Real Implementation of a Single Sensor based PFC with Novel Converter Fed BLDC Motor Drive

Transcription

1 GRD Journals- Global Research and Development Journal for Engineering Volume 1 Issue 8 July 2016 ISSN: Real Implementation of a Single Sensor based PFC with Novel Converter Fed BLDC Motor Drive Sreehari. B St. Thomas College Of Engineering And Technology, Kerala, India Jenopaul. P St. Thomas College Of Engineering And Technology, Kerala, India Abstract This paper proposed scheme for the Sensor-less BLDC motor drive fed by a Zeta based PFC converter operating in DICM. The front end Zeta DC-DC converter maintains the DC link voltage to a set reference value. Switch of the Zeta converter is to be operated at high switching frequency for effective control and small size of components like inductors. A sensor-less approach is used to detect the rotor position for electronic commutation. A blind startup is used for starting the BLDC motor. A high frequency MOSFET of suitable rating is used in the front end converter for its high frequency operation whereas an IGBT s (Insulated Gate Bipolar Transistor) are used in the VSI for low frequency operation. The proposed scheme maintains high power factor and low THD of the AC source current while controlling rotor speed equal to the set reference speed. Keywords- Zeta Converter, BLDC, DICM, power factor correction I. INTRODUCTION The requirement of improved power quality of the AC mains is becoming essential for any appliance as imposed by the International PQ (Power Quality) standards like IEC The requirement of power factor above 0.9 and THD (Total Harmonic Distortion) below 5% for Class-D (under 600 W, <16 A, single phase) applications, recommends the use of improved power quality converters for BLDC (Brush Less DC) motor drive[1]. There are many AC-DC converter topologies reported in the literature to meet the recommended PQ standards.bldc motor when fed by an uncontrolled bridge rectifier with DC link capacitor results in highly distorted supply current which results in low PF (Power Factor) and high THD (Total Harmonic Distortion); hence various improved power quality AC-DC converters are used in these drives. BLDC motor is an ideal motor for low-medium power applications because of its high efficiency, high torque/inertia ratio, low maintenance and a wide range of speed control[2]. It consists of three phase windings on the stator and permanent magnets on the rotor. Being an electronically commutated motor, the commutation losses in the BLDC motor are negligible[3].two stage PFC converters are widely in practice in which first stage is used for the power factor correction which is preferably a boost converter and second stage for voltage regulation which can be any converter topology depending upon the requirement [4-7].This two stage topology is complex and results in higher cost and more losses; hence a single stage Zeta converter is proposed in this paper which is used for DC link voltage control and power factor correction. The operation is studied for a Zeta converter working in DICM (Discontinuous Inductor Current Mode) hence a voltage follower approach is used. A voltage follower approach requires a single voltage sensor for the DC link voltage regulation while in case of CCM (Continuous Conduction Mode); current multiplier approach is normally used [8-9].This DC-DC converter has to operate over a wide range of output DC voltage for the speed control of BLDC motor unlike many applications which require a constant DC link voltage. Moreover a sensor less control of BLDC motor is used, to eliminate the requirement of Hall Effect position sensors and making the drive more cost effective. All rights reserved by 69

2 II. PROPOSED SPEED CONTROL SCHEME OF SENSOR LESS BLDC MOTOR DRIVE Fig. 1: Proposed speed control scheme The proposed scheme for the Sensor less BLDC motor drive fed by a Zeta based PFC converter operating in DICM mode is shown in Fig. 1. The front end Zeta DC-DC converter maintains the DC link voltage to a set reference value. Switch of the Zeta converter is to be operated at high switching frequency for effective control and small size of components like inductors. A sensor less approach is used to detect the rotor position for electronic commutation. A blind startup is used for starting the BLDC motor. A high frequency MOSFET of suitable rating is use d in the front end converter for its high frequency operation whereas an IGBT s (Insulated Gate Bipolar Transistor) are used in the VSI for low frequency operation. The proposed scheme maintains high power factor and low speed. A voltage follower approach is used for the control of Zeta DC-DC converter operating in DICM. The DC link voltage is controlled by a single voltage sensor. Vdc (sensed DC link voltage) is compared with Vdc* (reference voltage) to generate an error signal which is the difference of Vdc* and Vdc. The error signal is given to a PI (Proportional Integral) controller to give a controlled output. Finally, the controlled output is compared with the high frequency saw tooth signal to generate PWM (Pulse Width Modulation) pulse for the MOSFET of the Zeta converter. Arate limiter is used to limit the stator current during step change in speed. A. Zeta Converter Fig. 1(a): Zeta Converter 1) Principle of Operation When analyzing Zeta waveforms it shows that at equilibrium, L1 average current equals IIN and L2average current equals IOUT, since there is no DC current through the flying capacitor CFLY. Also there All rights reserved by 70

