A Modified Bridgeless LUO Converter For Simultaneous Torque ...

Journal of Engineering Technology Volume 6, Special Issue (Emerging Trends in Engineering Technology) March. 2018, PP. 71-81

A Modified Bridgeless LUO Converter For Simultaneous Torque Ripple Minimization And Power Factor Correction In BLDC Motors

Venkata Reddy Kota, Bapayya Naidu Kommula Department of Electrical and Electronics Engineering, University College of Engineering Kakinada, Jawaharlal Nehru Technology University Kakinada, India. Abstract. Brushless DC motors (BLDCM) suffer from the higher torque ripples and low supply power factor. Recently, Bridgeless Luo converter (BLLC) is used for BLDC motor to improve the supply power factor. In this paper, a modified BLLC is proposed for BLDCM for simultaneous supply power factor enhancement and torque ripple minimization of the motor. The phase current ripples and hence the ripples in commutation torque of the motor are minimized by inserting additional voltage sources in non-commutating phases of the motor. The existing BLLC is modified by inserting controlled voltage source in non-commutating phase. The proposed converter configuration for BLDCM is developed and simulated in Matlab/Simulink environment to examine its performance. The performance is also compared with conventional BLLC at different speeds. Results show the significant reduction in motor torque ripple and improvement in the supply power factor near to unity. Keywords: Brushless DC motor (BLDCM), Bridgeless Luo Converter (BLLC), Power Quality (PQ), Power Factor Correction (PFC) Converter, Torque Ripple Minimization.

1. Introduction Nowadays, BLDC motors are more popular in many areas such as automobile industry, medical equipment, space applications and robotics. The main attracting features of these motors are higher power density, compact in size, higher efficiency and higher reliability [1]. BLDC motors are similar to three phase synchronous motors but consist permanent magnets on the rotor. In a BLDCM, three phase windings are kept in stator which is energized by a voltage source inverter (VSI). The rotor contains permanent magnets. Working of BLDC motor depends on the position of the rotor. Hall Effect sensors are utilized for finding out the rotor’s position. Due to the absence of brushes and commutator in BLDC motors, the complications like sparking, wear and tear of brushes are eliminated [2]. Brushless DC motors (BLDCM) suffer from the higher torque ripples and low supply power factor. Hence, there is need to develop effective control schemes to reduce torque ripples and to improve the supply power factor. A typical BLDCM drive consists of a front-ended diode bridge rectifier (DBR) with a dc link capacitor and a VSI controlled by PWM pulses. This type of arrangement gives a lower power factor (PF) around 0.75 and input current consists of higher harmonics at supply mains. According to the international standards of power quality (PQ) like IEC 61000-3-2, these PQ indices are not acceptable. Hence, power factor correction (PFC) converters are to be used for BLDCM to improve the PQ at AC mains. These PFC converters run either in continuous conduction mode (CCM) or discontinuous conduction mode (DCM). The CCM provides minor stress over a converter switch which needs two regulating loops for voltage and current regulation to attain 71


