Department of Electrical Engineering, Syed Babar Ali School of Science and Engineering. Lahore University of Management Sciences (LUMS), Lahore, Pakistan.
Solar assisted, enhanced efficiency, induction motor EV drive with soft phase conversion Muhammad Awais, Muhammad Anees, Nauman Zaffar Department of Electrical Engineering, Syed Babar Ali School of Science and Engineering Lahore University of Management Sciences (LUMS), Lahore, Pakistan {15060039, 16060066, nauman.zaffar}@lums.edu.pk
Abstract— Inclusion of renewable energy resources in the energy mix is one of the fastest growing trends in both developing and industrialized societies to reduce dependency on fossil fuels and other non-renewable resources. This work proposes the design of solar assisted motor drive that can be used with single-phase (1φ) or three-phase (3φ) induction motors with a soft phase conversion without requiring a change in hardware. The complete system allows enhanced efficiency operation through regenerative braking and a proposed architecture that allows utilization of available solar energy at all times. The system is evaluated with battery bank having simultaneous charging capability through EV mounted solar PV, Grid, and energy recovered from the induction motor during regenerative braking. The overall system consists of a MPPT SEPIC converter for power extraction from solar PV, A bi-directional buck/boost converter to regulate the DC link voltage and a dual-capability (1φ/3φ) induction motor drive. A constant V/f (volts/Hz) control with voltage-fed SPWM inverter is used for the speed control of induction motor for simplicity of control and slow variations anticipated in the experimental setup not necessitating vector control of drive. The application considered in this paper is electric vehicle but inductions motor drives (1φ/3φ) have widespread use in many other applications. Keywords—Electric Vehicle, VFD, induction motor drive, V/f control, SPWM, Bidirectional converter, SEPIC converter
I. INTRODUCTION With growing energy demand, utilization of renewable energy resources is one of the emerging trends to move towards a cleaner energy mix. The declining cost of solar PV also makes it increasingly more attractive as a choice, especially in regions with high solar potential. It is also a particularly attractive choice in developing countries with a current energy generation mix dependent on non-indigenous fuels. Pakistan is one such developing country with high potential of the different renewable resources. Comparing the major renewable energy resources such as wind, solar and hydroelectric, Solar PV is the most scalable and low cost solution for clean energy production at a household level with some areas of Pakistan like Jacobabad in Sindh province having a minimum energy density of 4.45 kWh/m2/day; which is higher than the world average solar intensity (3.61kWh/m2/day) [1]. Along with a trend of increasing energy mix of renewable resources, the focus is also on the efficient and sustainable utilization of available energy. In this regard, a lot of development has occurred in the realm of electric vehicles (EVs). Solar electric vehicles thus combine the best of both generation and efficient utilization of energy with a huge
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positive impact in reducing air pollution, global warming and addressing the rapid decrease of the fossil fuel resources [2]. The proposed architecture of solar PV assisted electric vehicle further includes batteries for energy storage and converters with optimal control for efficient utilization of available energy along with the recovery of energy during braking. Considerable efforts are being made in electric vehicles for optimal selection of motors based on required specifications such as high torque/inertia and power/weight ratios, high efficiency, low cost and high reliability. Prior work suggests that PMSM, BLDC, switch reluctance motor (SRM) and induction motor (IM) are good options with induction motor a good choice due to reliability, low cost, low maintenance and ready availability.[3] The control of induction motors is significantly more complicated in comparison to other motor types but, due to rapid development in power electronics and digital control, that complexity is no longer a barrier in the development of electric vehicle drives. Induction motors are generally more suitable for industrial and motor drive applications. Presently, the induction motor drive is the most popular technology among the commutator-less motor drives as its advantages far exceed the benefits provided by other motors [4]. Variable speed control in many applications such as conveyer belts, packaging industry and especially in EV applications have different control requirements. Different speed control techniques have different relative merits in ease of control, complexity and dynamic response. The simplest of these control techniques is constant V/f (volts/Hz) control with voltage-fed SPWM inverter which is widely used for speed control of induction motors [5, 6]. The constant V/f control provides good running and transient performance with reduced complexity and regenerative braking from zero to above base speed. Single-phase induction motor drives are inherently more complicated and are explored because of the widespread use in many applications and ready availability of motors at low power ratings for retrofit applications. One important consideration for the work presented in this paper for motor drivers is to ensure enhanced efficiency. Our proposed design utilizes a bidirectional dc-dc converter to boost the battery voltage level at the dc-link which both provides the requisite voltage for inversion to full rated ac voltage and reduces the current along with the associated losses. This bidirectional DC-DC converter provides reverse power flow to the battery storage during regenerative braking, increasing the overall efficiency of operation. These characteristics of the bi-