3 Fig. 2: Zeta converter during MOSFET ON time 2) Stage-1[M1ON] The switch M1 is in ON state, so voltages VL 1 and VL 2 are equal to Vin. In this time interval diode D1 is OFF with a reverse voltage equal to - (V in + V O ). Inductor L 1 and L 2 get energy from the voltage source, and their respective currents IL1 and IL2 are increased linearly by ratio Vin/L1 and Vin/L2 respectively. Consequently, the switch current IM 1 =IL 1 +IL 2 is increased linearly by a ratio Vin/L, where L=L 1.L 2 / (L 1 +L 2 ). At this moment, discharging of capacitor C fly and charging of capacitor C 0 take place. 3) Stage-2 [M1 OFF] There is no DC voltage across either inductor. Therefore, CFLY sees ground potential at its left side and VOUT at its right side, resulting in DC voltage across CFLY being equal to VOUT.In this stage, the switch M1 turns OFF and the diode D1 is forward biased starting to conduct. The voltage across L1 and L2 become equal to Vo and inductors L1 and L2 transfer energy to capacitor Cfly and load respectively. The current of L1 andl2 decreases linearly now by a ratio V0/L1 and V0/L2, respectively. The current in the diodeid1=il1+il2 also decreases linearly by ratio V0/L. At this moment, the voltage across switch M1 is VM=Vin + V0. Figure 4 shows the main wave forms of the ZETA converter, for one cycle of operation in the steady state continues mode. III. DESIGN OF PFC WITH ZETA CONVERTER The proposed drive system is designed for Zeta converter as PFC converter fed BLDC motor drive operating in DICM. The output inductor value is selected such that the current remains discontinuous in a single switching cycle. The average input voltage Vin after the rectifier is given as [1], Vin = 2 2.V S / π (1) where VS is the rms value of the supply voltage. Fig. 2. Waveform of output inductor current in DICM control The duty ratio D for the Zeta converter (buck-boost) is given as, D = V dc / (V in + V dc ) (2) where Vdc represents the DC link voltage of Zeta converter.if the permitted ripple of current in input inductor Li and output inductor Lo is given as ili and ilo respectively, then the inductor value Li and Lo are given as [3-4], Li = D.Vin / {fs.( ili)} (3) Lo = (1-D) Vdc / {fs.( ilo)} (4) where fs is the switching frequency. For the critical conduction mode, ilo = 2.Idc i.e. Lo(critical) = (1-D) Vdc / {fs. (2.Idc)} (5) The value of intermediate capacitor C1 is given as [6], C1 = D.Idc / {fs. ( VC1)} (6) where VC1 is the permitted ripple in C1.The value of DC link capacitor Cd is given as [8-11], Cd = Idc / (2.ω. Vdc) (7) where ω = 2π fl; fl is the line frequency. Equations (1)-(7) represent the design criteria of the Zeta converter in discontinuous conduction mode. B. Performance 1) Efficiency- Open Loop Zeta Converter Input Voltage Output Voltage Output Power Efficiency in % All rights reserved by 71