DC link voltage management with power factor correction [3]. This necessitates three sensors. It is an expensive choice and suitable for higher power applications. For lower power applications, it is normally operated in DCM. Several PFC based converter configurations feeding BLDCM have already documented in a literature [4-5]. In general, most extensively utilized configuration is boost-PFC based converter. This approach has higher switching losses. Hence, it significantly decreases the effectiveness of the whole system. Furthermore, it entails more sensors and control has become complex. In [6], a PFC based adjustable bridgeless converter topology has proposed to regulate the speed of BLDCM and also it enhances the power factor (PF) at AC mains. This topology needs only one voltage sensor. To enhance the PF at AC supply mains and regulate the speed of BLDCM, a modified Zeta converter has intended in [7]. A new PFC converter has implemented in [8] which is fed to BLDC motor for low power home appliances. To get the improved power factor, a bridgeless CUK converter has intended in [9] that is fed to BLDC motor. This converter has lower conduction losses. A bridgeless Luo Converter topology has designed for feeding the BLDC motor in [10]. This converter has improved the power factor nearly unity and achieves the smooth control of BLDC motor. It is required to lessen the torque ripple in BLDCM to eradicate vibration, acoustic noise, and unwanted speed ripple. Thus, from the last few decades, researchers are trying to lessen these kinds of unwanted torque ripple in BLDCM. A cascaded H bridge topology has designed in [11] for suppressing the torque ripple in BLDCM. For low cost applications, an approach of commutation torque ripple minimization has presented in [12]. A different strategy for cutting down torque ripple resulting from commutation of phase current has proposed in [13]. A new converter topology has designed in [14] to mitigate the torque ripple in BLDCM. To obtain the torque ripple free operation of BLDC motor, a new current synthesizing method has developed in [15]. A new control algorithm has proposed in [16] to suppress the torque ripple. A new strategy for minimizing the torque ripple in BLDCM has discussed in [17]. To regulate the torque of BLDC motor instantaneously, a current source chopper has proposed in [18]. A hybrid converter topology has designed in [19] to obtain the minimum torque ripple in BLDC motor. Some of these methods require sensor circuits and implementation in real time is difficult which makes the system complex. This paper proposes a modified BLLC topology combined with a torque ripple minimization scheme. This scheme has exact formulae for torque ripple minimization and it works effectively for both low and high speed operation of motor. This low cost topology simultaneously improves the supply power factor and minimize the torque ripple of the motor. The approach provided in this paper is according to fact that the compensation of current waveform considerably minimizes the torque ripple of BLDCM. The compensation of currents is achieved by placing additional voltage sources in non-commutating phases. 2 Proposed BLLC Scheme with Torque Ripple Minimization The basic structure of the proposed BLLC fed BLDCM drive with torque ripple minimization technique is depicted in Figure 1. The proposed BLLC is provided with single phase supply along with a filter. Its output is connected to voltage source inverter (VSI) which drives BLDCM. This proposed BLLC is working in DCM which acts as PF corrector inherently. By altering VSI’s dc link voltage, the speed of BLDCM can be regulated. It requires only one voltage sensor. This makes it possible for VSI to work at an elementary switching frequency.

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3 phase VSI

Modified BL Luo Converter Lf

Phase A

D1

1φ AC Supply

X Y Y X

Lo1 Sw1

AC

T1

Cf

D2

Lo2

S1

S3

S5 X Y

Phase B Sw2 Li2

Li1

Dp

Cd

C1

Dn

C2

x Vcomn y

S4

S6

S2

Y X

T2

Phase C

X Y Y X

6 To S1 - S6 Sw1

BLDC Motor

Sw2

PWM Generator

Sawtooth generator

Voltage controller

Electronic commutation

Vdc

3

Hall Effect Rotor position Sensing

_

Vdc*

ω*

Ref. voltage (Reference speed) Generator

Figure. 1. Basic structure of the proposed control scheme for BLDC motor Therefore, it has minimal switching losses. These switching losses are substantially lower compared to PWM dependent VSI. Here, to minimize the ripple in torque, along with this BLCC an additional voltage source is placed in non-commutating phases of BLDCM. At the non-commutating period of time, switches T1, T2 are on and capacitor is charged up to its preset value of voltage. Later on, T 1, T2 are off, capacitor is separated from the primary voltage source. At commutation period, capacitor could be placed in series with non-commutating phase using suitable switches. The potential of capacitor can be determined from acceptable voltage drop for the period of commutation. To attain the dc link voltage management with power factor improvement, the control unit of proposed BLLC produces PWM signals for BLLC switches (Sw1 and Sw2). Voltage follower method is used for BLLC operation in DCM. Reference dc link voltage V *dc can be produced as V *dc  k v w *

(1)

where k v denotes voltage constant, w * denotes reference speed. The difference between V *dc and actual dc link voltage V dc is represented as voltage error (Ve) i.e. Ve (k 0 )  V* dc (k 0 )  Vdc (k 0 )

(2)

For getting controlled voltage output (Vcc), Ve is given to PI controller. V co ( k0 )  V co ( k0  1 )  k p { V e ( k0 )  V e ( k0  1 )}  k i V e ( k0 )

(3)

Here, Kp , Ki are PI controller’s proportional and integral gains respectively. Lastly, by comparing Vcc with saw tooth wave ( m ds ) a high frequency PWM signals are generated as given below If mds ( t )  Vco ( t ) then S w1  S w 2 = ‘ON’

(4)

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If mds ( t )  Vco ( t ) then S w1  S w 2 = ‘OFF’ Sw1, Sw2 denotes switching pulses of BLLC.

