Abstract--- This paper presents the development of a state-of-the- art remotely controlled single phase Sinusoidal Pulse Width Modulated (SPWM) inverter with the incorporation of RISC microcontroller. A provision of remote control from a PC in addition to the local control is incorporated in order to control the functionality of the inverter. The microcontroller itself generates the required pulse according to the remote command, to get variable output frequency from the inverter using the SPWM scheme. Finally the same is experimentally tested for resistive and inductive load and satisfactory result is obtained.

Keywords: Micro controller, MOSFET, SPWM, RS485 and RS232 Protocol, THD.

I. INTRODUCTION Pulse width modulation techniques have been the subject of intensive research during the last few decades towards the betterment of electrical power flow control to various applications. Nowadays there is a growing interest in development of microcontroller based PWM system compared to other conventional ones like dedicated analog and digital control [1]. The stand alone mode working features of microcontroller simplifies the hardware with reduced components, improves performance enhances reliability of the system, offers less aging than analog devices etc[2]. More precisely the standalone micro controller based system offers modularity which can readily be integrated with the power electronics with proper isolation in-between. This single chip module can provide to control the power electronics devices along with other features like protection of the devices and also all kind of annunciations. Thus the flexibility in control with cost effectiveness is one of the major advantage of any micro controller based dedicated module [3].

This work is based on Sinusoidal Pulse Width Modulation(SPWM) technique to produce a pulsed waveform that can be filtered relatively easy way to achieve a good approximation to a sine wave. The significant advantage of the SPWM approach is relatively highly efficient inverter with variable frequency sinusoidal output for all kind of loads. The inherent advantage of this method is that lower order

harmonics are almost eliminated. Instead small magnitude higher order harmonics are introduced in the system, which can easily be eliminated or minimized with the help of smaller size filter. By increasing number of pulses per half cycle the process is also enhanced. The SPWM technique, however, inhibits poor performance with regard to maximum attainable voltage and power [4]. Today, in the era of automation and digital control, remote operation facilitates a system to control it from a far end. Remote monitoring, control and analysis plays an important role in the automatic system which has been reported in different papers [5,6]. With the technological advancement, the control of different systems from a central location becoming possible. Recently, different research groups have been developing their own remote maintenance, control and diagnostic units on engineering systems [7]. The remote control of a system can be done by PC serial communication technique. In general RS 232 protocol is used. But to attain a greater distance for remote wired communication from the machine to the central location, RS485 protocol can be incorporated.

In this work, an attempt has been made to develop a PC based remote controlled sine wave output inverter. This remote controlling system consists of mainly a microcontroller based control unit, Power drive unit, serial communication link and a PC based user friendly software. Here the output frequency of the inverter is controlled from the central PC. The microcontroller gets required frequency command signal from remote PC to set proper frequency of the inverter through serial communication channel. Accordingly it generates a sinusoidal modulating wave and also generates a fixed frequency triangular carrier wave. The controller itself compares both this modulating signal with the carrier signal and generates SPWM pulses to drive the devices of the Inverter. H-bridge configuration inverter is used here.

II. THEORETICAL BACKGROUND

a. SPWM Sinusoidal Pulse Width modulation technique (SPWM) is one of the multilevel PWM technique used to get near sinusoidal current waveform across the load. SPWM technique is characterized by series of constant amplitude rectangular pulses with different duty cycle for each period.

Development of Microcontroller based Single Phase SPWM Inverter with Remote Control

Facility Abhisek Maiti#1, Sumana Choudhuri #2, Jitendranath Bera #3, , Tista Banerjee #4 ,Shaunak Maitra#5

Department of Applied Physics, Calcutta University 1abhra4u@gmail.com, 2sumana_cu05@rediffmail.com, 3jitendrabera@rediffmail.com,

4tista.banerjee@gmail.com, 5maitra.shaunak@gmail.com

Thus, a series of constant amplitude and unequal width rectangular pulse waves which is equivalent to sine waves are called SPWM waveform [8]. In SPWM control strategy a sinusoidal modulating signal is compared with a repetitive switching frequency triangular or saw tooth waveform to generate the switching signals for the inverter devices ( Fig 1). By changing the control signal magnitude the width of the gate drive for the devices can be changed and hence output voltage magnitude also [9]. By changing the frequency of the modulating wave the fandamental frequency at the inverter output changes.The inverter output voltage will not be of a perfect sinewave and will contain voltage components at harmonics frequency of f1. The amplitude modulation ratio is defined as

(1)

where is the peak amplitude of the control

modulating signal and the peak amplitude of triangular signal, which is generally kept constant. Generally ma≤1.

