorless brushless DC motors. K. Wang M.J. Jin J.X. Shen H. Hao College of Electrical Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China 241~248. 老師 : 王明賢 學生 : 方偉晋. Abstract.

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K. Wang M.J. Jin J.X. Shen H. Hao

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  1. Study on rotor structure with differentmagnet assembly in high-speed sensorlessbrushless DC motors K. Wang M.J. Jin J.X. Shen H. Hao College of Electrical Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China 241~248 老師:王明賢 學生:方偉晋

  2. Abstract • High-speed permanent magnet (PM) brushless DC motors have gained more and more interests for many applications. • The technique of detecting the electromotive force (EMF) zero-crossings is a common method in sensorless operation of PM brushless DC motors. • Based on these analyses, another method which can improve both the third harmonic and fundamental components in the airgap field, by segmenting the parallel-magnetised PMs, is employed and studied.

  3. Outline • Introduction • High-speed motor configurations • Enhancement of third-harmonic back-EMF • Experimental verifications

  4. Introduction • Sensorless operation is able to greatly strengthen the system reliability and diminish the performance variations caused by discrete rotor position sensors. Among various sensorless control techniques, the most common one is based on the detection of zero-crossings of the phase back-EMF. • The free-wheeling diode conduction has no influence on the method of detecting the third-harmonic back-EMF zero-crossings. Therefore, in this paper, the sensorless control using the third-harmonic EMF instead of the phase EMF will be studied at the motor design stage.

  5. High-speed motor configurations • For the above-mentioned three-phase PM brushless DC motor, two configurations with the same dimensions are studied, as given in Fig. 1. • A two-pole six-tooth stator with nonoverlapping windings is directly used, since in such a stator structure the third-harmonic winding factor is 1, which is beneficial to maximise the third-harmonic EMF. Figure 1 Motor configurations a Using one magnet per pole b Using two magnet segments per pole

  6. Enhancement of third-harmonic back-EMF • Fig. 2 shows the open-circuit magnetic field distributions in a two-pole six-slot motor, where the rotor has two magnets with the magnet pole-arc to pole-pitch ratio (ap) being 1.0, 0.9, 0.7 and 0.5, respectively. • Fig. 3 shows the airgap field distribution waveforms produced by the parallel-magnetised magnets with different ap. As can be seen, when the pole-arc to pole-pitch ratio (ap) decreases, the airgap field distribution waveform becomes farther away from sinusoidal waveform, containing more harmonics.

  7. Enhancement of third-harmonic back-EMF Figure 2 Field distributions in two-pole six-slot high-speed motors with different pole-arc to pole-pitch ratio ap a ap=1 b ap=0.9 c ap=0.7 d ap=0.5

  8. Enhancement of third-harmonic back-EMF Figure 3 Comparison of FEM and analytical predictions of airgap field distribution

  9. Enhancement of third-harmonic back-EMF • Fig. 4 shows the typical relationship between the airgap field harmonics and the magnet pole-arc to pole-pitch ratio (ap), which is obtained from finite-element analysis. Figure 4 Airgap field harmonics with different magnet pole-arc

  10. Enhancement of third-harmonic back-EMF • Influence of the number of magnet segments on the airgap field distribution is also investigated with FEM, as shown in Fig. 5, where the number N in the curve legend denotes the number of segments per pole. • It should be pointed out that the pole arc of each segment is p/Nrads, and the stator slot opening is neglected in the FEM analysis. It is seen that the number of segments has a significant effect on the airgap field waveform.

  11. Enhancement of third-harmonic back-EMF Figure 5 FEM prediction of airgap field distribution with different number of magnet segments per pole (N)

  12. Enhancement of third-harmonic back-EMF • From Fig. 6, it can be seen that the structure with two magnet segments per pole has the highest third harmonic in the airgap field, and even the fundamental component is higher than that with one segment per pole. This is beneficial to improve the motor performance. Figure 6 Variation of fundamental airgap field and thirdharmonic airgap field with number of segments per pole (N)

  13. Experimental verifications • From the comparison shown in Fig. 7, it is seen that the analytically calculated and FEM-predicted back- EMF waveforms are very similar. The third-harmonic back-EMF component must be high enough, usually over 20% of the fundamental, in order that the sensorless control can be realised.

  14. Experimental verifications Figure 7Comparison of FEM predicted and analytically calculated phase back-EMFs at 120 krpm a Structure of Fig. 2a with ap ?0.9 b Structure of Fig. 2b

  15. Experimental verifications • Ideally, it is preferable that a BLDC motor has a trapezoidal phase back-EMF and a square-wave phase current, such that a constant electromagnetic torque can be achieved. • However, in practical BLDC motors, especially in the high-speed ones, the phase current is usually not regulated, hence, is typically far away from the square-wave. This will certainly cause torque ripples.

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