3.

(5)

Proposed Torque Ripple Minimization Scheme

It is assumed that BLDC motor is working in the typical two-phase conduction mode for circuit analysis of ripple in commutation torque. The torque generated by motor is expressed as follows

T

E a i a  E b ib  E c i c 2E p I o   

(6)

Here, Ep denotes magnitude of back EMF, Io is the magnitude of phase current,  is speed of motor. At the time of conduction, back EMF is considered to be constant for analyzing the commutation torque ripple, phase ‘A’ current is assumed to be decaying, phase C current is rising via a freewheeling diode and non-commutating phase is B. Each phase voltage of stator winding is expressed as below equations during commutation

v an  R i a  E a  L

di a di di ; v bn  R i b  E b  L b and v cn  R i c  E c  L c dt dt dt

(7)

Phase currents of motor are expressed as follows with negligible voltage drops across switches [20]

1 2 di 1 4 di 2 di a  2 Ep E p and c   E p ; b  V dc  V dc  V dc  dt 3 L 3L dt 3L 3L dt 3L 3L

(8)

As per the above equations, current in non-commutating phase can be increasing, decreasing or being constant. Hence, at the time of commutation, the slope of current in non-commutating phase at low speed and high speed is positive and negative respectively. Thus, this slope can be compensated by injecting an extra voltage to the non-commutating phase during commutation, at both speeds. The objective of this approach is to determine excess voltage source that is certainly placed in noncommutating phase within a duration of commutation period of time to minimize current ripples. When commutation shifts from phase A to C, the phase current waveforms are depicted in Figure 2. Here, three cases describe the variation of phase current at the time of commutation. Whenever current in noncommutating phase is similar to Figure 2a, the rate of change of phase A current greater than rate of change of phase C current i.e., di a  di c . Thus, Vdc1< 4Ep and di b  0 , i.e., current in phase B reduces. Therefore, an excess dt dt dt voltage source needs to strengthen non-commutating phase current. When, di a  di c the value of dc link dt dt di voltage as Vdc1>4Ep and b  0 , i.e., the phase B current rises as shown in Figure 2b. Hence, an excess voltage dt

source needs to be made weaker the non-commutating phase current. If the increasing rate and decreasing rate of both phase currents are equal i.e., di a  di c the magnitude of Vdc is equal to 4Ep and di b  0 i.e., current in dt

dt

dt

phase B is constant as depicted in Figure 2c. In this case, this excess voltage source is not desired.

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In this work, the additional voltage injected in non- commutating phase is supplied from the modified BLLC converter. The amount of the voltage injected in non-commutating phase during commutation interval is given by [21] ib ib I I 3RI o  Vdc1 ktcom  (9) Vdc 2  2 E p     2  2 4  t

t

Where Ep is back emf, R is resistance of phase winding, Io is the magnitude of phase current, Vdc1 is the dc ia ic voltageiainput to VSI, tcom is icthe commutation-Iperiod and k is (a) constant  k  EPN  . -I (a) 10



ib

ib

I

I

-I

ia

ia

t

t

ic (a)

ib

I

t

-I



ia

-I

ic

ic

(b)

(b)

I

ib ib

II

ib Ea

t

-I

Ec

t ia

-I

Eb

ia

-I

ia (b)

ic

ic

(c)

ic

ia

ic

Time

(c)

