Development of Microcontroller Based ISPWM Switching Technique for Single Phase Inverter

Development of Microcontroller Based

ISPWM Switching Technique for Single

Phase Inverter

Abhisek Maiti

Department of Applied Physics, University of Calcutta, 92, Acharya Prafulla Chandra Road, Kolkata - 700009

abhra4u@gmail.com http:// www.caluniv.ac.in

Sumana Choudhuri

Department of Applied Physics, University of Calcutta, 92, Acharya Prafulla Chandra Road, Kolkata - 700009

sumana_cu05@rediffmail.com http:// www.caluniv.ac.in

Jitendranath Bera

Department of Applied Physics, University of Calcutta, 92, Acharya Prafulla Chandra Road, Kolkata - 700009

jitendrabera@rediffmail.com http:// www.caluniv.ac.in

Abstract :

This paper presents the development of a state-of-the- art single phase Inverse Sinusoidal Pulse Width Modulated (ISPWM) inverter with the incorporation of RISC microcontroller. The proposed switching techniques uses a sinusoidal reference signal and inverted sine as a carrier signal.The ISPWM techniques produces the lower Total Harmonic Distortion(THD) comparison with conventional sinusoidal PWM technique. The microcontroller is used to generate switching pulses using the ISPWM scheme of variable frequency and also provides the protection against shoot-through fault of castom power devices. Finally the same is experimentally tested for resistive and inductive load with satisfactory dynamic and static performance. . A provision of remote control in addition to the local control is also incorporated in order to improve the functionality of the inverter.

Keywords: Micro controller, MOSFET,ISPWM,THD, De-Saturation.

1.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]. Thus the flexibility in control with cost effectiveness is one of the major advantage of any micro controller based dedicated module [3].

PWM methods reduce the harmonics by shifting the frequency spectrum to the vicinity of high frequency band of carrier signal. In normal Sinusoidal PWM(SPWM) scheme, the control signal is generated by comparing a sinusoidal reference signal and a triangular carrier. The SPWM technique, how ever, inhibits poor performance with regard to maximum attainable voltage and power.[4].

switching technique has been developed for controlling power electronics devices of VSI based converter with better performance.

2.Theoretical Background

a.)ISPWM

In conventional SPWM control strategy a sinusoidal modulating signal is compared with a repetitive switching frequency triangular or saw tooth waveform as a carrier to generate the switching pulses[5]. The proposed ISPWM has new forms of carriers, carrier1 and carrier2, as shown in Fig. 1.In each case, equivalent triangular carriers have been shown by dashed lines in Fig. 1.

The firing control signals have been generated by comparing sinusoidal reference signal (with the frequency fmod and magnitude Vmod ) with the inverted-sine carrier signal (with the frequency ftri and magnitude Vtri), as shown in Fig.2. By changing the frequency of the modulating wave the fundamental frequency at the inverter output changes[6]. The inverter output voltage will not be of a perfect sinewave and will contain voltage components at harmonics frequency of f1.

Fig.2Firing pulse generation in proposed ISPWM Fig. 1Proposed ISPWM carriers

The amplitude modulation ratio is defined as

(1)

where is the peak amplitude of the control modulatingsignal and the peak amplitude of triangular signal, whichis generally kept constant. Generally ma≤1.

The frequency modulation index, mf is defined as

(2)

where ftriis the frequency of Carrier signal and fmodis the frequency of modulating signal. generallymf≥1 to

reduce the harmonics at the output.

Theoretically the frequencies at which high voltage harmonic occur will be for unipolar full bridge inverter at 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 fmodis the fundamental frequency[10].

.

Considering angle

θ

i

is the width of i

th

pulses in degree as an intersection angle of carrier and reference

signals, the following equations can be calculated:

1 sin[ ( 1)] sin , 1, 3, 5...(3)

2

f i a i

mθ π i m θ for i

− − − = =

1 sin[ ( 2)] sin , 2, 4, 6...(4)

2

f i a i

mθ π i m θ for i

+ − − = =

When m

f is an odd number, the half cycles of the phase voltage Vao are the same but with opposite sign and each half cycle is symmetrical with respect to half cycle midpoint. Therefore, 1

2 f

m −

angles should be determined In

following way.

3 3

1 1 1 1

2 2 2 2

3 3

3 3 3 3 ,...(4)

2 2 2 2

f f f f

f f f f

m m m m

m m m m

π π π π

θ( ) / 2 = θ( ) / 2 − θ(3 ) / 2 = θ(3 ) / 2 −

π π π π

θ( ) / 2 = θ( ) / 2 − θ(3 ) / 2 = θ(3 ) / 2 −

− − + = − − +

− − + = − − +

2

, 2

mf mf

θ =π θ = π .

Based on Fourier analyais, all harmonics of output voltage waveform can be calculated. Fourier expantion of the output waveform when m is also an odd number, consists of only odd harmonic orders.

1sin 3sin 3 5sin 5 ... sin

Vao=A wt+A wt+A wt+ An nwt…………..(5)

where

2 4

0

sin ( )

n Vao

A nwtd wt

π

π

=



=

2

0 1) / 2

4

[ sin ( ) sin ( ) sin ( )]

2 Vdc

m

wtd wt wtd wt wtd wt

π

θ1 θ2

θ1 θ(

π



−



±



₋

1 / 2 1 4

* * (1 2 cos cos cos )

2

Vdc

n m

A n n n

n π θ1 θ2 − ... θ −

= − + ±

Using the same analysis, Fourier series for V

bo can be expressed as follows: 1sin( 2 ) 3sin 3( 2 ) 5sin 5( 2 ) ... sin ( 2 )

3 3 3 3

Theoretically the frequencies at which high voltage harmonic occur will be for unipolar full bridge inverter at 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 fmodis the fundamental frequency[7].

