Current Controlled Bridgeless Buck Converter For …...Current Controlled Bridgeless Buck Converter...

Current Controlled Bridgeless Buck Converter For Power Factor Correction in Permanent Magnet

Brushless DC Motor Drive Systems

P. Sarala Research Scholar, Dept of EEE, JNTUA, Ananthpur, AP, India. e-mail: [email protected]

Dr. S. F. Kodad Professor and HoD, Dept of EEE

PESITM, Sivamogga, Karnataka, India.

Dr. B. Sarvesh Professor, Dept of EEE

JNTUA, Ananthpur, AP, India.

Abstract—This paper presents a Bridgeless Buck converter for power factor correction in brushless motor drive connected power systems. Current control technique is used to control static switches in bridgeless Buck converter. Need of additional converter before brushless DC motor may create power system quality issues. This paper presents a Bridgeless Buck converter for power factor correction in brushless motor drive systems. Control strategy with Bridgeless Buck converter for power factor correction was tested for different conditions like different DC link voltages and step-change in DC link voltage. Results prove the applicability of control scheme for different conditions and power factor was shown along with total harmonic distortions in source current of the system.

Keywords: Power factor; current control; bridgeless buck; brushless motor.

I. INTRODUCTION

Motor is an electro-mechanical machine which converts electrical energy to mechanical energy. DC motor is motor supplied with DC type of supply and exhibits good performance characteristics. Electric commutation in DC motors was done using mechanical commutator and brush assembly. Presence of mechanical commutator induces additional losses due to wear and tear, sparks and mechanical operation. Motors without commutator and brushes can give better performance with improved efficiency and led to development of brushless DC (BLDC) motors [1-3] . Electric commutation in BLDC motor is done with the use of electronic commutation.

BLDC motors are constructed with windings on its stator and permanent magnets on its rotor part. Use of permanent magnets in BLDC motor makes this type of machine costlier but can give high torque with simple construction, less maintenance and less losses improving the efficiency. Induction motor is used often in many applications due to simple construction, less maintenance [4-5]. But low power factor and efficiency makes induction motor limited in some applications. BLDC motor is superior and viable option having high efficiency, high torque, high power to weight ratio and high speed. BLDC exhibits difficulty in its control and modeling and also usage of non-linear solid state devices creates power system disturbances. Though the name suggests BLDC motor as a DC motor in which the actual supply in of

DC type but in practical BLDC motor should have a voltage source converter as front-end converter for its operation. The supplied DC is inverted to AC to feed BLDC stator windings by energizing two phases at a time. Voltage source converter to feed BLDC is an integral part of machine and hence named as brushless DC motor as shown in figure 1. Commutation in BLDC is done using an electronic commutator where the need of mechanical commutator and brushes are eliminated and hence called brushless motors. When BLDC motor acting as drive motor and as general available supply is AC, power electronic converters are needed and use of non-linear power electronic converters might affect the power system quality [6-8]. Use of additional accessories for power factor correction is viable option and can increases system performance. A systematic control of an opted converter can correct the power factor in the system. Conventional control employs voltage follower technique but current control strategy can be a good alternative for power factor correction [9-12].

This paper presents bridgeless buck converter controlled with current follower for power factor correction in BLDC motor drive connected system. Operation of bridgeless buck converter was explained with the aid of its working modes. Current control strategy was explained in detail and the bridgeless buck converter with current control was depicted in following sections of this paper. Validity of current control strategy with bridgeless buck converter for power factor correction in BLDC motor drive connected system was explained considering different cases of DC link voltages and also for step-change in DC link voltage.

II. BRIDGELESS BUCK CONVERTER WITH BLDC MOTOR

A bridgeless buck converter is a converter which avoids a bridge circuit before buck converter for conversion of AC supply to DC. Bridgeless buck converter directly converts AC to DC and charges the DC link capacitor directly through its operation eliminating a separate bridge circuit for AC to DC conversion. A bridgeless buck converter consists of four diodes, two coupled inductors along with a switch as shown in figure 2. The DC source feeding bridgeless buck converter can be a DC voltage source or can be extended to renewable energy source. The operation of bridgeless buck converter can be explained in six modes:

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 1 (2017) © Research India Publications. http://www.ripublication.com

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Controller

Inverter

Driver

BLDC

Position

Information

DC

source

Fig 1: BLDC motor

Vs

D1

D2D3

D4

Cf

Lf1

Lf23-phase

VSC feeding

BLDC motor

Switch1

Switch2

Fig 2: Bridgeless Buck converter with BLDC motor

A. Positive sequence mode-1:

Vs

D1

D2D3 D4

Cf

Lf1

Lf2

Switch2Three

phase VSC

feeding

BLDC motor

Fig 3: Conduction in positive sequence mode 1

Conducting path in positive sequence mode 1 is shown in figure 3. Initially switch S1 is ON and S2 is in OFF position and diode D2 will be in forward bias. Supply current charges the DC link capacitor from source through inductor Lf1 and diode D2. The charged capacitor drives VSC fed BLDC.