directional converter make it a better choice for power conversion. The roof of the EV is covered with solar PV panels that keep the car cool thereby reducing the need for air-conditioning. In addition, the energy available through these panels supplements the battery storage and also reduces the energy required during charging through the grid, thereby increasing the efficiency further [7,8]. The proposed solution ensures complete utilization of solar energy at all times through MPPT extraction and either feeding to the motor or charging the battery. It thus reduces the overall cost, size, and weight of the system to improve efficiency and achieve renewable energy integration in the energy mix available to the EV. II. PROPOSED IDEA The block diagram of the proposed architecture is presented in Figure 1. The system processes dc power through a three-port hybrid dc-link with bi-directional energy flow to/from the motor and battery storage ports and unidirectional flow from grid and solar PV through an interchanging port connection. The main source of energy for propulsion of motor drive is through the battery storage with an understanding that the EV rooftop solar PV may not provide the required energy at most times. The DC-link acts both as the integrating point for energy from different ports and also provides the hysteretic directional control of energy to the motor drive or battery depending on motor or generator operation of the drive. The architecture also supports charging by the grid through possibly a PFC boost rectifier when connected to the dc-link. Solar PV connection is established with the dc-link port when the drive is operational to provide available energy directly to the motor and the connection is switched to the battery storage when the drive is turned off to ensure optimal utilization of solar energy. The constant V/f control dual capability (1φ/3φ) induction motor drive converter is fed from the dc-link. The bidirectional converter provides buck/boost operation and regulates the dc link voltage to a set value suitable for (1φ/3φ) operation. We selected a value of 400V for driving a 1φ induction motor. This high voltage dc link provides a margin for hysteretic control through voltage sensing and is used in regenerative braking and battery charging through the grid. A dc-dc SEPIC converter for solar PV ensures smooth charging of the battery bank through the Solar PV panels when the drive is not being used. When the drive is operational in the motoring
Figure 1: Block diagram of the proposed idea
mode, the output is connected to the dc-link for direct feeding of energy to the induction motor, reducing the power requirement from the battery storage. Three phase variable frequency drive ensures a soft start and smooth running of 1φ or 3φ induction motors. The dual mode operation of this single drive is achieved by varying the phase of half-bridge outputs which is explained further in the subsequent sections of this paper. III. DESIGN METHODOLOGY The system includes a mechanical and electrical portion where the mechanical structure used in the experimental validation includes the vehicle design capable of carrying solar PV panels at its roof. The complete electric drive system is analyzed and simulated using PowerSim. Electrical design includes the SEPIC MPPT converter for solar PV panels, a bi-directional Buck-Boost converter for controlled flow of desired energy to/from the battery and the variable frequency drive used to run three phase or single phase induction motor. Step by step design procedure is described below: A. MPPT SEPIC Converter Single Ended Primary Inductor Converter is a dc-dc converter with continuous input current that is inherently suitable for MPPT and buck/boost operation. It can be used for downconversion as a buck converter and for up-conversion as boost converter with a positive and regulated output voltage. SEPIC is an optimal choice for battery charging in given scenario because it ensures boost mode MPPT based feeding of energy to the dclink and buck mode MPPT for battery charging when the EV is not operational. The battery storage is designed at 120Vdc with advantages in available protection components off-the-shelf. Different MPPT algorithms are studied [9-12] and finally perturb and observe is implemented. B. Bidirectional dc-dc converter Bidirectional converter is required for regulating dc link voltage to a desired value such as 400V for driving a single-phase induction motor. It is also required for battery charging during reverse power flow in regenerative braking mode. Since the isolated bidirectional dc-dc converters are generally used for higher conversion ratio, have a higher switch count, are more bulky and costly than the non-isolated counterparts, we chose to implement a non-isolated version. The isolation between two buses is not required in EV applications [13] and the overall efficiency of the non-isolated bidirectional converters is much higher as compared to the isolated converters. Comparison between different non-isolated Bidirectional converters was carried out and is presented below: 1) Buck-Boost Converter: Bidirectional topology can be directly derived from the conventional buck-boost topology by the introduction of the bidirectional conducting switch that allows buck-boost operation in both directions. Buck-Boost based bidirectional topology has greater loss and saturation of inductor due to high voltage stresses on the semiconductor switches as the input voltage, Vin, plus the output voltage, Vo [14] and both switches are at high side.