Hence it is obvious that the line voltage V

ab has no triple harmonics. In addition, if mf is equal to 3k for k=1, 2… then the line lowest harmonic orders are m

f-2, mf+2, 2mf-2 and 2mf+2 (e.g., for mf=9 the order of these harmonics are 7, 11, 17 and 19).

.

b.) H-Bridge Inverter

Fig.3 H-Bridge inverter circuit.

A single -phase H bridge voltage source inverter(Fig.3) consists of four devices (S1, S2, S3, S4). When switch S1 and S4 turned on simultaneously the input voltage Vdc 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 Inverse 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.

1. 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.

b. Control, Ckt.

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.

c.Protection and Isolation Ckt.

MOSFETs in a bridge configuration requires special protection against shoot-through fault, introduced by either dv/dt induced turn on or turn on onto a shorted complementary device. As the dc bus short circuit current capacity is very high ,the simultaneous turn on of both devices in an arm of the bridge will lead to failure of both these devices unless protection are taken.

Over current protection of an MOSFET is an essential feature to be incorporated into the gate drive circuit. Such protection fast enough to protect the device from overcurrent,the effective solution is to sense the collector – emitter voltage drop. MOSFET exhibits a desaturation features above a certain collector current level ,depending on its gate voltage .In such a situation the collector emitter voltage increases rapidly ,leading to excessive power dissipation that can be destroy the device.This feature is used here to provide over current protection ,by selecting the collector emitter voltage for the current level at which turned off is forced[8].A typical such circuit is shown in fig6. 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. Thus only selected type of opto isolators are suitable for high speed MOSFETs drives, specially ones with photodiode sensors where signal pulses turns on the driver transistor and causes to current flow through the driver LED.The photo diode sensor turns on which results the base current through the integral amplifier transistor .The transistor is is operated with an external pull up resistor from where the signal is further amplified and used to drive MOSFET. This is done by using high speed Optocoupler.(4N35)

Fig.6:circuit with over current protection

d. Power Circuit

e. Communication channel

Remote operation facilitates a system to control it from a far end. With the technological advancement, the control of different systems from a central location becoming possible. [9]. 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 ISPWM signal from two pins of the controller. The inverse 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 shaped function of 1Khz frequency is generated from the 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.7).

(7)

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.

Desird

Freque

ncy

(in Hz)

No. of discrete pts. on sin function

Delay betwee n 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

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

2. Experimental results

a)practical results

Tektronix TDS 1001B, 2channel digital storage oscilloscope is used to measure the experimental results.

Fig 7:ISPWM as generated by microcontroller

Fig.8:Inverter load current for resistive load Fig9: Inverter load current for inductive load

b)simulationl results

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 (Fig.10 and Fig.11).

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

-2 -1 0 1 2 3

0 500 1000 1500 2000 2500

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0

100 200 300 400 500 600

frequency

RM

S

m

a

gni

tude

Fig.12: FFT of current for R-L load

4.Conclusion

The single phase ISPWM 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 12) 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 8% 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.

Acknowledgement

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.

References

Books:

[1] “Modern Power Electronics and AC Drives”, Bose, B. K.,Prentice Hall, Inc., NJ 07458, 2002E.

[2] “Power electronics: converters, applications, and design” Ned Mohan, Tore M. Undeland, William P. Robbins,3rd _edition, (paperback)1989.

Technical Reports:

[3] “Discrete Time-Based Model of the Sinusoidal PulseWidth Modulation Technique”, S. A. Saleh and M. A. RahmanFaculty of Engineering and Applied Sciences Memorial University of Newfoundland St. John’s, NL, Canada A1B 3X5.

Papers:

[4] “ A novel PWM scheme for harmonic reduction in power converters", D. Quek and S. Yuvarajan, Proc. of International Conference on Power Electronics and Drive systems, Singapore, February 1995.

[5] “Development of a Single Phase SPWM Microcontroller-Based Inverter”, B. Ismail, S.Taib MIEEE, A. Mohd Saad, M. Isa, C. M. Hadzer: First International Power and Energy Coference PECon 2006. November 28-29, 2006, Putrajaya, Malaysia.

[6] “A new ISPWM Switching Technique for THD Reduction in Custom Power Devices”, S. Esmaeili Jafarabadi, G. B. Gharehpetian . Department of Electrical Engineering, Amirkabir University of Technology, 15914 Tehran, Iran.

[7] “Effects of Harmonics on PWM Inverter fed InductionMachines”, S Ekram , SIG College of Engineering, Mumbai and Dr B Sarkar , SGS Institute of Technology and Science, Indore 452 003.

[8] “Gate drive methods for IGBTs in bridge configurations”, Biswas, S.K. Basak, B. Rajashekara, K.S. Dept. of Electr. Eng., Jadavpur Univ., Calcutta. Industry Applications Society Annual Meeting, 1994., Conference Record of the 1994 IEEE On page(s): 1310 - 1316 vol.2