Research on field weakening based on reactive power with BLDC motor for electric vehicle application

At present, research on field-weakening of BLDC motor all obtains the best electric current vector to control the inverter to obtain certain effect under the different rotational speed ,

Research on Field-Weakening Based on Reactive Power with

Jinsong Kang Department of Electrical Engineering, TongJi University,Shanghai,P.R.C., Email:kjs@mail.tongji.edu.cn Department of Electrical and Computer Engineering,

Ryerson University,Toronto,Canada

Guoqing Xu Department of Electrical Engineering, TongJi University,Shanghai, P.R.C

Email:xugqdrr@vip.sina.com Shenzhen Institute of Advanced Technology

ShenzhenP.R.C

Bo Hu Department of Electrical Engineering,

TongJi University, Shanghai, P.R.C

Email:bobo _hu@mail.tongji.edu.cn

Zhouyun Zhang and Jun Gong Ananda Drive Technology Corporation,

Shanghai, P.R.C

Email:{zzy & gjun}@ananda.com.cn

Abstract— Brushless Direct Current (BLDC) motor with the

high power density and the high efficiency characteristic is been

used to Electric Vehicle The mathematical model of BLDC

motor under rotor flux linkage direction reference frame was

given The chart of field-weakening vector control and basic

principle of field-weakening control strategy were analyzed.

The theory of field-weakening based on reactive power with

BLDC motor was proposed The drive system of field-weakening

based on reactive power using BLDC motor was designed.

The experiment and result was test the validity of

field-weakening based on reactive power with BLDC for Electric

Vehicle Application.

Index Terms— BLDC motor; Electric vehicle;

field-weakening; reactive power;

I INTRODUCTION The electric vehicle(EV) is the cleanenergy saving and

environmental protection transportation vehicle, which

hav-ing no pollution, low heat radiation , the noise small, not

consumed the gasoline in the travel process It applying many

kinds of energy, is called ”star of the tomorrow” The drive

system is one power core of the electric vehicle, realizing the

vehicles power performance Along with the new material

technology, the computer technology, the power electronics

technology and the microelectronic technology rapid

de-velopment, the electric vehicle mostly uses the alternating

current (AC) machine It is the motive research hot spot that It

is the motive research hot spot that Brushless Direct Current

(BLDC) motor with the high power density and the high

efficiency characteristic is been used to EV The motor has

inherit the predominant timing performance of the traditional

motor, as well the less volume, lighter weight, high efficiency

less moment of inertia and without exciting wastage, and also

discard the commutator and brush Therefore it’s widely used

∗

and has good prospect[1]-[3]

At present, research on field-weakening of BLDC motor all obtains the best electric current vector to control the inverter

to obtain certain effect under the different rotational speed ,

in the foundation of not modifying the electrical machinery structure, resting on these characteristic curve such as the biggest torque/current path, the electric current and the volt-age limit ellipse Vector control technology is not suitable for weak magnetic control of BLDC motor with trapezoidal wave permanent magnetism, because not realizing electrical machinery complete decoupling[4]-[6] This article proposes transient powerless torque theory from the basic electric and magnet correlation, the theory is brought forward to meet the phase advancing control method Without complicated vector transform, the method calculated current instruction and angle instruction based on the given torque and motor speed feedback, and achieves high control precision in the high speed field-weaken area

II BASIC PRINCIPLE OF FIELD-WEAKENING CONTROL

STRATEGY WITHBLDCMOTOR The permanent magnet is produced the constant excitation magnetic field when not considering the temperature influ-ence of rotor permanent magnet It is only can carry on the equivalent field-weakening through the stator magnetic field to the air gap magnetic field Basic principle of field-weakening control strategy with BLDC motor all is uses the armature reactions of the stator current Through the stator magnetic potential and the rotor magnetic potential composing, causing the air gap magnetic potential to reduce, the induced potential of stator winding reduces When ne-glecting the saturation effect of stator inductance parameter