Figure 2: Simulated SEPIC Converter
2) ûuk Converter: This topology is obtained by replacing the unidirectional switches of the conventional ûuk converter with bi-directional switches. It can step up or step down the input voltage like a buck-boost converter but with inverted polarity. This is an indirect converter topology and has lower efficiency and higher switch [15] and component count than other direct topologies. 3) Half bridge DC-DC converter: In this topology, the Buck and the Boost converters are connected in antiparallel to each other but the resulting circuit has the same topology as Buck-Boost converter. But it requires only one high-side gate driver (as a source of one switch is high side and of other switch is at the low side ). Half bridge dc-dc converter is found to be most suitable for our proposed design [16-19] and is implemented for experimental results. C. Variable frequency Drive (VFD) Sinusoidal pulse width modulation based hex-bridge inverter is used for implementation of induction motor drive. As the objective of the inverter is to create a rotating flux vector, the current for both winding of single phase induction motor needs to be in quadrature and so does the voltage [20, 21]. The relation for the generation of appropriate voltage across the main and auxiliary winding is given below [22] ܸܽሺݐሻ ൌ ܸሺͳ ݊݅ݏሺ ݐݓ ͻͲሻሻȀʹ ܸܽሺݐሻ ൌ ܸሺͳ ݊݅ݏሺݐݓሻሻȀʹ ܸܽሺݐሻ ൌ ܸሺͳ ݊݅ݏሺ ݐݓെ ͻͲሻሻȀʹ Such that ܸ ሺݐሻ ൌ ܸܽሺݐሻ െ ܸܾሺݐሻ ൌ ܸȀʹሺܿݏሺݐݓሻ െ ݊݅ݏሺݐݓሻሻ ൌ ܸȀξʹܿݏሺ ݐݓ Ͷͷ ሻ ሺݐሻ ܸ௨௫ ൌ ܸܿሺݐሻ െ ܸܾሺݐሻ ൌ െܸȀʹሺܿݏሺݐݓሻ െ ݊݅ݏሺݐݓሻሻ ൌ ܸȀξʹܿݏሺ ݐݓ ͳ͵ͷ ሻ
------- 1) ------- 2) ------- 3)
Figure 4: Hex Bridge based SPWM VFD
IV. SIMULATION AND IMPLEMENTATION The proposed architecture and its associated converters were simulated in PowerSim. All subsystems are shown in Figures 2-4. PowerSim simulations are carried out for three phase induction motor and drive idea is implemented on hardware which is valid for three phase and single phase drive and tested for both configurations. On the input side, SEPIC converter is connected to the string of solar PV (thin film in proposed scenario) and its output can be connected to 400V dc link directly through boost mode. It is also capable of supplying power to 120V battery bank directly in buck mode when required. As the input of SEPIC is connected to solar all the time so it is capable of extracting power from PV string at all times independent of the amount of power available or required. Inductor and capacitor values of SEPIC converter are chosen appropriately for CCM operation. The SEPIC converter is designed to handle 300W power. Switching frequency for the SEPIC converter is chosen to be 30 kHz. The values of both inductors are calculated with 20% ripple the output current and the ripple voltage for the capacitor is chosen to be 5% of the output voltage. Calculations are carried out as described in [23] with values given in Table 1 and the simulation result for output power is in Figure 5. Inductor L1 Inductor L2
Capacitor C1 Capacitor C2
380.22 μH 442.88μH
1667μF 1667μF
Table 1: Parameter Values
------- 4)
-------5) The three outputs of three half bridge are differentially connected to single phase induction motor is such a way to obtain the desired two-phase power output. The selected hexbridge inverter topology also allows the generation of a balanced three-phase output voltage if the dc rail of the dc-dc converter is maintained at an appropriate level and the threephase voltage generated are 120o out of phase.
