Boost Converter Fed High Performance BLDC Drive for Solar PV Array Powered Air Cooling System

This paper proposes the utilization of a DCDC boost converter as a mediator between a Solar Photovoltaic (SPV) array and the Voltage Source Inverters (VSI) in an SPV array powered air cooling system to attain maximum efficiency. The boost converter, over the various common DC-DC converters, offers many advantages in SPV based applications. Further, two Brushless DC (BLDC) motors are employed in the proposed air cooling system: one to run the centrifugal water pump and the other to run a fan-blower. Employing a BLDC motor is found to be the best option because of its top efficiency, supreme reliability and better performance over a wide range of speeds. The air cooling system is developed and simulated using the MATLAB/Simulink environment considering the steady state variation in the solar irradiance. Further, the efficiency of BLDC drive system is compared with a conventional Permanent Magnet DC (PMDC) motor drive system and from the simulated results it is found that the proposed system performs better.


Introduction
Over recent years, renewable energy sources have become an indispensable part of world energy consumption.Among all the available renewable energy sources, solar energy is regarded as the most viable source of energy since it is abundantly available [1], [2] and [3].India has an average annual temperature that ranges from 25-27.5 • C which means that India has huge solar potential.Although several researches have been carried out on SPV array fed automotive and irrigation systems combining various DC-DC converters and motor drives, very less work has been done on Photovoltaic (PV) based home appliances [4] and [5].One of the foremost challenges observed by the developing countries like India is the energy supply to far-flung areas.In lieu of this, the SPV air cooling system is designed which can be easily employed in remote and isolated areas for home appliances.
The PMDC motors are widely used in various applications due to its low cost construction, simple and inexpensive control and requirement of no external controller.But some of the flaws of the PMDC motor make it unsuitable for SPV array based appliances since at higher speeds, brush friction increases, thus reducing useful torque.Other demerits include higher armature reaction, lower speed range owing to mechanical limitations on the brushes, and since the field in the air gap is permanent and restricted, it cannot be controlled externally.In recent years, BLDC motors have gained popularity in a plethora of applications.The BLDC motor is far more dependable than the DC motor because it uses an electronic commutator instead of a mechanical commutator.The advantages of brushless motors over brushed DC motors include: better speed versus torque characteristics, high efficiency and reliability, noiseless operation, longer lifetime, eradication of ionizing sparks from the commutator and an altogether trimming of ElectroMagnetic Interference (EMI).Due to all above paramount advantages, BLDC motor has been chosen to develop SPV fed air cooler [6] and [7].
Maximum efficiency of the SPV array is attained via a Maximum Power Point Tracking (MPPT) algorithm using the DC-DC converters [8].Numerous DC-DC converters have been employed for MPPT in different SPV array based applications.However, most of the classical converter topologies have the highest values of reactive components resulting in an increased cost, size and weight and therefore, in the proposed system, a boost converter with desired features such as good switch utilization, high conversion efficiency, low stress on semiconductor devices and minimal number of reactive elements is selected [9], [10] and [6].The SPV array is chosen such that its MPP always occurs within the bounded MPP region of the boost converter so that the power is invariably optimized immaterial of the variation in solar irradiance and loading conditions.Several non-identical MPPT algorithms have been recommended and the simplest one is to operate the PV array at constant voltage equal to the MPP voltage of the array [11] and [12].Perturb & Observe (P&O) MPPT method is utilized in this paper because of its sharp fidelity of tracking even under the abrupt change in the environmental state.The perturbation size is prudently selected such that the oscillation around the peak point is prevented [13] and [14].
The structure of the paper is as follows: First section provides the introduction of the paper.Section 2. describes the various stages in the configuration of the conventional and the proposed system and Sec. 3. deals with the design of the PV array.Section 4. and Sec.
5. discuss about the design of DC-DC boost converter and Sinusoidal Pulse Width Modulation (SPWM) technique based inverter.Section 6. illustrates the BLDC motor drive system and Sec. 7. presents the control techniques of the system.Section 8. shows the results and the simulation work carried through MATLAB/Simulink and the final section exemplifies the conclusion.