International Conference on Integration Technology

March 20 - 24, 2007, Shenzhen, China

of BLDC motor, it’s mathematical model under rotor flux

linkage direction reference frame is like as the formula 1

u d = R1i d + pψ d − ω e ψ q

u q = R1i q + pψ q + ω e ψ d

ψ d = L d i d + ψ m

ψ q = L q i q

(1)

In the formula, u d and u q represent direct axis

compo-nent and quadrature axis compocompo-nent of the stator voltage

respectively (V), i d and i qrepresent direct axis component

and quadrature axis component of the stator current

respec-tively (A);ψ d and ψ q represent direct axis component and

quadrature axis component of the stator flux linkage (Wb);

ω q represents the synchronization angular speed (rad/s)L d

andL q represent direct axis component and quadrature axis

component of the stator inductance (H);ψ m represents rotor

permanent flux linkage (Wb) [7]

The space vector chart of BLDC motor is shown as Figure 2

through stator voltage, stator current and stator flux linkage

respective expressed by space vector form The vector control

of DLDC motor is advance control to the stator current

under the rotor flux linkage direction frame Field-weakening

control is achieved by reducing gap flux linkage , throughi d

direct axis component the stator current multiplyingL d (the

direct axis inductance) counteract permanent flux linkage

d q

A

→ m

ψ

→

s

d

i

q

i

ϕ

θ β

q

qi L

d

di

L

Fig 1 Chart of field-weakening vector control of BLDC motor.

In figure 1, ϕ is the angle between stator current vector

and permanent magnetism rotorq’s axis, is called the

inter-nal power factor angle; β is the angle between permanent

magnetism rotor flux linkage and the stator flux linkage,

calling the power factor angle;θ is the phase angle between

rotor d axis and A phase winding middle line in the stator

winding reference The influence of stator armature reaction

on rotor magnetic field increases magnetism after degausses

first when the angle maintains at90◦ about between the stator

magnetic field and the rotor magnetic field of BLDC motor,

causing each magnetic flux mean value maintenance to be

extremely invariable, namely only changing its peak-to-peak

value not to change its phase Equivalent field weakening

can be achieved when the demagnetization function of stator current armature reaction is bigger than the increasing action through phase change ahead of time Field-weakening and speed increasing is achieved by equivalent weakening the winding magnetic flux through the stator current armature reaction When BLDC motor using PWM control, the stator winding is at unceasingly the power on and off condition periodically, the air gap magnetic flux is weakened along with the rotor position changing[8]

III THE THEORY OF FIELD-WEAKENING BASED ON REACTIVE POWER WITHBLDCMOTOR The electrical machinery power is composed by the active power and the reactive power[9]-[10] In the static α − β

frame, the active powerP eand reactive powerQ eof BLDC motor which with the stator winding opposite electromotive

is respectively shown as formula 2,3:

P e=u →1· → i1≈ → e1· → i1= |e1| · |i1| cos ϕ = |e1| · |i1| cos (β − α)

= |e1| · |i1| (cos β cos α + sin β sin α) = e α · i α + e β · i β

(2)

Q e=u →1× → i1≈ e →1× → i1= |e1| · |i1| sin ϕ = |e1| · |i1| sin (β − α)

= |e1| · |i1| (sin β cos α − cos β sin α) = e β · i α − e α · i β

(3)

In the above formula, the electromagnetism torque com-ponent T e and reactive power torque component S e can be obtained by formula 4,5

T e= P e

ω1 =

e α · i α + e β · i β

S e= Q e

ω1 =

e α · i β − e α · i β

The electromagnetism torque component T e and reactive power torque componentS eare scalars, not through the com-plex vector transformation.T e andS e under different angle are calculated through simple 3/2 mathematics operation of three-phase counter-electromotive force and the three-phase current under the certain speed Torque control commandT e

can directly obtained in operating mode of vehicle If the relation between i α andi β can be obtained through current control strategy, reactive power torque component S e can

be calculated The magnetic torque of BLDC motor is fully obtained by the phase advance control in low speed area and field-weakening control with permanent over speed area The principle of stator control based on reactive power theory for BLDC motor is described by formula 5



i α

i β



f2

α (θ) + f2

β (θ)



f α (θ) f β (θ)

−f β (θ) f α (θ)



·



T e

S e

 (6)

Fig 2 Chart of field-weakening vector control of BLDC motor.