The Bi-directional converter is designed to handle 1kW at a switching frequency of 25kHz while regulating the dc link voltage at 400V with the input voltage of 120V from the batteries. The inductor designed for a ripple of 20% of the input current using the relation ܮൌ ቀ
ሺଵିሻ்௦ ሺଶכ௱ሻ
ቁ
------ 6)
Lൌ ͷ݉ܪ
The output DC link capacitor was chosen to be 1500μF to allow margin for hysteretic control. The input and output voltages were sensed using voltage sensors that allowed changes in duty
Figure 5: Output power curve of SEPIC converter (MPPT)
Figure 3: Simulated bi-directional half bridge dc-dc converter
Figure 8: SEPIC converter
Figure 6: DC link and battery Voltage
cycle to set the output. The bi-directional converter controls the duty cycle based on the desired output voltage. The simulation results of the bidirectional converter are shown in Figure 6 which demonstrates the variations of voltage levels at the DC link (400V) and the battery side (at 120 volts). The battery voltage lowers, as the charge is taken out of it and at the same time the boosted output voltage has a small voltage ripple of around 2%. Sinusoidal pulse width modulation technique is used to control the output of the inverter used in the drive. Three PWM signals are produced through a sine wave reference such that they are 120o degrees out of phase with respect to each other. In V/f control, the frequency of the output signal is controlled by changing the frequency of sine wave generated and amplitude of the output voltage is controlled by changing the modulation index[24]. This process is summarized in the code below: ݐ݊ܽݐݏ݊ܥൌ ݂Ǣ ݂ ൌ ݊݁ݕܿ݊݁ݑݍ݁ݎ̴݂ݓǢ ܸ݊݁ ݓൌ ݂ ;ݐ݊ܽݐݏ݊ܿ כ ݅ܯൌ ܸ݊݁ݓȀ݁݃ܽݐ݈ݒ݈݇݊݅ܥܦǢ ݁ݒܽݓ݁݊݅ݏ݂݁݀ݑݐ݈݅݉ܣ ൌ ݁ݒܽݓݎ݁݅ݎݎ݂ܽܿ݁݀ݑݐ݈݅݉ܽ כ ݅ܯǢ
Figure 9: Efficiency vs. output power of SEPIC converter.
The result of the VFD in Figure 7 shows v/f control for the increase in speed with increase in modulation index. A limit for maximum modulation is set to avoid ‘over modulation’ which can create harmonics. V. HARDWARE RESULTS Prototype of the proposed idea was successfully tested in the laboratory. Hardware prototype is shown in Figure 9 for Single phase induction motor. Hardware implementation of each individual component is discussed in the section A. SEPIC converter SEPIC converter is implemented for hardware prototype as shown in Figure 8. It has a maximum power processing capability of 300 W. 200W to 220W of power has been observed with the MPPT through Solar panel. Experimental result has been plotted in the Figure 9 with efficiency that shows that implemented SEPIC converter is above 80% for maximum power tracking though solar panels. B. Bi-directional Converter Bi-directional converter is implemented as shown in Figure 1o by using the half bridge DC/DC converter topology by a
Figure 10: Bi-directional converter hardware implementation
Figure 7: VFD Results
Figure 11: Efficiency vs. load power
connected antiparallel diode (dsep-30-60a) with each IGBT (IRGB30b60k). The DC bus formed by the series combination of two 350V 3500uF capacitors to maintain the DC rail 400V for the inverter stage. Maximum duty cycle has been set to 0.69 for the boost mode to avoid the stresses on switching device. The inrush current has been controlled through soft switching technique. During buck mode operation, the converter charges the battery from DC link and battery voltage at the source of IGBT (Vs) reduces the voltage buildup across the bootstrap capacitor which doesn’t allow IGBT to turn on. A 15V Zener diode with startup resistor is added to the bootstrap capacitor to charge it from input source of Buck mode. A window comparator has been used for the hysteresis control of DC link voltage and for regenerative braking operation [25]. It has two sets limit, an upper limit 420V and a lower limit 380 V. Whenever the DC link voltage cross the upper limit, the window comparator allow the bi-directional converter to operate as buck mode until DC link voltage becomes near to lower limit. The Efficiency variation with output power is shown in Figure 11. C. Three half bridge Inverter For hardware implementation of the three half bridge inverter, an intelligent power module (Mitsubishi PS22A78-E) is used as shown in Figure 12. This module has internally isolated gate driver circuitry for each IGBT along with snubbing circuit and the requisite antiparallel diode to allow proper commutation. Dspic30f4011 has been used for the inverter control circuitry. This controller has six independent pulse width modulation
