direct torque control of matrix converter fed bldc motor

Direct Torque Control of Matrix Converter fed Brushless DC motor

P. Geeth Prajwal Reddy

L. A. Abishek Rajaraman

P. Ganesh

Dr. Ranganath Muthu

Dr. M. Senthil Kumaran

SSN College of Engineering

1IEEE India International Conference on Power Electronics, NIT Kurukshetra, 2014

(a) BLDC Motor (b) DC Motor

Fig.1.ComparisonofBLDCandConventionalDCMotor

Brushless DC MotorWhy BLDC Motor• Conventional DC motors, though with its numerous advantages, have a major

drawback with the commutator and brushes which are subject to wear and tear.• In Brushless dc motors, they are replaced by solid-state switches and lead to a longer

life span with all the inherent benefits of conventional dc motors.

Construction of BLDC Motor• Stator is similar to that of induction machine• Stator windings are distributed in such a way that they generate trapezoidal back EMF

and phase currents in the machine. (120o conduction mode)• Rotor is made of permanent magnets.• Based on the required magnetic field density in the rotor, the proper magnetic

material is chosen to make the rotor.

Fig.2.ConcentratedandDistributedWindingsofBLDCmotor

IEEE India International Conference on Power Electronics, NIT Kurukshetra, 2014 2

Brushless DC MotorOperation of BLDC Motor•BLDC motor employs an electronic commutator and uses Hall Effect sensors to sensethe position of the rotor and switch the phases accordingly.•The output from Hall sensors changes for every 60° of rotation thus defining sixconduction zones.•The switching of the inverter is arranged to give symmetrical current pulses IU, IV, IW of120° duration in both directions through each phase winding of the motor.•The position sensor and control logic ensure that the applied currents are in phasewith the motor back EMF in order to give maximum torque at all times.

Hall Effect Principle

N

N

N

S

S

S

N

N

N

S

S

Hall Element

2-Pole 4-Pole

Fig. 3. Arrangement of Hall ICs for 2 and 4 pole Machines at a

displacement of 120° and 60°respectively

Fig. 4. Operation of BLDC Motor


Brushless DC MotorApplications of BLDC Motor•BLDC motors have come to dominate many applications, particularly devices such ascomputer and hard drives and CD/DVD players. Small cooling fans in electronicequipment are powered exclusively by BLDC motors.•They can also be found in cordless power tools where the increased efficiency of themotor leads to longer periods of use before the battery needs to be charged.

Apart from these, they are also used in

•Transport – in electric vehicles and hybrid vehicles. E.g.: Segway Scooter, Vectrix Maxi-scooter, electric bicycles.

•Industrial Engineering - used in positioning or actuation systems.

•Motion Control – used as pump, fan and spindle drives in adjustable or variable speedapplications. They are also used in automated remote control systems.

•Heating and Ventilation – A growing trend in the HVAC and refrigeration industriesto use BLDC motors due to reduced power consumption than AC motors


Matrix ConverterThe matrix converter is a single-stage converter with m x n bidirectional power

switches, designed to connect an m-phase voltage source to an n-phase load withoutusing any DC link or large energy storage elements.

Salient Features• Simple and compact power circuit.• Generation of load voltage with

arbitrary amplitude and frequency.• Sinusoidal input and output currents.• Operation with unity power factor.• Regeneration capability.• Increased power density.

SaA SaCSaB

SbA SbB SbC

ScA ScB ScC

Va

Vb

Vc

VB VC

ICIBIA

VA

Fig. 5. Matrix Converter Topology

Space Vector ModulationSVM is an algorithm for the control of

pulse width modulation (PWM) used forthe creation of variable amplitude andfrequency AC.


Space Vector Modulation• The objective of SVM is to synthesize the output voltages from the input voltages

and the input currents from the output currents.• A 3Φ Current and Voltage reference is tracked on the current (input side) and voltage

hexagons (output side), shown in Fig.6 and Fig.8 respectively. Based on the locationof reference in a sector in the SVM hexagon, the appropriate switching vectors areapplied with the sufficient duty cycle to match the reference.

• Zero vectors are used to alter the magnitude of the output as per the reference.

