STUDY ON ROTOR STRUCTURE WITH DIFFERENTMAGNET ASSEMBLY IN HIGH-SPEED SENSORLESSBRUSHLESS DC MOTORS

K. Wang M.J. Jin J.X. Shen H. HaoCollege of Electrical Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China241~248

老師 :王明賢學生 :方偉晋

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.

OUTLINE Introduction High-speed motor configurations Enhancement of third-harmonic back-EMF Experimental verifications

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.

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 configurationsa Using one magnet per poleb Using two magnet segments per pole

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.

ENHANCEMENT OF THIRD-HARMONIC BACK-EMF

Figure 2 Field distributions in two-pole six-slot high-speedmotors with different pole-arc to pole-pitch ratio apa ap =1b ap =0.9c ap =0.7d ap =0.5

ENHANCEMENT OF THIRD-HARMONIC BACK-EMF

Figure 3 Comparison of FEM and analytical predictions ofairgap field distribution

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 magnetpole-arc

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/N rads, 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.

ENHANCEMENT OF THIRD-HARMONIC BACK-EMF

Figure 5 FEM prediction of airgap field distribution withdifferent number of magnet segments per pole (N)

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 thirdharmonicairgap field with number of segments per pole (N)

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.

EXPERIMENTAL VERIFICATIONS

Figure 7 Comparison of FEM predicted and analyticallycalculated phase back-EMFs at 120 krpma Structure of Fig. 2a with ap ?0.9b Structure of Fig. 2b

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.