(d)

ib

I

ib

t

Figure. 2. During commutation, variations of current at (a) Vdc14Ep (c) Vdc1 = 4Ep (d) ideal waveforms for eradicating ripple in commutation t 4. Simulation Results and Analysis ia ic -I (c) The essential simulation models are developed and the analysis is carried out in Matlab/Simulink environment. The specifications of BLDC motor and BLL converter are given in Table 1. Initially, the BLDC motor fed from Diode Bridge Rectifier (DBR) is simulated and the supply current waveform along with its harmonic spectrum are shown in Figure 3. From Figure 3, it can be observed that the supply current is deviated with a THD of 25.14% with a very low power factor of 0.73. Table 1. BLDC motor and BLLC parameters Bridgeless Luo Converter

BLDC motor Parameter

Value

Parameter

Value

Resistance per phase

14.56 Ω

Input inductors ( Li1 and Li2)

40 H

Inductance per phase Rated speed Rated torque No. of poles

25.71 mH 2000 rpm 1.5 Nm 4

Intermediate capacitors Output inductors (Lo1 and Lo2) DC link capacitor Filter capacitor Filter inductor

0.44 F 1.78 mH 2200 F 330 nF 3.77 mH 75


(a) (b) Figure. 3. (a) Supply voltage (b) Supply current (c) Harmonic spectrum of supply current for traditional configuration

Figure. 4. Simulink model of BLCC fed BLDCM drive Now, the DBR is replaced with the basic Bridgeless Luo converter and simulations are carried out as depicted in Figure 4. Figure 5a describes waveform of current at supply mains. From this figure, it can be noticed that current is in sinusoidal shape which is also in phase with voltage and also attained an enhanced power factor of 0.98 at supply mains. The amount of harmonics in supply current is also less and %THD of supply current is found to be reduced to 1.65% as depicted in Figure 5b.

(a) (b) Figure. 5. (a) Supply current (b) Harmonic spectrum of supply current for proposed BLLC fed BLDCM The torque ripple in BLDC motor can be calculated by Torque Ripple%  

T max  T min 

 100

(10)

T avg

When the DBR is used to feed the BLDC motor, the amount of ripple in torque waveform is 48.27% which is very high. This makes the BLDC motor not suitable for several applications. Figure 6a depicts stator current of phase A and Figure 6b depicts electromagnetic torque attained at a speed of 2000 rpm when the BLLC is 76


used. It can be noticed that ripples in torque are reduced compared to conventional method and % ripple is observed as 37.31% which is also not desirable for smooth operation of BLDC motor. To overcome this, BLLC converter is modified with an additional voltage in the non-commutating phases of the motor as shown in Figure 1.

(a) (b) Figure. 6. (a) Stator phase ‘A’ current (b) Torque In this approach, the torque ripple of the motor is minimized by inserting an excess voltage source in non commutating phase in addition to PQ improvement. The overall effectiveness of this approach is evaluated in Matlab/Simulink environment. By employing this proposed topology, current is in phase with supply voltage and gives an improved power factor of 0.98. The supply current contains less amount of harmonics. The %THD of supply current found to be 1.61%. The line voltage vab of inverter is shown in Figure 7a and the line voltage vab of BLDCM, which is the summation of inverter output voltage and additional voltage inserted is shown in Figure 7b. This recommended approach obviously signifies that ripple in stator phase current is reduced and is very much nearer to a shape of a rectangle as depicted in Figure 8a. For a reference load torque of 1.5 Nm, it can be observed that torque ripple can be substantially reduced and attains an effective torque response. In this proposed BLCC fed BLDCM drive with torque ripple minimization scheme, by placing an excess voltage source in the non-commutation phase of BLDCM, the subsequent ripple in torque is significantly minimized. As a result, a smooth response of torque is attained with a minimal torque ripple. The zoomed view of torque response from 1.47 to 1.48 seconds for a reference torque of 1.5 Nm is depicted in Figure 8b. From this figure, it can be noticed that the amount of ripple in torque is very low which is of 4.56%. It is very less as compared to techniques as mentioned earlier and makes the BLDC motor suitable for several applications that require smooth torque.

