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By ANJU K P

Edited BySarath S Nair

www.technologyfuturae.com

MODELING AND SPEED CONTROL OF

BRUSHLESS DC MOTOR DRIVESBY LOAD TORQUE ESTIMATION

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OVERVIEW

Introduction BLDC Advantages, disadvantages and applications

of BLDC Modeling of BLDC and current control method Conclusions References

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mINTRODUCTION

DC motors have many attractive properties such as high efficiency and linear torque speed characteristics .

The control of dc motors is simple and their hardware is less.

The main drawback of dc motor is the need of periodic maintenance

Thus DC motors have been gradually replaced by the BLDC

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The PI controller is usually employed in a BLDC motor control, which is simple in realization. But it is difficult to obtain the sufficiently high-performance applications.

Used a feedforward compensation method through load torque estimation

The estimated load disturbance by the state observer is compensated as the equivalent current

The d-q transform of phase currents were possible by the Fourier series summation method

This current control method was used to get compensation current of BLDC motor.

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BLDC

Permanent magnet synchronous motor, having permanent magnets on the rotor and trapezoidal shape back EMF.

The BLDC motor employs a dc power supply switched to the stator phase windings of the motor by power devices, the switching sequence being determined from the rotor position.

BLDC is driven by a three-phase inverter with, six-step commutation

Commutation timing is determined by the rotor position, which can be detected by Hall sensors

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mADVANTAGES, DISADVANTAGES AND APPLICATIONS OF BLDC

ADVANTAGES More Efficient Less electrical energy is wasted Reduced Noise Longer Lifetime Reduced EM Interference. Torque, Voltage, And RPM Linearly Related DISADVANTAGES High cost More complex controller

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mAPPLICATIONS

They are having different power ratings from very small motors as used in hard disks to larger motors in electric vehicles

hard drives, CD/DVD players, and PC cooling fans use BLDC motors.

High power BLDC motors are found in electric vehicles and some industrial machinery.

A number of electric bicycles use BLDC motors

Certain HVAC systems, especially those featuring variable-speed and/or load modulation, use ECM motors .

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MODELING OF THE BLDC MOTOR AND CURRENT CONTROL METHOD

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MODELING OF BLDC MOTOR

The BLDC motor is an AC synchronous motor The BLDC motor has three stator windings

and permanent magnets on the rotor. The modeling is based on the following

assumptions:(1) The motor is not saturated.(2) Stator resistances of all windings are equal

and self and mutual inductances are constant.

(3) Power semiconductor devices in the inverter are ideal.

(4) Iron Losses are negligible.

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BLOCK DIAGRAM OF BLDC MOTOR DRIVE

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Under the above assumptions, a BLDC motor can be represented as

where ia, ib , and ic are rectangular shaped phase currents and ea, eb and ec are trapezoidal shaped back-EMFs. The electromagnetic torque is expressed as 

where, wm is mechanical speed

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CURRENT CONTROL METHOD

Input current reference value was calculated from the summation method of the Fourier series.

Phase current is rectangular shape and then, it's Fourier series is expressed as

Where a0=0 and an=0, because it is odd function

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using above eqn d-q transform of phase current can be done

the electromagnetic torque is rewritten as

α is proportional constant for feedforward compensation, λm is linkage flux of permanent magnet, andiq is q axis stator current

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The interaction of T, with the mechanical dynamics

where TL is the load torque, J is the inertia, w is the rotor speed, and B is the viscous damping.

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BLOCK DIAGRAM OF PROPOSED CONTROL ALGORITHM

BLDCM

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LOAD TORQUE OBSERVER The proposed speed control method requires information

of the load torque for feedforward compensation. A direct measurement of the load torque is difficult

because other high cost equipment is required. Therefore, we use state observer to estimate value. Input

value of the state observer is position information of rotor. The system equations of a motor can be described as

where y is the rotor position

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m The dynamic equation of the given system

can be expressed as

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A state observer for the load torque estimation can be represented as follows

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Estimated state vector can be represented as

is the proportional gain vector to be chosen so as to determine the error dynamics of the observer

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Estimated outputs of the observer are rotor position, speed,and load torque.

Estimated load torque is used for feedforward compensation  

Reference current iq* is compensated as iq,com by estimated

motor speed quickly converges the reference shortly after startup and recovers very well from the load torque disturbance as well as parameters variation

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CONCLUSIONS

Though BLDC is costly and its control circuit is complex, it replaces DC motors in many applications due to its wide advantages . The basics of BLDC was discussed.

The feedforward compensation control improves the overall speed control performance in low speed controller gain. This technique was also discussed.

It is regarded as the best control method to get fast and accurate speed response.

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REFERENCES

[1]Ki-Hong Park, Tae-Sung Kim, Sung-Chan Ahn, Dong-Seok Hyun,”Speed Control of High-PerformanceBrushless DC Motor Drives by Load Torque Estimation”.

[2]K. Ohishi, M. Nakao, K. Ohnishi and K. Miyachi,“Microprocessor-Controlled DC Motor for Load-Insensitive Position Servo System,” IEEE Trans. Ind. Electron., vol. 34, no. 1, pp. 44-49,1987.

[3]Jong Sun KO, Jung Hoon Lee, Se Kyo Chung, and Myung Joong Youn,“A Robust Digital Position Control of Brushless DC motor with Dead Beat Load Torque Observer,” IEEE Trans. Ind. Electron., vol. 40, no. 5,pp. 512-520, May 1993.

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[4]P.Pillay and R. Krishnan, “Application Characteristics of Permanent Magnet Synchronous and Brushless dc Motors for Servo Drives,” IEEE Trans. Ind. Appl., vol. 27, no.5, pp. 986-996 Sept./Oct. 1991.

[5]Guchuan Zhu, Dessaint, L.-A., Akhrif, O., and Kaddouri, A., “Speedtracking control of a permanent-magnet synchronous motor with state and load torque observer,” IEEE Trans, vol. 47, no. 2, pp. 346-355, Apr.2000.

[6]Stefán Baldursson,”BLDC Motor Modelling and Control – A Matlab®/Simulink®Implementation” Master Thesis work , Institutionen för Energi och Miljö International masters program in Electric Power Engineering , Chalmerstekniska HogskolaGöteborg, Sverige, 2005

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