Proposed System Description
The block diagram of the conventional PMDC motor based air cooling system and the proposed BLDC motor drive based air cooling system are depicted in Fig. 1 and Fig. 2 respectively.
In the conventional system, as shown in Fig. 1, the solar panel produces the required electrical power and feeds the DC-DC boost converter through the MPPT controller.The DC output power of the boost converter is utilized to energize the two PMDC motors to drive the centrifugal water pump and a fan blower.The speed of the fan blower is controlled through a speed controller and since MPPT and speed control cannot be achieved through one single converter, additional boost converter is utilized to the PMDC motor coupled to the fan blower.
In the proposed system, the SPV array output is fed to the DC-DC boost converter which is operated through the MPPT controller.The two VSI's converts the DC input power fed from the DC-DC converter to AC power to run the two BLDC motors coupled to the centrifugal water pump and a fan blower.The inbuilt encoder appended in the BLDC motor encodes the signal generated by the Hall sensors.These signals are converted by the encoder depending on the position of the rotor and accordingly gate pulses are generated to control the stator currents of the BLDC motor.The electronic commutator takes these signals and controls the gate pulses of the inverter.The speed of the fan blower coupled to one of the BLDC motor is controlled through a speed controller.As the BLDC motor VSI's operate at the fundamental frequency, the high frequency switching losses and the brush losses are eliminated, contributing to the reliable and high efficiency operation of the proposed air cooler system.The thorough design of various components at various stages such as SPV array, DC-DC boost converter, SPWM technique based inverter, PMDC motor, BLDC motor, water pump and fan blower are described as follows.

Design of PV Array
The conversion of solar energy to electrical energy is done through solar PV cells.The equivalent circuit of a practical PV cell is shown in Fig. 3, which consists of a current source I ph , a series resistance R s and a parallel resistance R sh [2].For an ideal PV cell the parallel resistance is infinite and the series resistance is zero.Hence the output current of an ideal PV cell is given by the Eq. ( 1) and Eq. ( 2): where I s is the saturation current, q is the electron charge (1.6 )), A is the diode ideality factor, T is the PV cell temperature (K).To increase the output voltage or the output current, the PV cells are connected in series or parallel combinations to form solar arrays [15].Let N s be the number of cells connected in series and N p the number of cells connected in parallel, then the output current of an ideal PV cell is given by the Eq.(3) [16].
The capacity of SPV array is selected slightly greater than the capacity of the PMDC and BLDC motors, which perpetuates the smooth functioning of the system despite the power losses.Hence, the selected capacity of SPV array is P mpp = 900 W which is slightly more than what is required by the motors and also, all its parameters are framed accordingly.The voltage of the SPV array at MPP is selected considering BLDC motor voltage rating which is similar to the VSI DC link voltage rating.Table 1 summarizes the estimation of the various parameters to design an SPV array of appropriate size [6].

Design of Boost Converter
SPV systems require the use of DC-DC converters to regulate and control the sporadic output of the solar panel.In addition, the boost converter topology is the only one that allows follow up of the PV array MPP notwithstanding the temperature, irradiance, and connected load [17] and [18].The SPV array voltage at MPP, V pv = 108 V appears as the input voltage source, whereas DC link voltage of VSI, V dc = 231 V appears as the output voltage of the boost converter.The duty ratio D, of the boost converter is evaluated by employing the input-output relationship [14] as shown in Eq. ( 4): The average current flowing through the DC link, I dc is estimated by the Eq. ( 5): The design calculations of inductor L and DC link capacitor C [6] are done as shown in Eq. ( 6) and Eq. ( 7): where f sw is the switching frequency of the boost converter, I L is the average current flowing through the inductor, ∆I L is the amount of ripple permitted in the current and ∆V dc is ripple permitted in the voltage.