Vα

Vβ

dβVβ V

REF

dαVα

θr

12

3

5

64

VA

VB

VC

V1(100)

V2(110)V3(010)

V4(011

)

V5(001) V6(101

)

Fig. 6. SVM Voltage Hexagon - Inverter Fig. 7. Reference Voltage Vector Synthesis

I1[ab]

1

23

5 6

4

I2[ac]

I3[bc]

I4[ba]

I5[ca]

I6[cb]Fig. 8. SVM Current Hexagon - Rectifier

Iγ

Iδ

dδIδ

IREF

dγIγ

θc

Fig. 9. Reference Current Vector Synthesis

δ1

δ1+δ2

T1

2

T0

TS

Vx

Vy

V0

Triangular Wave (Carrier)

Zero Vector

Active Vectors

Time

Du

ty

Cy

cle

T2

2

T2

2T1

2

Fig. 10. SVM symmetric Switching


Space Vector ModulationIndirect Modulation Strategy

• The basic idea of the indirect modulation technique is to decouple the control of theinput current and the control of the output voltage.

• Therefore, the Matrix Converter switching function is split into the product of theswitching function of the rectifier and inverter stage.

VA

VB

VC

VDC+

S1 S3 S5

S2 S4 S6

S7 S9 S11

S8 S10 S12

IDC+

VDC

Va

Vb

Vc

VDC-Rectifier stage Inverter stage

IDC-

Fig. 11. Indirect Modulation Principle Equivalent Circuit

Switching FunctionSwitch SAa – SCc – Matrix ConverterSwitch S7 – S12 – Inverter StageSwitch S1 – S6 – Rectifier Stage

• Due to the decoupling of the Matrix Converter, it is not necessary to device a DirectTorque control strategy for the entire Matrix converter.

• While the rectifier stage uses SVM to ensure sinusoidal input currents, DTC isimplemented for the inverter stage alone.

• The switching strategy of the Matrix Converter is obtained using the above product.IEEE India International Conference on Power Electronics, NIT Kurukshetra, 2014 7

Direct Torque Control• Independent control of electromagnetic torque and flux linkage using hysteresis

controllers.• Hysteresis Controller restricts the torque and flux within a set hysteresis band, as

shown in Fig. 12.

IEEE India International Conference on Power Electronics, NIT Kurukshetra, 2014

Fig. 12. Trajectory of stator flux linkage in the dq plane as restricted by the hysteresis controller

Salient Features and Advantages• Detailed machine equations and parameters are

not need.• Easy implementation, complex transformations

and calculations are not required, ideal forMicrocontrollers.

• Presence of look up table simplifies the algorithmand reduces process time and memoryrequirement.

• No current control loops.• Rotor Position information is not required.

Two Phase Conduction Scheme• BLDC motors have trapezoidal back EMF with 120° flat top and phase current

injection occurs only in this region.• At any given instant only two phases conduct, therefore the conventional DTC

algorithm is altered to generate the “Two Phase Conduction Scheme”8

Reasons for maintaining constant stator flux linkage•Control of stator flux linkage requires knowledge of its exactshape, but it is considered too cumbersome due to the 120°current conduction, which causes sharp dips in the stator fluxlinkage at every commutation instant.•In the constant torque region (below base speed), there is nonecessity to alter the stator flux linkage.

•By intentionally keeping the stator flux linkage almost

constant, DTC of BLDC motor is simplified to a torque

controlled drive.

Operation of Two Phase Conduction Scheme• Important step of DTC is the estimation of the electromagnetic torque of the BLDC

motor, this is given by


Two Phase Conduction Scheme

• Absence of rotor speed term in the second equation enables torque estimation atzero and near zero speeds.

• The hysteresis controller gives an output of +/- 1 based on the between theestimated and reference torque.

where ωe is the electrical rotor speed and kα (θe), kβ (θe), eα and eβ are the back EMF in αβ coordinates

Fig. 13. Actual (solid lines) and ideal (dotted lines) stator flux linkage trajectories αβ reference frame.

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Operation of Two Phase Conduction Scheme•Based on output of the hysteresis torque controller and the location of the statorvoltage space vector, the appropriate switch states of the inverter are obtained from the‘Two Phase Conduction Voltage Vector Selection’ look up table.



Φ τ ϴ

Θ1 Θ2 Θ3 Θ4 Θ5 Θ6

1

1 V1(100001) V2(001001) V3(011000) V4(010010) V5(000110) V6(100100)

-1 V6(100100) V1(100001) V2(001001) V3(011000) V4(010010) V5(000110)

0

1 V2(001001) V3(011000) V4(010010) V5(000110) V6(100100) V1(100001)

-1 V5(000110) V6(100100) V1(100001) V2(001001) V3(011000) V4(010010)

-1

1 V3(011000) V4(010010) V5(000110) V6(100100) V1(100001) V2(001001)

-1 V4(010010) V5(000110) V6(100100) V1(100001) V2(001001) V3(011000)

Table. 1. Two Phase Conduction Voltage Vector Selection for BLDC Motor

Note : the vectors given in grey are not used in below base speed operation ( constant stator flux)

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Block Diagram representation of Two Phase Conduction Scheme• Fig. 14. represents the block diagram representation of this scheme for the inverter

stage of the Matrix Converter



Fig. 14. Block diagram of Two Phase conduction scheme for BLDC Motor.