4 2) Efficiency-Closed Loop Zeta Converter Input Voltage Output Voltage Output Power Efficiency in % C. Modfied DC- DC Converter Bi directional DC DC Converters are useful in applications where power transfer takes place in either direction i.e power transfer between two DC DC sources. These converters are widely used in hybrid electric vehicles, photovoltaic hybrid power Systems, Fuel- cell hybrid power systems, uninterruptible power systems and battery charges. Many bi directional DC DC Converter topologies are proposed in literature out of the available models, bi - directional DC DC flyback converters are found to be simple in structure and easy in control. It is observed that the switches used in the switches used in these converters subjected to high voltage stress due to leakage energy released by transformer during energy transfer phase. For minimization of voltage stress of converter switches due this leakage energy release by transformer literature suggests energy regeneration techniques. These techniques suggest that the leakage inductor energy is recycled by clamping the voltage stress on the converter switches. In some of the literature isolated bi directional DC DC converters are proposed, these converter technologies includes half bridge, full bridge types. These technologies make use of adjustable turns transformers as a result of that these converters provide high step up and step down voltage gains. For non isolated applications non isolated bi-directional DC DC Converters are suggested. These converters include topologies like buck / boost, multilevel level converters, Three level Converters, Sepic / Zeta, Switched capacitor and coupled inductors. Three Level and Multi Level converters suffer with low step up and step down voltage gains. Sepic / Zeta converters uses two stages for power conversion, this results in more losses as a result conversion efficiency decreases. Multi-level type converters make use of magnetic less converter concept, and require more number of switches for energy conversion. This makes this topology with complicated structure and control circuit. If more step up and step down voltage gains are required the number switches are to be increased. This makes the control more complicated. The switched capacitor and coupled inductor converters can provide higher step up and step down voltage gains. And the voltages appearing across switches used in these topologies can be made minimum. Fig. 3.1: Circuit Diagram of Conventional Bi directional DC DC Converter. Figure 3.1 Show conventional DC DC converter with two switches S1 and S2. A modification is made to the above circuit such that the inductor is replaced with a coupled inductor and one more switch is added. New configuration is shown in figure 2. The preceding sections will discuss the modeling issues involved, results obtained. Fig. 3.2: Proposed DC DC Converter model diagram working as boost converter. All rights reserved by 72

5 D. Step Up Mode The proposed converter in step-up mode is shown in Fig 5. The pulse width modulation (PWM) technique is used to control the switches S1 and S2 simultaneously. The switch S3 is the Synchronous rectifier. 1) CCM Operation 1) Mode 1: During this time interval, S1 and S2 are turned on and S3 is turned off. The current flow path is shown in Fig 5(a). The energy of the low-voltage side VL is transferred to the coupled inductor. Meanwhile, the primary and secondary windings of the coupled inductor are in parallel. The energy stored in the capacitor CH is discharged to the load. Thus, the voltages across L1 and L2 are obtained as ul1=ul2=vl (8) By substituting above equations we get dil1(t)dt=dil2(t)dt=vl 1+k L, (9) 2) Mode-2: During this time interval S1 and S2 are turned on and S3 is turned off. The energy of the low-voltage side VL is transferred to the coupled inductor. Meanwhile, the primary and secondary windings of the coupled inductor are in parallel. The energy stored in the capacitor CH is discharged to the load. Thus, the voltages across L1 and L2 are obtained as il1=il2 ul1+ul2=vl VH (10) By substituting above equations we get dil1(t)dt=dil2(t)dt=vl VH2 1+k L, (11) By using the state-space averaging method, the following equation is derived from DVL 1+k L+ 1 D (VL VH)2 1+k L=0 (12) By simplifying we get GCCM(step up)=vhvl=1+d/1 D 2) DCM Operation 1) Mode 1: During this time interval, S1 and S2 are turned on and S3 is turned off. The operating principle is same as that for the mode 1 of CCM operation I L1p=I L2p=VLDTs /(1+k L) (13) 2) Mode 2: During this time interval, S1 and S2 are turned off and S3 is turned on. The low-voltage side VL and the coupled inductor are in series to transfer their energies to the capacitor CH and the load. Meanwhile, the primary and secondary windings of the coupled inductor are in series. The currents il1 and il2 through the primary and secondary windings of the coupled inductor are decreased to zero at t = t2. From eqn, another expression of IL1p and IL2p is given by IL1p=IL2p=(VH VL)D/2Ts2 1+k L (14) 3) Mode 3: During this S1 and S2 are still turned off and S3 is still turned on. The energy stored in the coupled inductor is zero. Thus, il1 and il2 are equal to zero. The energy stored in the capacitor CH is discharged to the load. From above equation, is derived as follows D2=2DVL/VH VL (15) From Fig, the average value of the output capacitor current during each switching period is given by IcH=12D2TsIL1p IoTs/Ts=1/2(D2IL1p Io) (16) By substituting above values we get IcH={D2VL2Ts/ 1+k L(VH VL)} VH/RH (17) Since IcH is equal to zero under steady state, above equations can be rewritten as follows: D2VL2Ts/ 1+k L(VH VL)=VH/RH (18) Then, the normalized inductor time constant is defined as TLH L/RHTs=Lfs/RH (19) where fs is the switching frequency. Substituting above equations we get, the voltage gain is given by GDCM(step up)=vh/vl=1/2+ ( 14+D2(1+k)τLH) (20) 3) Boundary Operating Condition of CCM and DCM When the proposed converter in step-up mode is operated in boundary conduction mode (BCM), the voltage gain of CCM operation is equal to the voltage gain of DCM operation. From above equations, the boundary normalized inductor time constant τlh,b can be derived as follows τlh,b=d(1 D) 2/ 2 (1+k) (1+d) (21) The curve of τ LH,B is plotted in Fig. If τlh is larger than τlh,b, the proposed converter in step-up mode is operated in CCM. All rights reserved by 73

