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A Novel Adaptive Commutation Angle Method forSingle Phase BLDC Motor

Jian Ni, Lijian Wu, Bo Zhang, Wanbing Jin, Jianping YingDELTA ELECTRONICS(SHANGHAI) CO., LTD.

No.238 Minxia Road, Pudong, Shanghai, 201209, China

Abstract- Single-phase BLDC motor is widely applied incooling fan, pump and blower. Due to high energy price,efficiency is gotten more attention. Commutation angle is one of key factors determining efficiency of BLDC motor. The optimalcommutation angle increases with speed increasing. Generallycommutation angle is adjusted by software, but the cost of suchsystem is high. In this paper, a novel low-cost adaptivecommutation angle method is proposed to improve theperformance of single phase BLDC motor. Analysis andsimulation verify its effectiveness. Experimental results show themotor efficiency using adaptive commutation angle is aboutaverage 4% higher than that without adaptive commutation

angle.

I. I NTRODUCTION

Single-phase BLDC motor is widely used in cooling fan, pump and blower for its simple structure, easy assembly andlow cost. A typical structure of the single-phase BLDCM isshown in Fig.1. The control system of traditional single phaseBLDC motor is shown in Fig.3. A position detector such asHall sensor is employed to detect the rotor position. Hall IC’s

position is shown in Fig.2. And the drive system switches thecurrent in the winding according to the position signal. The

interaction between two magnetic fields produced by stator winding and permanent magnet drives the rotor to rotate.

Commutation angle [1], which is an angle between voltagecommutation in motor windings and zero-crossing point of

back EMF, is one of key factors determining BLDC motor’s performance. Optimal commutation angle depends on windingvoltage, speed, load and winding inductance etc. Generallyoptimal commutation angle increases with speed increasing [2].

If a large commutation angle fit for high speed is set, thecommutation is advanced at low speed. It will bring on highefficiency at high speed but low efficiency at low speed.

Otherwise if a small commutation angle fit for low speed isset, the commutation is lagged at high speed. It will lead tohigh efficiency at low speed but low efficiency at high speed.

Based on above analysis, it’s important to auto-adjust motor commutation angle according to speed changing. Somemethods have been introduced such as adaptive commutationangle based on software. But the cost of such system is high.So developing low-cost adaptive commutation angle issignificant.

In this paper a novel adaptive commutation angle method based on RC circuit is developed. The operating principle isanalyzed in detail below. Using this method, system cost islow.

II. ADAPTIVE COMMUTATION A NGLE METHOD

For BLDC motor, regulating average voltage of motor winding ( Vm) means to tune the rotational speed. The generalmethod is to set an original commutation angle firstly. Anddelaying an angle makes commutation optimal. The delayangle is implemented by Delaytime in (1). The needed delaytime could be gotten by simulation or experiment. From(1), it is also founded that needed delaytime that makescommutation angle optimal decreases as speed increasing. In

DriveCircuit Hall

Signal

Vm 12V

ZD

Diode

TriodeTriode

Fig. 3. Control system

HallIC

Rotation

α

N S

Fig. 1. Cross-section of single phase BLDC motor

N

S S

N yokerotor

corestator

Fig.2. Expansion figure of Hall element position

Proceeding of International Conference on Electrical Machines and Systems 2007, Oct. 8~11, Seoul, Korea

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traditional adaptive commutation angle method, software isused to implement needed delaytime .

ω α α ×−= Delaytime Needed orig opt _ (1)

In this paper needed delaytime is implemented by hardware,not software. There exists a delay circuit between Hall signaland Drive circuit. Hall signal is from Hall element. Controlsignal is connected to drive circuit. Delay time is generated by

a delay circuit according to Vm (represent speed). Fig.4 showsthe fundamental principle during Hall signal jumping fromzero to a positive signal. If Hall signal is zero, the capacitor voltage is zero too and control signal is zero. If Hall signal

jumps to a positive value, the capacitor voltage begins toincrease. When the capacitor voltage reaches Vref , the controlsignal will step into a positive signal. There exists a Delaytime

between Hall signal and control signal.When Hall signal jumps from zero to a positive value,

capacitor starts being charged. The capacitor voltage isdescribed as

)1( / τ t C eVmV −

−= (2)

Whereτ

=R*C When V C reaches Vref , the Control signal will step into a positive value.

Vc=Vref (3)By (2) and (3), the hardware delaytime between position

signal and control signal can be described as

)ln( _ Vref Vm

Vmdelaytime Hardware

−= τ (4)

Where Vref is a reference voltage.Fig.5 shows the function of delay circuit. From (4) it is

founded that hardware delaytime decreases due to Vm (represent speed) increasing. It’s the same changing rule as

needed delaytime . Equati on (5) shows least squaretechnology is used to get optimized a orig , τ and Vref that makehardware delaytime closing needed delaytime more. By thismethod optimal commutation angle is gotten in whole speedrange.

−∑=

2

1) _ _ ( delaytime Hardwaredelaytime Needed Min

n

i(5)

III. MODEL VERIFICATION

The proposed model was applied to the design andsimulation of a prototype single-phase BLDCM. The cross-section of the motor is shown in Fig.1 and its drive circuit isshown in Fig.3. The rated voltage of the motor is 12V.

A.

Needed DelaytimeIn this paper, the needed delaytime of the motor is obtained by experiment. When initial commutation angle is set a valuefit for high speed running ( a orig = a0, ,a0 is an angle fit for highspeed running), the needed delaytime is shown in table.1.