DTC Two Phase Conduction scheme algorithm for Matrix Converter• Thus, the switching function for the DTC of BLDC motor using the Matrix converter is

obtained through the product of the Two phase conduction scheme for the virtualinverter stage and the current SVM for the virtual rectifier stage.

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The simulations were carried out using the following parameters: Supply voltage of230 V, reference speed = 100 rad/s . The value of torque hysteresis band was set at 0.01Nm. Based on trial and error method, PI speed controller was set to KP=0.04 and KI=0.9.


Simulation Results

Stator Phase Currents and Back EMF waveforms Fig. 15 shows the stator phase current and back EMF waveforms of the BLDC motor.Smooth rectangular output current waveform is obtained with a small dip in betweendue to commutation at every 60° electrical.

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Fig. 15. Stator phase currents and back EMF waveforms of the BLDC Motor fed by Matrix Converter.


Simulation Results

Speed Characteristics of the BLDC MotorThe reference speed for the motor was set at 100 rad/sec. After the initial overshoot,

the speed settles at the set reference of 100 rad/sec. Once the load is applied, the speedinitially dips and then returns to the set reference value. Further on, the speed varies,subject to load variation but promptly returns to the set reference.

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Fig. 16. Rotor speed characteristics of the BLDC Motor subject to Load variations


Simulation Results

Torque Response of the BLDC MotorDue to the use of DTC, the torque of the BLDC motor is restricted to a narrow band,

except in the instances where current commutation occurs but this torque dip iscomparatively less when a matrix converter is used instead of the convention AC-DC-ACconverters.

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Fig. 17. Torque Response of the BLDC motor fed by a Matrix converter implementing DTC

Simulation shows that the drive system has a good dynamic response to changes inthe load. Due to the hysteresis controller, the motor torque is restricted in a narrow band.Improved tuning of the PI controllers using Ziegler Nicholas method or Lambda tuningmethod, will further enhance the torque response.


Simulation ResultsStator Flux of the BLDC Motor

Fig. 18. shows the plot of the BLDC motor operating under the Two Phase Conductionscheme. Despite the lack of explicit Stator Flux, the Two Phase Conduction schemeexercises control over the Stator Flux of the Machine and restrict it within a narrow band (0.04 Vs). The dips in the Stator Flux linkage occur due to the current commutation every60° .

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This result quantifies the ability of the Two Phase Conduction to exercise control overthe Stator Flux of the BLDC motor. Especially during operation above the base speedoperation.

Fig. 18. Stator Flux Linkage of BLDC Motor operating under DTC Two Phase Conduction Scheme


Simulation Results

Source Current drawn by the Matrix ConverterOne of the major advantages of the Matrix Converter is that the source current drawn

is sinusoidal and has reduced THD.

16

Fig. 19. Source current drawn by the Matrix Converter

After the initial high starting current is drawn by the motor, the current settles down at theits no load value. The current drawn varies in accordance to the motor torque requirement.However, in all cases, sinusoidal nature of the current and a power factor of 0.96 ismaintained at the source.


Simulation Results

Source Current drawn by the Matrix Converter

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Fig. 20. 3 Φ Source currents

• Conventional AC-DC-AC converter requires 12 switches, six each for the rectifier andinverter. Whereas the Matrix converter employs 12 bidirectional switches in total.Thereby reducing switching losses.

• Filter requirements for a Matrix converter are comparatively smaller than that forconventional AC-DC-AC converters.

• Lack of bulky energy storage elements, reduces the size and the cost of the Matrixconverter and offers higher power density

Total Harmonic distortion

• In drive application, the current drawn by the converters from the source must havereduced THD. Injection of Harmonics will give rise to increased power loss andconsequent heating in machines.

