IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 4, APRIL 2010 1123
Optimal Driving Efficiency Design for the Single-PhaseBrushless DC Fan Motor
Chun-Lung Chiu�, Yie-Tone Chen�, You-Len Liang�, and Ruey-Hsun Liang�
Graduate School of Engineering Science & Technology, National Yunlin University of Science and Technology,Yunlin 640, Taiwan
Department of Electrical Engineering, National Yunlin University of Science and Technology, Yunlin 640, Taiwan
One of the methods to improve efficiency and torque performance of the single-phase brushless dc (BLDC) motor is to find out theoptimum commutation angle at each different speed. We used the finite-element method (FEM) to simulate the back-EMF voltage andthe coil current for the single-phase BLDC motor, and then adjust the conduction time of switches by detecting the waveform of coilcurrent. The motor can improve its efficiency, noise, and vibration when it obtains the optimal shift angle of each speed. We used PSPICEto verify the exactness of FEM simulation results of the single-phase BLDC motor. We adopted Microchip’s dsPIC30F4011 digital signalprocessor (DSP) to process the Hall signal and the driving signals of switches of the driving system prototype of the single-phase BLDCfan motor. Finally, we used the related experimental results to confirm the feasibility and effectiveness of the proposed driver.
Index Terms—Finite-element method (FEM), optimum commutation angle, single-phase brushless dc motor.
I. INTRODUCTION
C OMPARED with an induction machine, a brushless dc(BLDC) motor has a higher efficiency. The BLDC motor
also has lower maintenance and higher speed bandwidth than adc brush motor because it uses the electrical commutating de-vices to replace the mechanical commutator and brush gear ofa dc brush motor [1], [2]. Therefore, in low-cost and low-powerfan applications, the single-phase brushless dc (BLDC) motor,which is less expensive and easier to fabricate, is widely used[3]–[11].
The single-phase BLDC motor is widely used in many appli-cations. An important market of BLDC motor is the applicationof an air-cooling fan. The air-cooling fan is normally used inmany different occasions to cool the system such as the server,storage, power supply, heat exchanger, and so on. In these ap-plications, high efficiency and low vibration are required. Amethod to improve the efficiency and vibration of the singlephase BLDC motor is to change the commutation angle whileit rotates at each different speed. The commutation angle is animportant factor to affect the characteristic of the single phaseBLDC motor, especially for use in wide speed range. This is be-cause the BLDC motor belongs to an inductive load. Therefore,its current lags behind voltage. The lag angle will change withthe variation of speed, so the exact shift degree at the differentspeed is very important. Because the BLDC motor has an ade-quate shift angle, the overlap quantity of current and back-EMFvoltage will be increased and the output power will then be in-creased. Therefore, the relative efficiency will become high too.Moreover, if the angle is shifted correctly, the current will alsobe relatively smooth. It makes the possible damage of the semi-conductor switch be reduced too. Until now, the so-called phase
Manuscript received July 03, 2009; revised October 04, 2009; accepted Oc-tober 06, 2009. First published November 03, 2009; current version publishedMarch 19, 2010. Corresponding author: Y.-T. Chen (e-mail: [email protected]).
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMAG.2009.2035051
Fig. 1. Structure of a four-pole external rotor of the single-phase BLDC motor.
advance methods are proposed to come to the aim [4]–[7]. How-ever, for the phase advance methods, the use of the hardwarecircuit is not very accurate and also increases the cost and com-plexity. As for the conventional use of the software, it tries theoptimal angle in writing the program and this leads to the pro-gram to be often revised.
In this paper, the finite-element method (FEM) is first usedto simulate the optimal angle at every speed and these optimalangles can be written in the software program in advance.And PSPICE is also used to further verify the exactness ofFEM simulation results of the single-phase BLDC motor. Bythe proposed method, the single-phase BLDC motor can becost-effective and get the phase advance effect easily. Finally,to verify the exactness and effectiveness of the optimal angles,Microchip’s dsPIC30F4011 digital signal processor (DSP)was used to process the Hall signal and the driving signals ofswitches in the laboratory prototype circuit.
II. BASIC ELECTRIC CIRCUIT ANALYSIS BY FEM
The single-phase BLDC motor with 4-pole external rotormotor is shown as Fig. 1. The flowing direction of the flux ofmagnetic pole is also illustrated in Fig. 1. When the definingposition of the motor in this paper is 0 , the structure in its
1124 IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 4, APRIL 2010
Fig. 2. Winding plan of a four-pole external rotor of the single-phase BLDCmotor.