The relationship between Vm and n (speed) is also given byexperiments, as in Fig.6.

This relationship can be approximated asn=230.77*Vm (6)

Then table.1 can be described as the relationship betweenneeded delaytime and Vm, as in Fig.7.

TABLE IRelationship between needed delaytime and speed

Speed(rpm) Needed Delaytime (us)

2800 0

2400 75

2000 150

1600 225

1203 300

1098.1 350

991.1 375

895 425

796.4 475

703.9 575

601 625499.8 750

Fig. 6. Relationship between speed and

COMPVref

Vm

ControlSignal

Vc2.5v COMP

Fig. 4. Delay circuit principle

Hall signalR

C

ControlSignal

HallSignal

Fig. 5. Function of delay circuit

time

time

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B. Delay Circuit Frame

Fig.8 shows the integral frame of delay circuit. The Hallsignal is from Hall IC. The Control signal is connected to drivecircuit.

If Hall signal jumps from zero to positive, the C1 voltage ischarged and Vout1 is connected to Control signal. When the

C1 voltage reaches Vref , the control signal will step into a positive signal. There exists a delay time between Hall signaland Control signal. And when Hall signal is positive, C2 isdischarged

Otherwise if Hall jumps from positive to zero, the C2voltage is charged and Vout2 is connected to Control signalthrough a negation. When the C2 voltage reaches Vref , thecontrol signal will step into zero. There exists a delaytime

between Hall signal and control signal. And when Hall signalis zero, C1 is discharged.The hardware Delaytime is decided

by (4). Fig.9 shows the relationship between hardwaredelaytime and Vm.

C. Parameters Calculation With 0α α =

orig

aorig = a0 means origin commutation angle is set a value fitfor motor high speed running. Method of least squares is usedhere to minimize error between needed delaytime andhardware delaytime in whole speed range.

In this condition, the optimal parameters are Vref=1.488V and τ =880us . The result of curve approaching is shown asFig.10.

Considering practical condition, tolerance rank of resistor and capacitor should be taken into account. ±5% tolerancecapacitor and ±1% tolerance resistor are selected. Consideringtolerance, the result of curve approaching is shown as Fig.11.

When aorig = a0, the absolute error between needed delaytime and hardware delaytime is shown as Fig.12.

Fig. 7. Relationship between needed delaytime and Vm

Vref

Vm

Vout2COMP2.5v

Vref

Vm

Vout1COMP2.5v

Hall signal

S1

EN-Hall signal

ControlSignal

R

C

R

C

COMP

COMP

Fig. 9. Relationship between hardware delaytime and Vm

Dotted: needed delaytimeCurve: hardware delaytime

Fig.11. Optimized result considering tolerance whena orig = a 0

Fig. 8. Delay circuit

Fig. 10. Relationship between needed delaytime and hardware

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D. Parameters Calculation With 0α α >orig

aorig > a0 means origin commutation angle is set a value largethan the value fit for high speed running. If aorig > a0, needed delaytime is different from table.1.

Through calculation we find that as aorig increasingEuclidean distance decreases. Through calculation, theoptimized parameters are Vref=0.9444V , τ =4930us and aorig = a 0+10 0. The result of curve approaching is shown asFig.13.

Considering tolerance, the result of curve approaching isshown as Fig.14.

Then absolute error of curve approaching is shown as Fig.15.

Comparing Fig.15 with Fig.12, if resistor and capacitor hadno tolerance or very small tolerance rank, moving Hall elementto a proper position ( aorig = a 0+10 0) would get a smaller Euclidean distance. Considering component tolerance, movingHall element to such a “proper” position couldn’t keep asmaller absolute error and error range. So aorig = a 0 is selected.

IV. CONCLUSIONS

This method makes it possible to improve efficiency withlow cost. By this method, the motor efficiency (shown inFig.16) is about average 4% higher than the efficiency withoutadaptive commutation angle. This paper presented a novelcommutation angle auto-adjustment method based on RCcircuit. This technique makes it possible to improve efficiencywith low cost. The relationship between commutation angleand delay time is given. The operating principle of this methodis detailed analyzed and simulated. With this technique, themotor efficiency is about 4% higher than the efficiency of atraditional motor.

Experimental results verified that the commutation angleauto-adjustment method is high efficiency and low cost.

R EFERENCES

[1] Z.Q. Zhu, S. Bentouati and D. Howe "Control of single-phase permanentmagnet brushless dc drives for high-speed," IEE Power Electronics and Variable Speed Drives , 18-19, September 2000.

[2] C.M. Chao, C.P. Liao, D.R. Huang and T.F. Ying "A New AutomaticPhase Adjustment of Optical Drive Signal," IEEE TRANSACTIONS ON MAGNETICS , vol. 34, No. 2, March 1998

Dotted: needed delaytime Curve: hardware delaytime

Fig. 13. Optimized result when aorig = a 0+10 0

Fig. 15. Absolute error when aorig = a 0+10 0

0.2

0.3

0.4

0.5

0.6

0.7

0 500 1000 1500 2000 2500 3000speed (rpm)

e f f i

c i

e n c y

Large commutation angle

Small commutation angle

Our solution

Fig. 16. Efficiency comparison of three methods

Fig. 14. Optimized result considering tolerance whenaorig = a 0+10 0

Fig. 12. Absolute error when aorig = a0

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