• Fast Fourier Transform (FFT) Analysis of the source currentMatrix Converter fed drive THD : 0.88%,AC-DC-AC converter fed drive THD : 3.70%


Comparison of Matrix Converter fed and conventional AC-DC-AC converter fed BLDC Drive implementing DTC

18

Power Factor at the SourceThe plot of the current drawn by Matrix Converter and the current drawn by the

conventional converter system is shown in Fig. 21. The corresponding power factors weremeasured to be

Matrix Converter : 0.963 AC-DC-AC converter : 0.213



19

Fig. 21. Power Factor Comparison

Rotor Torque Ripple caused due to current commutation every 60°Use of AC-DC-AC Converter resulted in a dip in electromagnetic torque ripple in•No-load condition : 0.5 Nm•Loaded Condition : 1.1 Nm – 1.5 NmWhereas, with the Matrix converter•No-load condition : negligible ripple•Loaded Condition : 0.7 Nm – 1 NmThus the torque ripple is reduced in matrix converter fed BLDC drives.



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Fig. 22. Torque ripple in Matrix converter fed BLDC drive Fig. 23. Torque ripple in AC-DC-AC converter fed BLDC drive

• This paper has successfully demonstrated the application of the proposed two-phaseconduction direct torque control (DTC) scheme for BLDC motor drives in the constanttorque region using matrix converter producing rectangular output currents whilemaintaining sinusoidal input current at unity power factor.

• Compared to the three phase DTC technique, this approach eliminates the flux controland only torque is considered in the overall control system.

• For the proposed DTC scheme, A look-up table for the two-phase voltage vectorselection is designed to provide faster torque response both on rising and fallingconditions.

• Performance under Matrix Converter resulted in reduced THD in current, reducedtorque ripple and improved power factor on the source side than the one with theinverter, thereby making it the better choice for the above proposed Direct TorqueControl of BLDC Motor.


Conclusion

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Future Scope

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• A position estimation technique can be used in six-step DTC of BLDC motor driveinstead of an expensive and bulky position encoder for a cost-effective system.

• Implementation of sensor less control methods for the control of BLDC Motor drive.

• Power factor control technique can be coupled with the proposed two-phaseconduction DTC of BLDC motor drive to improve the current and torque performanceat high dc-link voltage conditions while keeping the dc-link voltage fluctuations atminimum and power factor at maximum level.

• Use of improved PI tuning methods, such as Ziegler-Nicholas method, Coon Cohenopen loop method, etc. will significantly improve the dynamic response of the drivesystem.


References

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1. T. J. E. Miller, Brushless Permanent Magnet and Reluctance Motor Drive, Clarenford, UK: Oxford, 1989.

1. A. Purna Chandra Rao, Y. P. Obulesh, and Ch. Sai Babu, “Mathematical modeling of BLDC motor with closed loop speed control using PID controller under various loading conditions,” ARPN Journal of Engineering and Applied Sciences, vol. 7, no. 10, pp. 1321-1328, October 2012.

2. P. W. Wheeler, J. Rodriguez, J. C. Clare, L. Empringham, and A. Weinstein, “Matrix converters: a technology review," IEEE Transactions on Industrial Electronics, vol. 49, no. 2, pp. 276-288, April 2002.

3. M. Przybylski, B. Slusarek, and J. Gromek, “Brushless DC motor with a bonded permanent magnet and powder magnetic core,” Proceedings of the 2010 XIX International Conference on Electrical Machines (ICEM), pp. 1-4, 6-8 Sept. 2010.

4. A. R. Paul and M. George, “and ,f the , ading conditions,"Brushless DC motor control using digital PWM techniques,” Proceedings of the 2011 International Conference on Signal Processing, Communication, Computing and Networking Technologies (ICSCCN), pp. 733-738, 21-22 July 2011.


References

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6.H. J. Cha, “Analysis and design of matrix converters for adjustable speed drives and distributed power sources,” Doctoral Dissertation, Texas A&M University, 2004.

7.S. B. Ozturk and H. A. Toliyat, “Direct torque control of brushless DC motor with non-sinusoidal back-EMF,” IEEE International Electric Machines and Drives Conference 2007 (IEMDC '07), vol. 1, pp. 165-171, 3-5 May 2007.

8.P. Devendra, Ch. Pavan Kalyan, K. Alice Mary, and Ch. Saibabu, “Simulation approach for torque ripple minimization of BLDC motor using direct torque control,” International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering (IJAREEIE), vol. 2, no. 8, pp. 3703-3710, August 2013.

9.B. K. Bose, Modern Power Electronics and AC Drives, Prentice Hall: New Delhi, India, 2008.

Thank you and a pleasant evening to all

direct torque control of matrix converter fed bldc motor

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