Figure 13: Hardware prototype
Figure 14: Integration on Electrical Vehicle
signal with the independent setting of dead time that allows the proper turn on and turn off time without shoot-through. A feedback loop is designed to control the V/F constant ratio for the motor drive. Through a feedback loop, the speed of induction motor can be change with the change of modulation index. The images of hardware implementation and setup in an EV are shown in Figure 13 and 14 respectively. The control architecture also allows the running of three phase induction motor through a soft select option in the control scheme that generates control signal of 120o out of phase. Single phase induction motor drive configuration has been tested that runs in split phase configuration by exciting both windings 90o out of phase to each other. The phase shift can be seen in the SPWM voltage is shown in Figure 15. Winding sinusoidal currents are shown in Figure 16. Phase shift of 90o can be seen from the current waveforms. Variable frequency drive gives a soft start to single phase induction motor. In the
Figure 15: Phase voltages of the induction motor.
Figure 12: Hex-bridge inverter
Figure 16: Phase currents of the motor
soft start, frequency and current of the motor increase gradually. Soft start of the motor can be seen in Figure 17.
Figure 17: Soft start of the variable frequency drive
VI. ACKNOWLEDGEMENTS The authors would like to thank the National Grassroots ICT Research Initiative - National ICT R&D Fund, providing the seed funding for the initial work on this project. VII. CONCLUSION Propose idea of single phase variable frequency drive is successfully tested in the laboratory. Hardware and simulation results are in close agreement with each other. Peak efficiency of the SEPIC converter observed to 93.5% in figure 9. SEPIC converter ensures MPPT based safe charging of the battery bank by solar panel. By the efficient design of single-phase induction motor drive, we are able to run induction motor based loads. For high power processing, three phase induction motor can be used with the same drive setup. Extension to this work may be on ways to use newer PV technologies that allow integration in the EV body thus making the design selfsustained for decreased dependence on storage. REFERENCES [1] E. T. Elahi, M. H. Mushtaq, H. M. U. Shafique and S. A. Ali, "Solar power generation using concentrated technology," 2015 12th International Conference on High-capacity Optical Networks and Enabling/Emerging Technologies (HONET), Islamabad, 2015, pp. 1-4. [2] K. Yeager, "Electric Vehicles and Solar Power: Enhancing the Advantages," in IEEE Power Engineering Review, vol. 12, no. 10, pp. 13-, Oct 92. [3] M. Zeraoulia, M. E. H. Benbouzid and D. Diallo, "Electric Motor Drive Selection Issues for HEV Propulsion Systems: A Comparative Study," in IEEE Transactions on Vehicular Technology, vol. 55, no. 6, pp. 1756-1764, Nov. 2006. [4] R. E. Araujo, H. Teixeira, J. Barbosa and V. Leite, "A Low Cost Induction Motor Controller for Light Electric Vehicles in Local Areas," Proceedings of the IEEE International Symposium on Industrial Electronics, 2005. ISIE 2005., Dubrovnik, Croatia, 2005, pp. 1499-1504. [5] C. S. Kamble, J. G. Chaudhari and M. V. Aware, "Digital Signal Processor Based V/f Controlled Induction Motor Drive," 3rd International Conference on Emerging Trends in Engineering and Technology, Goa, 2010, pp. 345-349. [6] Pabitra Kumar Behera, Manoj Kumar Behera and Amit Kumar Sahoo. Article: Speed Control of Induction Motor using Scalar Control Technique.
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