B. Positive sequence mode-2:

Vs

D1

D2D3

D4

Cf

Lf1

Lf23-phase

VSC feeding

BLDC motor

Switch1

Switch2



315

In positive sequence mode pair of solid state devices S1 and S2 is turned OFF as shown in figure four. Throughout this period of operation as each the switches are turned OFF, no energy is provided from the supply. However the stored energy as a result of operation of positive sequence mode 1, the stored energy in inductance Lf1 freewheels through capacitor and diode D3.

C. Positive sequence mode-3:

Vs

D1

D2D3

D4

Cf

Lf1

Lf23-phase

VSC feeding

BLDC motor

Switch1

Switch2


Conducting path in positive sequence mode 3 is shown in figure 5. As both switches are turned OFF, only capacitor discharges through load.

D. Negative sequence mode-1:

Vs

D1

D2D3 D4

Cf

Lf1

Lf2

Switch1

Three

phase VSC

feeding

BLDC motor

Fig 6: Conduction in Negative sequence mode 1

In negative sequence mode 1 as in figure 6, switches S1 is turned OFF and S2 is ON. Current through switch S2 charges Lf2 through capacitor and diode D1 returning to source.

E. Negative sequence mode-2:

Vs

D1

D2D3

D4

Cf

Lf1

Lf23-phase

VSC feeding

BLDC motor

Switch1

Switch2


Conducting path in negative sequence mode 2 is shown in figure 7. During this period of time, as both the switches are OFF, no energy is supplied from the source. But

the stored energy due to operation of negative sequence mode 1, the stored energy in inductor Lf2 freewheels through capacitor and diode D4.

F. Negative sequence mode-3:

Vs

D1

D2D3

D4

Cf

Lf1

Lf23-phase

VSC feeding

BLDC motor

Switch2


Conducting path in negative sequence mode 3 is shown in figure 8. As both switches are turned OFF, only capacitor discharges through load.

III. BRIDGELESS BUCK FED BLDC DRIVE WITHOUT PFC

Bridgeless Buck converter for power factor correction fed BLDC motor drive without control was shown in figure 9. In bridgeless buck converter the bridge circuit was eliminated and AC supply is rectified to DC directly by bridgeless buck converter. The rectified DC is fed DC link capacitor. DC link capacitor acts as a DC source to the inverter of BLDC motor. Inverter excites the stator windings of brushless DC motor and thus rotates the rotor due to torque exerted. The rotor position of BLDC motor is continuously sensed by using hall sensors. The obtained information from the hall sensors about the position of the rotor is send to controller. Controller produces pulses for static switches in inverter through driver circuit thus exciting the phases of inverter. At a time two legs of inverter will be ON and only one switch in one leg will be turned ON at a time to avoid risk of short circuit in inverter circuit. BLDC motors are constructed with windings on its stator and permanent magnets on its rotor part. Use of permanent magnets in BLDC motor makes this type of machine costlier but can give high torque with simple construction, less maintenance and less losses improving the efficiency.

IV. CURRENT CONTROL STRATEGY FOR POWER FACTOR

CORRECTION WITH BRIDGELESS BUCK FED BLDC DRIVE

In bridgeless buck converter, the bridge circuit was eliminated and AC supply is rectified to DC directly by bridgeless buck converter. The rectified DC is fed DC link capacitor Figure 10 shows the bridgeless buck converter driven BLDC motor drive with power factor control strategy. The switching signal for bridgeless buck converter is obtained from driver circuit of controller. Speed reference is taken and is converted to voltage signal. Obtained reference voltage is compared with actual DC link voltage and error signal is sent to PI controller for error reduction obtaining current magnitude. From source voltage, information regarding sine is obtained and combined to current magnitude obtaining reference current signal. The obtained reference current signal is compared to actual current and error signal is sent to hysteresis current controller which produces gating pulses to two switches of bridgeless buck converter.