Fig. 3. External driving circuit of H-bridge.
entirety is shown in Fig. 1. The definition of rotating direction ispositive as the motor rotates in the counterclockwise direction.
The eight regions “PA1 MB2” construct the winding inFig. 1. The way to wind the windings is illustrated in Fig. 2. InFig. 1, the regions are marked as both symbols of a point anda fork for expressing the flowing direction of the windingcurrent. In Fig. 2, the defining direction for the winding cur-rent flowing is positive as the current flows in the directions ofarrows. In Fig. 1, the symbols are distributed among the eightregions according as the winding current flowing in Fig. 2.
However, the BLDC motor needs an external driving circuitto drive the motor. The external driving circuit used in this paperis shown in Fig. 3. Basically, it is an H-bridge converter con-sisting of four semiconductor switches ( , , , and ) andfour diodes ( , , , and ). The function of switches is toconverter dc source to ac source and the function of diodes is toprovide the path of energy release while the switches are turnedoff. As for the capacitor C, it is to absorb the released energyback from the motor and is to prevent the released energyback to disturb the input. To obtain the actual situations of themotor operation, the motor’s FEM model needs to couple withthe above external driving circuit in FEM. Due to this reason,the external driving circuit also needs to be built in the FEMsoftware.
The control signals for the switches in the external circuitneed to cooperate with FEM software likewise. The detaileddimensions of the motor and the corresponding process havebeen described in [10]. The detail parameters of the motor that issimulated and implemented in this paper are as shown in Table I.
The signals for the switches are controlled according to theposition of the rotor in the operation of BLDC motor. The re-lationship between the control signals for the switches and theposition of the rotor is illustrated in Fig. 4. The switches S1 and
TABLE IMOTOR PARAMETERS
Fig. 4. Controlling signals for switches as a function of the rotor electricalangle. (a) Original switching angle; (b) switching signals of phase advance; and(c) switching signals of phase lag.
Fig. 5. Equivalent circuit of a single-phase BLDC motor.
S4 conduct at the electrical angle 0 and cut off at the electricalangle 175 . The switches S2 and S3 conduct at the electricalangle 180 and cut off at the electrical angle 355 . As for thesituation of phase advance, the switching angle is shifted nega-tively from the original situation. The switching angle is shiftedpositively for the case of phase lag.
III. OPTIMAL ANALYSIS OF MOTOR CURRENT
Because the motor belongs to an inductive load, so the cur-rent lags behind the voltage when the motor speeds up. The lag
CHIU et al.: OPTIMAL DRIVING EFFICIENCY DESIGN FOR SINGLE-PHASE BLDC FAN MOTOR 1125
Fig. 6. Smooth current waveform.
Fig. 7. Not enough phase shift. (a) Current waveform and (b) waveforms ofmotor current and back-EMF.
amount of the angle will change with the variation of the speed.The lag angle in the BLDC motor can be illustrated and esti-mated with the following equivalent circuit as shown in Fig. 5
(1)
Fig. 8. Too much phase shift. (a) Current waveform and (b) waveforms ofmotor current and back-EMF.
From (1)
(2)
Therefore
(3)
(4)
(5)
Here, and . When the speed is increased,the frequency will be increased also. Once the frequency is in-creased, and will be larger. Because is increased, theangle of current can be seen to be far from the angle of thevoltage. The conclusion is while the speed is increased more, thelag amount of angle between the current and back-EMF voltagewill become larger.
From the formulas of (3)–(5), it can be known the is theneeded shift angle because the current and voltage will not be inthe same phase while the motor runs at different speed. There-fore, to make the overlap range of motor current and back-EMFvoltage to be as large as possible, the switch signals must beshifted in advance adequately. If the shift angle is more accu-rate, the current of motor can be found to be smoother. There-fore, whether it is the optimal current waveform or not can bedetermined based on how smooth the current waveform is. Fig. 6
1126 IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 4, APRIL 2010
Fig. 9. Optimal phase shift angle. (a) Current waveform and (b) waveforms ofmotor current and back-EMF.
explains the smooth current waveform that this paper defines. Inthe picture, the current of the single-phase BLDC motor has twopeak values at point A and point B. It is the basis that the valuescorresponding to point A and point B are as close as possible.It is defined that the error between them does not exceed 10%here. However, we make the further explanation with the cur-rent waveform of 4200 rpm. In Fig. 7, it shows the case thatthe shift angle is not enough. Fig. 7(a) shows the current wave-form of the single-phase BLDC motor while its shift angle isnot enough. The error between point A and point B is larger. Sothere is higher surge in the commutation, the coil loss and elec-tromagnetic loss is higher. In Fig. 7(b), it shows the current rel-atively lags behind the back-EMF voltage. Therefore, the motorefficiency drops because the back-EMF voltage and the motorcurrent are not in the same phase.