SPWM Technique Based Inverter
Inverter is the most important device to utilize the renewable energy sources efficiently.Also BLDC motors makes use of inverters as electronic commutators to establish alternating current in congruent with the rotor position to produce the required torque.An equivalent circuit of a three phase SPWM technique based inverter utilized in the proposed system is demonstrated in Fig. 4. The main objective of the circuit is to generate a sinusoidal output voltage with tractable magnitude and frequency.For this, the switching signals are produced in such a way as to get the desired output.To generate the switching signals, a sinusoidal control signal of desired frequency is compared with a triangular waveform.According to the generated switching signals the inverter switches are controlled to get a three phase output voltage [19].The Sinusoidal Pulse Width Modulation (SPWM) technique is one of the most popular techniques for harmonic reduction of inverters.In SPWM technique, three sine waves and a high frequency triangular carrier wave are used to generate PWM signal.Generally, three sinusoidal waves are used for three phase inverter.The sinusoidal waves are called reference signals and they have 120 • phase difference with each other.The frequency of these sinusoidal waves is chosen based on the required inverter output frequency [8].The three sinusoidal modulating signals should be balanced to get a balanced three phase output and also, it is important that the carrier waveform for all the three legs remains identical.

6.
Design of the Proposed BLDC Motor Drive 6.1.

Design of BLDC Motor
BLDC motors are considered to have various advantages [17], [18] and [19] and hence are used to develop a SPV fed air cooler, which can operate satisfactorily for longer time as compared to the PMDC motors under the dynamically changing atmospheric conditions.
Though PMDC motor is widely used in various applications, some of its drawbacks restrict its use in SPV array based appliances.This is because at higher speeds, brush friction increases, thus reducing useful torque.Other demerits include higher armature reaction, lower speed range owing to mechanical limitations on the brushes and since the field in the air gap is permanent and restricted it cannot be controlled externally.In view of this, BLDC drive system is deployed in the proposed work and its efficiency is compared with that of a PMDC drive system.It is shown that the former system gives high performance from the simulated results.
The voltage equations of the three windings of the BLDC motor are given by the Eq. ( 8), Eq. ( 9) and Eq. ( 10) [20], [21] and [22]: where V a , V b and V c are the phase voltages, i a , i b and i c are the phase currents, E a , E b and E c are the phase back-EMF, R is the phase resistance, L is the self inductance of the each phase.
The total output torque is the sum of the three torques in the three phases given by the Eq. ( 11): where ω r is the speed of the rotor.
The mechanical equation of the motor is given by the Eq. ( 12): where τ p and τ b are the pump load torque and blower load torque respectively, J is the rotor inertia, β is the friction constant (Nm•s•rad −1 ).
Since the conventional drive circuits are expensive, bulky and complex, this paper proposes a low cost, compact, user friendly and high performance BLDC drive system which employs solar module.The practical converters undergo various power losses and the functioning of the BLDC motors is affected by the mechanical and electrical losses associated with them.To compensate these losses, the maximum power capacity of SPV array is selected slightly more than that of the BLDC motors which facilitates the flawless operation of the proposed system despite the power losses [18] and [19].The parameters and ratings of the BLDC motor and the PMDC motor selected are indicated in the Tab. 2.

Design of Water Pump
One of the BLDC motors is coupled to the water pump which is driven as a load to the BLDC motor by giving a load-torque signal depending on the torque-speed characteristic equation of a pump [15].The rating of the pump is taken as 0.4 kW and the estimation of proportionality constant, K p for the selected water pump is taken by its torque-speed characteristics Eq. ( 13): where ω p is the mechanical speed of the rotor in rad•s −1 , τ p is the electromagnetic torque developed and P p is the rated power developed by the BLDC motor connected to the water pump under steady state for stable operation [23] and [24].The detailed specifications of the centrifugal water pump are given in Tab. 3.

Design of Fan Blower
A fan blower is realised as a load to the second BLDC motor and its torque-speed characteristics is related with the Eq. ( 14): where, τ p is the load torque offered by the blower which is equal to the electromagnetic torque developed by the BLDC motor under steady state for stable operation, ω b is the mechanical speed of the rotor in rad•s −1 and P b is the rated power developed by the BLDC motor connected to the blower fan under steady state condition.The detailed specifications of the fan blower are given in Tab. 4.

Control Schemes of the Proposed System
The air cooling system proposed is controlled through three control schemes viz.MPPT, electronic commutation and DC-link voltage controller [17].These schemes are detailed as follows: 7.1.