316

Electronic

commutation

GATE

PULSE

Vs

D1

D2

D3 D4

Cf

Lf1

Lf2

Switch1

Switch2S1

BLDC

Motor

Hall

signal

S3 S5

S4S6 S2

Fig 9: Bridgeless Buck converter fed BLDC without controller

Electronic

commutation

GATE

PULSE

Vs

D1

D2

D3 D4

Cf

Lf1

Lf2

Switch1

Switch2 S1

BLDC

Motor

Hall

signal

S3 S5

S4S6 S2

1/Vmax

Speed to voltage

convertion

Iact

Iref

Iact

Iref

Ish Imag Vdcref

Vdcact

Wref

V

V

PI

-+

Hysterisis

control

Hysterisis

control

+-

Fig 10: Bridgeless Buck converter fed BLDC with controller


317

V. RESULTS AND DISCUSSIONS

A. Analysis for Vdc = 50V

Fig 11: Source voltage and source current of grid system with Dc link voltage as 50V

Fig 12: DC link voltage of Inverter

Grid voltage and current of BLDC connected system was depicted in Figure 11. Source voltage is maintained at 360V and source current at 8A. Both source voltage and current are sinusoidal in shape.

Fig 13: BLDC motor speed in RPM

DC link voltage maintained at 50V at the inverter is given in figure 12. Brushless DC motor running at speed of 400RPM is given in figure 13.

Fig 14: Torque of BLDC motor

Operating torque in BLDC motor is shown Figure 14 shows torque is operated at 1.2 Nm but contains some ripples.

Fig 15: Stator current of BLDC motor

Brushless DC motor is supplied with stator current and is shown in figure 15. Due to non-linear nature of stator windings, stator current draws non-linear currents and is observed from the output waveform.

Fig 16: Power factor of system

Fig 17: THD in source current


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Power factor of the system is shown in figure 16 and is maintained nearer to unity as can be observed since there is no phase shift between source voltage and current waveforms. Total harmonic distortion in source current was shown in figure 17 indicating THD as 4.42% which is nominal value. Thus power factor is maintained in system with power factor correction converter which can be concluded from power factor and harmonic distortion shown.

B. Analysis for Vdc = 100V

Fig 18: Source voltage and source current of grid system

Grid voltage and current are shown in figure 18 where grid voltage is maintained at 360V and current at 1.2A. Voltage and current in grid are maintained sinusoidal.

Fig 19: DC link voltage of system

Inverter DC link voltage is shown in figure 19 and constant 100V DC link is maintained which is observed from the waveform.

Figure 20: Speed of BLDC motor

Figure 21: Torque of BLDC motor

Speed at which BLDC motor runs is shown in figure 20. BLDC runs at 900 RPM constant speed. Developed torque in BLDC motor is shown in figure 21. Constant torque of 1.25 is produced with small ripples in torque.

Figure 22: Stator current of BLDC motor

Figure 23: Power factor of system

Stator currents drawn by stator windings of BLDC motor is shown in figure 22. Since stator windings are of non-linear type, they draw non-linear components from current for excitation.

Figure 24: THD in source current


319

Power factor of the system is shown in figure 23 and is maintained nearer to unity as can be observed since there is no phase shift between source voltage and current waveforms. Total harmonic distortion in source current was shown in figure 24 indicating THD as 2.65% which is nominal value below 5%. Thus power factor is maintained in system with power factor correction converter which can be concluded from power factor and harmonic distortion shown.

C. Analysis for Vdc = 150V

Fig 25: Source voltage and source current of grid


Grid voltage and currents in connected system were shown in figure 25. Grid voltage is maintained at 340 V and current at 2A. Grid voltage and current are sinusoidal. DC link voltage in this case is too maintained at 150 V DC and is shown in figure 26. DC link voltage is maintained constant.

Fig 27: Speed of BLDC motor

Fig 28: Torque of BLDC motor

Fig 29: Stator current of BLDC motor

Speed at which BLDC motor runs at is represented in figure 27 and indicates motor is running at constant speed of 1400 RPM. Produced torque in BLDC motor was shown in figure 28 and is maintained constant with small torque ripples. Current drawn by the stator of BLDC is shown in figure 29. Stator currents draw non-linear currents since it is of non-linear type.

Fig 30: Power factor of system



320


D. Analysis for Vdc = 200V

Fig 32: Source voltage and source current of system


Grid currents and voltages in grid are shown in figure 32 which are sinusoidal in shape. Voltage is maintained at 340 V and current is at 3A. In this case, DC link voltage of inverter is to be maintained tat 200 V DC and is shown in figure 33. Constant DC link is provided to inverter.




Speed at which BLDC runs with 200 V DC link voltage is 1900 RPM shown in figure 34. Speed is constantly maintained. Torque produced in BLDC is shown in figure 35 and is maintained with small ripples with 1.2 Nm. Currents in stator of BLDC is shown in figure 36, stator currents are non-linear in shape due to their non-linear behavior.