In Fig. 8, it shows the degree of shift angle is too much.Fig. 8(a) shows the current waveform while its shift angle istoo much. The error between point A and point B is also larger.Therefore, the waveform is also not smooth in the commutation.In Fig. 8(b), it shows the waveform of the current leads rela-tively the back-EMF. The motor efficiency also drops becausethe back-EMF and the motor current are not in the same phase.
In Fig. 9, it shows the shift angle and commutation almosthappen at the same time. Fig. 9(a) shows the current wave-form while its shift angle is in accuracy. The values at pointA and point B are almost the same. So the current waveform issmoother.
Fig. 10. Relationship between the speed and the shift angle.
Fig. 11. Comparison of current consumption with three different kinds of phaseshift angles.
In Fig. 9(b), the waveform of the current is nearly totally over-lapped with the back-EMF voltage. Because the output poweris that the current multiplied by the back-EMF voltage, the ef-ficiency for the case in Fig. 9 will be better than the cases inFig. 7 and Fig. 8. So, the optimal current waveform that thispaper chooses is smoother.
Based on the above description, the point A and point B inthe current waveform not only can observe the relationship ofthe back-EMF voltage and motor current but also can define theappropriate shift angle.
From Figs. 7, 8, and 9, it can be identified while the point Ais higher than point B, the back-EMF voltage lags behind thecurrent of the motor and while the point A is lower than pointB, the current of the motor lags behind the back-EMF voltage.Therefore, it is easy to know the motor’s condition from thecurrent waveform.
In Fig. 10, it shows the relationship between the speed and theoptimal phase shift angle with FEM and PSPICE simulation re-sult. The shift angle is nearly zero at low speed. It is because theangle difference between back-EMF voltage and current of themotor is not obvious at low speed. In the PSPICE simulation,the back-EMF voltage information adopts from the FEM sim-ulation result. About the phase shift, we adjust the conduction
CHIU et al.: OPTIMAL DRIVING EFFICIENCY DESIGN FOR SINGLE-PHASE BLDC FAN MOTOR 1127
Fig. 12. Current waveforms of left shifted unit� � 0.5, 1, and 1.5 (3600 rpm).
Fig. 13. Smooth current waveforms for � � 0.5 and 1.5 (3600 rpm). (a) Shiftangle of 3 �� � ���� and (b) shift angle of 1 �� � ����.
time of the switches to get the result. In Fig. 10, it discovers al-most the same simulation angle with FEM and PSPICE models.
Fig. 11 shows while the input voltage is constant at 12 V dc,the relationship between the speed and the current with three dif-ferent kinds of phase shift. One is the optimal phase shift angle,others are the angles that lag or lead the optimal angle. The an-gles that lag or lead the optimal angle are the 5 electrical angle,
Fig. 14. Complete system integration.
Fig. 15. Practical experimental circuit.
respectively, at each different speed. The result can confirm thatthe lower current can be obtained by the optimal angle.
IV. ANALYSIS OF ASYMMETRIC STATOR STRUCTURE
The single-phase BLDC motor has the dead point where theexcitation net torque is zero, so it will be unable to start up if the
1128 IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 4, APRIL 2010
Fig. 16. Simulation and measured current waveforms at different speeds. (a) 1800 rpm; (b) 2400 rpm; (c) 3000 rpm; (d) 3600 rpm; (e) 4200 rpm; (f) 4800 rpm;(g) 5400 rpm; and (h) 6000 rpm.
motor stops at the dead point. The asymmetric air gap is thus de-signed so that the starting point of the motor can avoid the dead
position, but the asymmetric air gap deteriorates the motor char-acteristics in the cogging torque and electrical circuits. To solve
CHIU et al.: OPTIMAL DRIVING EFFICIENCY DESIGN FOR SINGLE-PHASE BLDC FAN MOTOR 1129
Fig. 17. Noise test at 1800 rpm. (a) Without phase shift and (b) with phase shift.