(a) (b) Figure .7. (a) Line voltage vab (b) Line voltage vab after adding an extra voltage source

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(a) (b) Figure. 8. (a) Stator phase ‘A’ current (b) Torque of the BLDCM with proposed scheme Here, the effectiveness of the proposed scheme is compared with conventional method with DBR. The commutation in stator phase currents with conventional and the proposed schemes are depicted in Figure 9. It is seen from Figure 9a that the rising and falling rates of phase currents during commutation are not equal and causing ripples in the non-commutating phase current. These current ripples are causing the ripples in the torque of the motor. In the proposed scheme, due to the injection of applied voltage in non-commutating phase, the rising of incoming phase current and falling of outgoing phase current occurs at the same time during commutation. Hence, there is less impact on non-commutating phase current of motor as depicted in Figure 9b and results low commutation torque ripple. Figure 10 depicts comparison diagram of torque ripple for both configurations of basic BLLC and proposed BLCC with torque ripple minimization scheme for a load torque of 1.5 Nm at a reference speed of 2000 rpm. In conventional control, the torque of motor varies between 1.2 to 1.8 Nm. With the proposed scheme, as depicted in Figure 10, the torque of motor is at the reference value of 1.5 Nm without much ripple. Table 2 gives the supply power factor and the amount of torque ripple with three different schemes at various speeds with a constant load torque of 1.5 Nm. From Table 2, it can be noticed that the proposed scheme not only improve that supply power factor but also significantly minimizes the torque ripple of the motor at all speeds.

Figure. 9. Commutation of phase currents in (a) conventional scheme (b) proposed scheme

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Figure. 10. Comparison chart of torque ripple for both methods Table 3. Comparison of different schemes Speed of Description BLDC motor (rpm) Supply 500 power 1000 factor 2000 500 Torque 1000 Ripple (%) 2000

Conventional scheme with DBR 0.749 0.751 0.753 28.7 37.8 48.27

BLLC fed BLDC motor 0.972 0.9801 0.9803 26.89 32.9 37.31

Proposed scheme fed to BLDC motor 0.974 0.981 0.985 1.32 2.89 4.56

5. Conclusion This paper proposes a modified BL Luo converter fed BLDC motor drive with torque ripple minimization approach by inserting an excess voltage source in non-commutating phase at the time of commutation. This BLLC is functioning in DCM and behaves as natural power factor corrector. In torque ripple minimization approach, an excess voltage is supplied by a capacitor that is connected to the BL Luo converter. This capacitor discharges at the time of commutation interval and charges in non-commutation periods through a suitable switching circuit. The proposed BLLC fed BLDCM drive is designed and its overall effectiveness is simulated in Matlab/Simulink environment to get an enhanced PQ for an extensive range of speed regulation with minimal torque ripple. Simulation results shows significant reduction in commutation torque ripple even at higher speeds and very good improvement in supply power factor near unity. Finally, this proposed drive attains an acceptable performance and it is suitable for most of the applications. References H. Li, S. Zheng, H. Ren, "Self-correction of commutation point for high-speed sensorless bldc motor with low inductance and non-ideal back emf", IEEE Trans. Power Electron., vol. 32, no. 1, pp. 642-651, 2017. [2]. J. Fang, X. Zhou, G. Liu, "Precise accelerated torque control for small inductance brushless dc motor", IEEE Trans. Power Electron., vol. 28, no. 3, pp. 1400-1412, 2013. [1].