The frequency modulation index, mf is defined as

(2) where ftri is the frequency of Carrier signal and fmod is

the frequency of modulating signal. generally mf ≥1 to reduce the harmonics at the output. Theoretically the frequencies at which high voltage harmonic occur will be for unipolar full bridge inverterat fh= ( jmf ± k) fmod , where h is the order of harmonic, j is the even multiple of the frequency modulation index and k is the sideband number and fmod is the fundamental frequency[10].

Fig.1 Generation of Sine, Triangular and SPWM wave

b. H-Bridge Inverter

Fig2.H-Bridge inverter circuit. A single -phase H bridge voltage source inverter consists of four devices (S1, S2, S3, S4). When switch S1 and S4 turned on simultaneously the input voltage Vin appears across the load and when the switches S2 and S3 are turned on then the voltage across the load is reversed and is –Vdc. The full bridge topology is chosen because of the fact that it is capable of delivering high current at low voltage and devices with low Peak Inverse Voltage (PIV) can be used. In case of multiple Pulse width modulated or Sinusoidal pulse width modulated inverter, the topology may be unipolar or bipolar. In case of Bipolar switching scheme ,the output voltage changes between positive and negative Vdc. In the unipolar switching scheme, the output voltage changes between positive, Vdc and zero, or between zero and negative voltage, Vdc in as in Fig 3. In this work Unipolar scheme has been selected.

Fig 3: Unipolar inverter gate drive signals & corresponding inverter output

III.DEVELOPED SYSTEM a. Block Diagram The schematic diagram is shown in fig 4. The system consists of a PC, Serial Communication Channel, Controller unit and Inverter circuit. The software in the remote PC generates the frequency command signal which is communicated through serial communication link to the

controller unit. The controller generates the multi pulses of different widths having fundamental frequency of demand speed. This signals are used to drive the gates of the inverter circuit. There is optical isolation between the controller and power circuit.

Fig 4: Schematic Diagram of the System

b. Control and Isolation Ckt. P To generate the SPWM signal an Atmel Atmega32L-24PI RISC microcontroller was used. The Atmega32L is a low voltage, high performance CMOS 8-bit microcomputer with 32K bytes of Flash programmable and erasable read only memory (EPROM). The device has some salient features like it has RISC architecture with mostly fixed-length instruction and 32 general purpose registers. There are up to 12 times performance speedup over conventional CISC controllers. The controller has up to 10-MHz clock operation. There are wide variety of on-chip peripherals, including digital I/O, ADC,EEPROM, Timer, UART, RTC timer, pulse width modulator(PWM) etc. To stop the driver switching elements from shoot through fault, a dead interval of sufficient idle time delay is provided through the microcontroller also. The Isolation circuit is used to isolate signals for protection and safety between low voltage control circuit to high voltage power circuit. This is done by using high speed Optocoupler. d. Power Circuit The power circuit topology chosen is a full H-bridge Inverter. It consists of a single phase rectifier and filter DC voltage source, four switching element (MOSFET IRF 840),freewheeling ultrafast diode and load. The four MOSFETs are supplied from a bridge rectifier connected to output of single Phase. Anti-parallel ultrafast diodes and properly designed snubbers are used across each MOSFET for providing the free wheeling path in idle transition and dv/dt protection purpose accordingly. e. Communication channel The communication channel is established by using RS485 protocol of serial interfacing technique because of its capability of working over long distance at high speed. On the PC side, RS232 port (COM) is utilized for data acquisition while the controller of the inverter is having its own dedicated micro controller for control signals