The instantaneous value of three-phase electric current is

obtained by the counter-Clark transformation according to the

above equation Field weakening control based on reactive

power with BLDC motor includes operating mode of biggest

torque/Current ratio control in low speed and operating mode

of weak magnetic control with permanent power in high

speed The operating mode using weak magnetic control

in low speed is for fully using permanent magnetism

mag-netic resistance torque, enhancing electrical machinery the

torque/Current ratio; The operating mode using the weak

magnetic control in high speed is in order to use the stator

current the straight axis component to realize the air gap

magnetic flux equivalent weak magnetism, realizing weak

magnetic control keeping permanent power

IV THE DRIVE SYSTEM OF FIELD-WEAKENING BASED ON

REACTIVE POWER USINGBLDCMOTOR

The principle of field-weakening based on reactive power

is realized by electrical current phase advance according

to the current torque instruction and the current rotational

speed The angle ahead of time can be obtained by the angle

of the stator current opposite the counter electromotive force, which are calculated by the d-axis component and q-axis component of stator current based on reactive power theory The stator current peak value serves as the electric current instruction value of the stator current with closed-loop control The control block diagram of field-weakening based

on reactive power theory using square-wave electric current control BLDC motor is shown as figure 2

In the chart, torque instructions is accepted by communication control unit through the CAN from vehicle controller The reactive power torque is calculated

by the control command and the speed The electric current function table and the field-weakening control angle function table are calculated by torque instruction signal and speed signal The PWM duty factor signal is to come from the electric current function table and field-weakening control angle function table The signal is also comes form the DC voltage signal, the phase current signal, the temperature signal, real-time trouble protect signal of drive system

V THE EXPERIMENT AND RESULT OF FIELD-WEAKENING

CONTROL WITHBLDCMOTOR FOR ELECTRIC VEHICLE

In order to test the validity of field-weakening control

based on reactive power with BLDC motor The experiments

are carried out in an AC drive system, which parameters

of the square-wave electric current control BLDC motor are

list as: rated power is 65kW, maximum speed is 11000 rpm,

maximum torque is 125Nm The three-dimensional chart of

stator electric current instruction calculated on reactive power

theory is shown as 3 The three-dimensional chart of angle

instruction obtained from field-weakening based on reactive

power theory is shown as 4

The rotor position signal, the corresponding PWM profiles

and the electric current profiles according to field-weakening

control based on above instructions under different speed

experiment are respectively shown as 5 and 6

Fig 3 Three-dimensional chart of stator electric current instruction

VI CONCLUSION

It is availability to field-weakening control based on

re-active power theory When the BLDC motor is in the low

speed with permanent torque control operating mode, Under

the same speed and same peak value torque, experimental

result is shown to obtain 5 percent higher torque under the

same electric current, more to enhance the drive system

controllability and control precision When in high speed

operating mode, the stator current peak can be reduced15 ∼

20 percent under same torque in the field-weakening control

region, thus the system reliability and the power module

security enhance greatly

VII ACKNOWLEDGMENTS

The authors would like to thank Dept of Electric

Engineer-ing, Tongji University and Ananda Drive Technology

Corpo-ration for their helpful support and constant encouragement

Fig 4 Three-dimensional chart of angle instruction obtained from field-weakening

Fig 5 Experiment waveforms around 1000 rpm with field-weakening control

Fig 6 Experiment waveforms around 10000 rpm with field-weakening control

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