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E. Analysis for step change in Vdc

Figure 39: Source voltage and source current of system

Source grid voltage and current are shown in figure 39 and current along with voltage are maintained sinusoidal. Since DC link voltage is variable, source current gets distorted and later settles at instant 0.3 sec.

Figure 40: DC link voltage of system




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Variation in DC link voltage from 80 V DC to 120 V DC is shown in figure 40. Variable DC link voltage is at 0.3 sec and can be observed. Speed at which BLDC runs with is shown in figure 41. Since variable DC link voltage is applied at 0.3 sec, the speed also varies at 0.3 sec accordingly. Torque generated in BLDC motor is shown in figure 42. Variable DC is applied at 0.3 sec and thus torque gets slightly varied at 0.3 sec and later is maintained constant. Stator currents drawn by BLDC are shown in figure 43. Only at 0.3 sec stator current gets distorted due to variable DC link variation.


Power factor of the system is shown in figure 44 and is maintained nearer to unity as can be observed since there is no phase shift between source voltage and current waveforms. Thus power factor is maintained in system with power factor

correction converter which can be concluded from power factor shown.

VI. CONCLUSION

Though conventional DC motors give good results in motor drive system the presence of commutator and brushes reduces the efficiency. Brushless DC motors are synchronous motors where in electrical commutation is done by electronic commutator. Use of front-end converters for conversion of power from one form to other for operation of BLDC motor creates power factor issues which is a cause for losses deteriorating efficiency. By employing a bridgeless buck converter and by properly switching static switches in bridgeless buck converter, power factor can be improved. Current control technique was employed to control bridgeless buck converter eventually increasing power factor of the system. For different DC link voltages and also for step variation in DC link voltage results prove the applicability of current controlled bridgeless buck converter for PFC. Buck converter with current control strategy was explained. Total harmonic distortion in source current was kept within normal limit and was shown. Power factor for different cases was maintained nearer to unity as no phase shift between source current and voltage was observed from shown results.

References

[1] Niasar A H, “Sensorless control of four switch, threephase brushless DC motor drives for low-cost applications,” Ph.D. dissertation, Dept. Electr. Eng., Iran Univ. Sci. Technol., Tehran, Iran, Dec. 2007.

[2] Pillay, Freere P, “Literature survey of permanent magnet ac motors and drives,” in Proc. IEEE IAS Rec., 1989, pp. 74–84.

[3] Lu C W, "Torque Controller for Brushless DC motors", IEEE Transactions on Industrial Electronics, 1999, 46, pp. 471–473.

[4] Ashwini M, Apte A, "Development of Brushless DC Motor Drive," Journal of Electrical and Electronics Engineering (ICAET-2014), PP 12-18.

[5] Kang K J, "Development of a highly efficient brushless dc motor utilizing both radial and axial air gaps", Journal of Applied Physics, 2012, 111.

[6] Nitin Sanadhya et al.," Reliability of Permanent Magnet Brushless D.C. Drives Using IGBT’s," International Journal of Innovative Research in Science, Engineering and Technology, 2013, 2 (3).

[7] Dasari R K, Madipalli S J and Javvadi S, "Power quality improvement of brushless permanent magnet and non-permanent magnet machines using DSTACOM," 2015 International Conference on Electrical, Electronics, Signals, Communication and Optimization (EESCO), Visakhapatnam, 2015, pp. 1-6.

[8] Chandra Rao et al., "Power Factor Correction in Two Leg Inverter Fed BLDC Drive Using Cuk Dc-Dc Converter," International Journal of Power Electronics and Drive Systems (IJPEDS), June. 2015, 6(2), p. 196-204.

[9] Tridibesh et al. "An efficiency optimization scheme for BLDC motor drive system," International Journal of Power Electronics and Drive Systems (IJPEDS), 2015, 6(4), p. 869-875.

[10] Srinivasan, K. et al. "Power Quality Analysis of Vienna Rectifier for BLDC motor Drive Application," International Journal of Power Electronics and Drive Systems (IJPEDS), 2016, 7(1), p. 7-16.

[11] P. Agrawal, et al., "Comparative Study of Fuzzy Logic Based Speed Control of Multilevel Inverter fed Brushless DC Motor Drive", International Journal of Power Electronics and Drive System, vol. 4, 2014.

[12] Sachin K. Kupati, MukulChavan, Sunil Bhattad, N. Arun ‘‘Average Current Mode Controlled Power Factor Correction Converter’’ IJST January 2015, 8(2).

[13] Jeya Selvan Renius et al., "Analysis Of Variable Speed Pfc Chopper Fed Bldc Motor Drive", IJST, DECEMBER 2014, 9(12), pp 2521-2527.


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