P&O-MPPT Algorithm
The salient features of all solar panels include a distinct Maximum Power Point (MPP) and a nonlinear voltagecurrent characteristic, which relies on the atmospheric conditions, such as temperature and irradiance.In order to continuously harvest maximum power from the solar panels, they have to be operated at their MPP despite the inevitable changes in the environment.The solar to electrical energy conversion efficiency of solar cell lies between 9-20 %.Hence, it is important to use MPPT controller to enhance the overall efficiency of system [13], [14] and [9].As per the maximum power transfer theorem, a source can deliver maximum power if the source and load impedance is same.Here DC-DC boost converter serves the purpose of impedance matching of source and load.To extract maximum possible power from a varying source, MPPT algorithms are used to match the source and the load properly.
In this work, an efficient, simple and common P&O MPPT technique is utilized.The operating voltage is sampled and the algorithm changes the operating voltage by changing the duty ratio in the required direction depending on the sign of the difference between the samples of power and voltage as shown in Fig. 5.
This algorithm is not suitable when the variation in the solar irradiance is high.The voltage actually never reaches an exact value but perturbs around the MPP.Selecting an optimum value of perturbation size not only avoids the oscillations around the MPP but also reduces the starting current of the BLDC motor [25], [26] and [27].An intellectual agreement between the tracking time and the perturbation size is also held to fulfill the required objectives [28].

Electronic Commutation
Electronic commutation of BLDC motors produces the switching signals for the VSI and decodes three Hall effect signals which are produced by an inbuilt encoder of the motor depending on the angular position of the rotor.BLDC motors need to operate more efficiently to reduce the energy and save cost.Moreover, to ensure greater efficiency, the correct Hall-effect sensors need to be selected for electronic commutation.These Hall effect signals are logically converted into 6 switching pulses which are further used to operate the 6 IGBT switches of the VSI.Table 4 depicts the switching states of VSI for each set of Hall-effect signal states.
Tab. 4: Switching states for electronic commutation of BLDC Motor.
For the fan blower, the hysteresis control method is used to obtain a preset speed.The reference speed when compared with the original speed of the BLDC motor, generates a torque reference which when divided by the constant K b obtained from Eq. (4) gives the current reference magnitude.To get current reference, this current reference magnitude is multiplied with current pattern generated through the Hall signals.The three current references thus produced are then compared with the actual current flowing in the stator and the error signal is then made to pass through the Proportional Integral (PI) controller which then generates two pulses each.Hence, the 6 pulses gener-ated are given to the gates of the VSI switches resulting in a desired speed of the BLDC motor [29] and [30].
The speed of the BLDC-motor for the water pump is regulated through the DC link voltage and the speed is fixed without using any external controlling circuit.Hence, by maintaining the power balance, it is ensured that the DC link voltage remains within limits [28] and [31].

DC-Link Voltage Controller
If only the air blower is to be operated and no controller is used, then the DC-link voltage increases gradually due to the lack of power balance.Hence to restrict this, the power balance (Generated power = Power consumed + Losses) should be maintained.
And the correct assessment of the losses is formidable.Now with the help of the DC-link voltage controller, the DC-link voltage is maintained; but this makes the SPV to be operated below MPP if the voltage reference to the DC-link voltage controller is other than Vmpp.Hence, less power is obtained.However, this will not give rise to a difficult condition as the mechanical load specified to be driven now is also diminished.

Summary of Simulation Results
Performance evaluation of the proposed air cooler is carried out using simulated results in MAT-LAB/Simulink considering the starting and steady state condition at the standard solar irradiance of 1000 W•m −2 and 600 W•m −2 [6] and [7].Further, the performance of BLDC motor is compared with a PMDC motor and the results are demonstrated on an individual basis in the following subsections.The Fig. 6 and Fig. 7 shows the output parameters of the PV array viz; PV power , PV voltage and PV current and the tracking performance of the P&O MPPT technique under the steady state condition of irradiance of 1000 W•m −2 of the PMDC motor and BLDC motor respectively.These indices correspond to the operation of the SPV array at MPP as PV power reaches 900 W at steady state.From the two figures, it can be illustrated that both the PMDC and BLDC motor's PV array output indices increase and reach their steady state values at the solar irradiance of 1000 W•m −2 .