the starting problem of a single-phase BLDC motor, the statorstructure is often designed to be asymmetric [8]–[10]. One ofthe important factors that affect the optimum phase angle of thesingle-phase BLDC motor is the asymmetric air gap. When theasymmetricairgapis larger, theoptimumphaseangle isrequestedto be smaller relatively; when the asymmetric air gap is smaller,the optimum phase angle needs to be larger. This is because in theasymmetric air-gap design, the motor’s characteristic is changed,so the optimum phase angle is changed also. In this section, theaffection of asymmetric stator structure on the phase shift angleis discussed. Due to the asymmetric structure, the back-EMF ofthe motor is changed. So because its current is different, the cog-ging torque and the starting torque are also different. In Fig. 12,it shows the current waveform at the speed of 3600 rpm with theshift angle of 2 . The asymmetric structure is defined accordingto the left horizontally shifted unit X [10]. From Fig. 12, the affec-tionofasymmetric stator structureon thecurrentcanbeseen. Thecurrent at 0.5 and 1.5 is not smooth. However, by changingthe shift angle to 3 for and changing the shift angle to1 for 1.5 as shown in Fig. 13, it can be seen their current be-come smoother. The similar conclusion can also be obtained forthe speed of 5400 rpm. The adequate phase shift angle is changedto 13 for and changed to 11 for . Therefore,when the motor runs at 3600 rpm and the asymmetric unit X ischanged from 0.5 to 1.5, the variation range of the phase shiftangle is about 2 due to FEM simulation.
V. EXPERIMENTAL RESULT
The FEM model is used to simulate the current waveformwith the adequate phase shift in this paper. To confirm the ac-curacy of FEM simulation results, a laboratory prototype circuit
Fig. 18. Relationship between speed and noise.
Fig. 19. Vibration test with and without the phase shift. (a) Vibration withoutthe phase shift (5400 rpm) and (b) vibration with the phase shift (5400 rpm).
was built to verify. The current can be seen whether it is smoothor not after the simulated phase shift angle is introduced in theswitching signals. Then the noise and vibration tests are con-ducted to see whether the performance is improved or not afterthe introduction of adequate phase shift. The simulation resultis totally confirmed by actual experiments to make sure the ac-curacy and feasibility.
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Fig. 20. Relationship between vibration and speed.
Fig. 21. Efficiency comparison.
Fig. 14 shows the completed driving system of the singlephase BLDC fan motor. Digital signal processor (DSP), thedsPIC30F4011 produced by Microchip, was adopted to processthe Hall signal and the driving signals of switches of the drivingsystem prototype of the single-phase BLDC fan motor. The ex-perimental prototype circuit is shown in Fig. 15. The load ofFig. 15 is using 120 mm square and 38 mm thickness fan whichis usually used in the applications of sever and switching powersupply for the cooling system.
Fig. 16 shows the results of simulation and actual mea-sured current waveform at speeds of 1800, 2400, 3000,3600, 4200, 4800, 5400, and 6000 rpm. By comparing the sim-ulation waveform with the measured result, one can find thetwo waveforms are almost the same. It verifies the accuracy ofsimulation.
The driving system of fan motor is also used to make the noisetest. Although the results at many speeds are tested, Fig. 17 onlyshows the result of noise test at 1800 rpm and the completed re-sults are summarized in Fig. 18. In Fig. 18, it can be found thatthe noise level with phase shift (modify) is lower than withoutphase shift (standard) while the speed is lower than 3600 rpm;the noise after 3600 rpm has been almost the same level. Thisis because the sound of the wind cut after 3600 rpm has al-ready overlaid the noise level that this electronic product hasproduced. So the noise test in this paper is only to 4200 rpm.
The prototype circuit is also used to make the vibration test.Similarly, the results at many speeds are tested; Fig. 19 shows
the vibration test with and without the phase shift at 5400 rpmand the complete results are summarized in Fig. 20.
Fig. 20 shows the relationship between the vibration andspeed. When the shift angle that the paper simulates is adopted,the vibration can be found to become much lower.
Fig. 21 shows the comparison of the efficiency regarding thestandard way and the proposed method in this paper. The im-proved efficiency is around 10% in average, and normally thestandard model usually used in industry will have a constantlead phase angle, and this will affect the efficiency result of thestandard part.
VI. CONCLUSION
This paper uses the finite-element method (FEM) to simulatethe optimal shift angle of a single-phase BLDC motor. PSPICEis also used to verify the exactness of FEM simulation results.Microchip’s dsPIC30F4011 digital signal processor (DSP) andDelta FFB1212EHE fan was adopted to implement the systemprototype of the single-phase BLDC fan motor. The special de-sign point used in this paper is to only observe the smooth de-gree of the motor current that can be used to find out the optimalshift angle. Then, not only is the overlapping rate between thecoil current and back-EMF voltage high, but the efficiency isalso improved. The procedure for software writing is more con-venient; it is not necessary to spend a lot of time looking for theoptimal shift angle. The paper also analyzes noise and vibration.The single-phase BLDC motor has lower noise than the standardproduct. The vibration with the optimal phase shift as a wholeis lower in the applicable speed range.
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