6 IV. SIMULATION RESULTS Fig. 4: Simulation Results A. Simulink Model of Proposed System 1) Output Voltage Fig. 5: Output Voltage 2) Dc Link Voltage Fig. 6: DC Link Voltage All rights reserved by 74

7 3) Rotor Speed Fig. 7: Rotor Speed 4) Stator Current & EMF Fig. 8: Stator Current & EMF 5) Inductor Currents a) Hardware Implementation Fig. 9: Hardware Implementation All rights reserved by 75

8 V. CONCLUSION A simple control using a voltage follower approach has been used for voltage control and power factor correction of a PFC Zeta converter fed BLDC motor drive. A novel scheme of speed control using a single voltage sensor has been proposed for a fan load. A sensor less operation for the further reduction of position sensor has been used. A single stage PFC converter system has been designed and validated for the speed control with improved power quality at the AC mains for a wide range of speed. The performance of the proposed drive system has also been evaluated for varying input AC voltages and found satisfactory. The power quality indices for the speed control and supply voltage variation have been obtained within the limits by International power quality standard IEC The proposed drive system has been found a suitable candidate among various adjustable speed drives for many low power applications. REFERENCES [1] Limits for Harmonic Current Emissions (Equipment input current 16 A per phase), International Standard IEC , [2] B. Singh, B. N. Singh, A. Chandra, K. Al-Haddad, A. Pandey and D.P. Kothari, A review of single-phase improved power quality AC-DC converters, IEEE Transactions on Industrial Electronics, vol. 50, no. 5, pp , Oct [3] T. Kenjo and S. Nagamori, Permanent Magnet Brushless DC Motors, Clarendon Press, Oxford, [4] T. J Sokira and W. Jaffe, Brushless DC Motors: Electronic Commutation and Control, Tab Books, USA, [5] R. Handershot and T.J.E Miller, Design of Brushless Permanent Magnet Motors, Clarendon Press, Oxford, [6] J. F. Gieras and M. Wing, Permanent Magnet Motor Technology Design and Application, Marcel Dekker Inc., New York, [7] N. Mohan, T. M. Undeland and W. P. Robbins, Power Electronics: Converters, Applications and Design, John Wiley and Sons Inc, USA,1995. [8] S. Singh and B. Singh, "Voltage controlled PFC Zeta converter based BLDC MOTOR drive for an air-conditioner," 2010 International Conference on Industrial and Information Systems (ICIIS), pp , 29th July st Aug [9] Bhim Singh, B.P.Singh and Sanjeet Dwivedi, AC-DC Zeta Converter for Power Quality Improvement of Direct Torque Controlled PMSMrive, Korean Journal of Power Electronics, Vol. 6, No. 2, pp , April [10] J. Uceeda, J. Sebastian and F.S. Dos Reis, Power Factor Preregulators Employing the Flyback and Zeta Converters in FM Mode, in Proceedings of IEEE CIEP 96, 1996, pp [11] D.C. Martins, Zeta Converter Operating in Continuous Conduction Mode Using the Unity Power Factor Technique, in Proceedings of IEE PEVSD 96, 1996, pp All rights reserved by 76