generation. Hence RS232 level has to be converted to TTL level compatible to the working voltage of microcontroller. Then for transmission and receiving data, TTL to RS485 and vice-versa are needed. A low cost 5 core S-video cable is used to construct the network bus where only two core is used for transmitting line from the PC and the one cable is for common ground. e. Design of the controller algorithm for frequency control Here the algorithm of microcontroller programmed is developed to provide to perform the key features of the whole circuit, ie. To generate SPWM signal from two pins of the controller. The sine wave of desired frequency is generated from the stored look up table of sine function. The desired frequency can be obtained simply by changing the time delay in between the points of the sine function. The carrier triangular function of 1Khz frequency is generated from the triangular look up table. Then in the software both this waves are compared for each half cycle of the sinewave. One port bit is activated for one half cycle and the other port bit is activated in the other half cycle and the process is repeated. The design of the controller for frequency control is a pretty task to have accuracy at the inverter output. As already considered that the modulating signal is generated from a lookup table of “s” number of samples of sine function, hence to generate particular frequency ( f ) of SPWM the delay (d) between the samples is to be estimated in the firmware of the controller (eqn.3).

(3) Here the command frequency is communicated to the controller and accordingly the delay is generated. The table 1 shows percentage error in frequency for different frequency of operation.

Table 1: percentage error as generated in frequency for different frequency of operation.

Desird Frequency (in Hz)

No. of discrete pts. on sin function

Delay between each pts (in μsec.)

Approx. delay (In μsec)

Calculated Frequency (in Hz)

% Error

50 200 100 100 50 0 45 200 111.11 111 45.045 0.1 40 200 125 125 40 0 35 200 142.85 143 34.965 0.1 30 200 166.66 167 29.94 0.2 25 200 200 200 25 0 20 200 250 250 20 0 15 200 333.33 333 15.015 0.1 10 200 500 500 10 0

IV. EXPERIMENTAL RESULTS Tektronix TDS 1001B, 2channel digital storage oscilloscope is used to measure the experimental results. SPWM1and SPWM2 (in Fig 5, upper and lower) are the pulses generated from the controller for 50Hz. frequency command signal, where SPWM1is leading SPWM2 by half cycle of the switching pulses. The number of pulses per half cycle here has taken odd number. Hence Fig. 6 shows the inverter current waveform for resistive load. The output current waveform from single phase inverter with inductive load have been shown in fig 7. The current waveforms is measured by using Hall sensor CSNE151.

Fig.5 .SPWM as generated by microcontroller.

Fig.6. Inverter load current for resistive load.

.

Fig.7..Inverter load current for inductive load. V.SIMULATION RESULTS

Sampled datas are collected from digital storage oscilloscope through Tektronix Openchoice software in form of Excel Comma Separated Values File(.csv file). Matlab7.1 is used to simulate the results for both resistive and inductive load .

0 200 400 600 800 1000 1200 1400 1600 1800 2000-3

-2

-1

0

1

2

3

Fig.8.. Simulated load current for resistive load.

0 500 1000 1500 2000 2500-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

Fig.9.. Simulated load current for resistive load.

0 500 1000 1500 2000 2500 3000 3500 4000 4500 50000

100

200

300

400

500

600

frequency

RM

Sm

agni

tude

Fig.10. FFT of current for R-L load V. CONCLUSION

The single phase SPWM AVR microcontroller based remote controlled inverter is designed and tested for both resistive and inductive load. The experimental results is very close to the computer simulation results for this scheme. The control algorithm produces very low THD (Fig 8) in output current for inductive loading and very fast response in transient of non linear loads for 50 Hz frequency. It is found that THD is less than 10% for current which employ IEEE standard. In order to achieve a more better performance in higher inductive load, different improvements is being made in control and driver stages like V/F controlling,, Soft switch technology etc and the work is progressed on. The work can be extended by using wireless technology in its remote control part.

ACKONWLEDGEMENT

The authors acknowledge University Grant Commission (UGC) of India for providing opportunity and infrastructural support to carry on this research under the auspices of UGCSAP-DRS-I Project.

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