2) Performance of Boost Converter at 1000 W•m −2
The behaviour of the boost converter at 1000 W•m −2 for the PMDC motor and BLDC motor are shown in Fig. 8. where the DC link voltage is presented.It is inferred from waveforms that the converter operates in CCM, for both the motors resulting in a delimited strain on the components.The peak voltage on the switch for both the motors is observed as 310 V but the steady state value is arrived at 2 seconds for the PMDC drive system and 0.5 seconds for the BLDC  Performance of the PMDC motor at steady state irradiance of 1000 W•m −2 connected to the two loads motor fan and pump are shown in Fig. 9 and Fig. 10.The variables viz. the current, the torque, the speed and the output power when plotted against time reach their steady state values within 2 seconds which is slightly more than that of the BLDC motor.The starting current of BLDC motor is bounded within the permissible range and hence the motor has a soft start.The starting and steady state behaviours of the BLDC motor fan and motor pump at 1000 W•m −2 are shown in Fig. 11 and Fig. 12 and the following points are inferred from the simulation results.All the motor indices of the BLDC motor such as the stator current, the electromagnetic torque, the speed and the output power offered by fan and pump reach their rated values within 0.5 s under steady state condition as MPP is tracked whereas the PMDC motor parameters takes 2 seconds to reach their steady state values.The BLDC motor draws it's rated current and attain the rated speed, resulting in the water pumping and fan operating with full capacity.However, a minute and admissible pulsation in the electromagnetic torque is seen due to the electronic commutation in the DC link current of  VSI.The starting current of BLDC motor is bounded within the the permissible range hence the motor has a soft start [17].The various performance indices of SPV array, boost converter and PMDC motor drive and BLDC motor drive under starting and steady state at the standard  The behaviour of the boost converter at 600 W•m −2 is shown in Fig. 15, where the DC link voltage of the PMDC and BLDC motor is presented.It is inferred from waveforms that the converter operates in CCM, irrespective of variation in solar irradiance.Also, at this irradiance, a DC voltage equal to 48 V for the PMDC motor and a DC voltage equal to 120 V for the BLDC motor sufficient to pump the water as well as to drive the fan blower is attained.in Fig. 18 and Fig. 19 and the following points are clearly observed from the presented simulation results.The variables viz. the stator current, the rotor speed, the electromagnetic torque and the output power offered by fan and pump reach their rated values within 0.5 s.Further, the motor attains a minimum speed of 700 rpm required to pump the water and drive the fan even at the irradiance of 600 W•m −2 .From the simulated results, it is seen that there is always torque balance between the BLDC motor and the loads irrespective of the solar irradiance.This torque balance between the BLDC motor and the loads irrespective of the solar irradiance variation verifies the stable operation of the proposed system.

Efficiency of PMDC Drive vs. BLDC Drive System
The efficiency evaluation of the proposed BLDC drive system and the PMDC drive system are shown in Tab. 5, subjected to the variation in irradiance, where η D and η B are the overall efficiences of the PMDC drive system and the BLDC drive system respectively.The graphical representation shown in Fig. 20, depicts the superior efficiency of the BLDC drive system over the PMDC drive system.It is observed from the efficiency values achieved that the BLDC drive attains an efficiency of 79.3 % at the minimum solar irradiance of 400 W•m −2 and as the irradiance increases beyond 600 W•m −2 , the efficiency of the system improves over 80 %, whereas the efficiency of the PMDC drive system varies between 60 % to 62 %, thus illustrating the advanced efficiency of the proposed BLDC system.

8. 1 .
Starting and Steady State Performances of PMDC Motor Drive System and BLDC Motor Drive System at Solar Irradiance of 1000 W•m −2 The parameters of SPV array, boost converter of the PMDC motor drive and BLDC motor drive connected to the two loads water pump and fan blower at a steady state solar irradiance of 1000 W•m −2 are exemplified in Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11 and Fig. 12, and detailed in the following subsections.