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Singh, B., Singh, S., Chandra, A., Al-Haddad, K.: ‘Comprehensive study of single-phase AC–DC power factor corrected converters with high-frequency isolation’, IEEE Trans. Ind. Inf., 7, (4), pp. 540–556, 2011. [4]. S. S. Bharatkar, R. Yanamshetti, D. Chatterjee and A. K. Ganguli, “Dual-mode switching technique for reduction of commutation torque ripple of brushless dc motor”, IET Electr. Power Appl., vol.5, no.1, pp.193-202, 2011. [5]. H. A. Toliyat and S. Campbell, DSP-based Electromechanical Motion Control, CRC Press, New York, 2004. [6]. V. Bist, B. Singh, "An adjustable speed PFC bridgeless buck–boost converter fed BLDC motor drive", IEEE Trans. Ind. Electron., vol. 61, no. 6, pp. 2665-2677, 2014. [7]. V. Bist, B. Singh, "A reduced sensor configuration of BLDC motor drive using a PFC based modifiedzeta converter", IET Power Electron., vol. 7, no. 9, pp. 2322-2335, 2014. [8]. V. Bist; B. Singh, “A PFC-Based BLDC Motor Drive Using a Canonical Switching Cell Converter”, IEEE Trans. Ind. Inf., vol. 10, no. 2, pp. 1207-1215, 2014. [9]. V. Bist, B. Singh, "A unity power factor bridgeless isolated Cuk converter-fed brushless DC motor drive", IEEE Trans. Ind. Electron., vol. 62, no. 7, pp. 4118-4129, 2015. [10]. V. Bist, B. Singh, “Power Factor Correction in Bridgeless-Luo Converter-Fed BLDC Motor Drive”, IEEE Trans. Ind. Appl., vol. 51, no. 2, pp. 1179-1188, 2015. [11]. M. A. Doss, E. Premkumar, G. R. Kumar, J. Hussain, "Harmonics and torque ripple reduction of brushless dc motor (BLDCM) using cascaded H-bridge multilevel inverter", Proc. 2013 Int. Conf. Power Energy Control, pp. 296-299, Feb. 2013. [12]. R. Carlson, M. Lajoie-Mazenc, and J. C. D. S. Fagundes, “Analysis of torque ripple due to phase commutation in brushless DC machines,”IEEE Trans. Ind. Appl., vol. 28, no. 3, pp. 632–638, May/Jun. 1992. [13]. Y. Liu, Z.Q. Zhu, and D. Howe, “Commutation torque ripple minimization in direct torque controlled PM brushless DC drives,” IEEE Trans. Ind. Appl., vol.43, no.4, pp. 1012 -1021, July/Aug.2007. [14]. V. Viswanathan, S. Jeevananthan, "Approach for torque ripple reduction for brushless dc motor based on three-level neutral-point-clamped inverter with dc-dc converter", IET Power Electron., vol. 8, no. 1, pp. 47-55, 2015. [15]. G. Buja, M. Bertoluzzo, R. K. Keshri, "Torque ripple-free operation of PM BLDC drives with petal-wave current supply", IEEE Trans. Ind. Electron., vol. 62, no. 7, pp. 4034-4043, Jul. 2015. [16]. T. Sheng, X. Wang, J. Zhang, Z. Deng, "Torque-ripple mitigation for brushless dc machine drive system using one-cycle average torque control", IEEE Trans. Ind. Electron., vol. 62, no. 4, pp. 2114-2122, Apr. 2015. [17]. H. K. S. Ransara, U. K. Madawala, "A torque ripple compensation technique for a low-cost brushless DC motor drive", IEEE Trans. Ind. Electron., vol. 62, no. 10, pp. 6171-6182, Oct. 2015. [18]. X. Wang, X. Wang, T. Fu et al., "Predictive instantaneous torque control for disc coreless permanent magnet synchronous motor with the current source chopper", IEEE Trans. Power Electron., vol. 30, no. 12, pp. 7100-7112, 2015. [19]. V. vaiyapuri, J. Seenithangom, “Hybrid converter topology for reducing torque ripple of BLDC motor”, IET Power Electron., vol. 10, no. 12, pp. 1572-1587, 2017. [20]. R. Carlson, M. Lajoie-Mazenc, and J. C. D. S. Fagundes (1992), “Analysis of torque ripple due to phase commutation in brushless DC machines,” IEEE Trans. Ind. Appl., vol. 28, no. 3, pp. 632–638. [3].

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[21]. Y.

Sangsefidi, S. Ziaeinejad, A. Shoulaie (2011), Torque Ripple Reduction of BLDC Motors by Modifying the Non-Commutating Phase Voltage, International Conference on Electrical, Control and Computer Engineering Pahang, Malaysia, pp 308-312.

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