ESTIMATION OF LOOKUP TABLE FOR POPULAR PULSE-WIDTH MODULATION SCHEMES BASED ON VISUAL INSPECTION FOR AC MOTOR DRIVE APPLICATIONS.
This research paper presents an estimation technique to build a lookup table to be embedded in a microcontroller for PWM three-phase inverter system. The application of microcontroller for PWM technique allowsreducing power electronic devices switching losses and reduction of circuit complexity.
Using the look-up table approach four PWM generation schemes that estimate to viz. sinusoidal third harmonic injected sinusoidal trapezoidal and triangular techniques have been implemented. Frequency spectrum of the estimated PWM schemes has also been presented. The presented experimental results show that the ac induction motor can successfully be operated by reducing the PWM waveform resolution. However some PWM schemes have advantage over other PWM schemes.
Keywords: AC Motor Drive PWM lookup table low-resolution estimated PWM waveform microcontroller based inverter.
The advents of low-cost microcontroller (e.g. microcontrollers from ATMEL(R) or Microchip(R)) have brought a revolution to the electronic-control industry. One concept of employing microcontrollers is to develop a software program which is implanted in the microcontroller that will do the same job as ananalog circuit.
Traditional PWM inverters generate a PWM wave by comparing a reference signal to a carrier signal. The output of the comparator is normally logic one or zero. Various reference signals may be sinusoidal third-harmonic injection sinusoidaltriangular or trapezoidal waveforms whereas the carrier wave is normally a triangular waveform of frequency kept many folds larger than the reference signal frequency . This traditional technique requires a separate analog circuitry of the reference signal and a separate analog circuit for the carrier signal. Thus the complete analog circuit will be large costly complex and will consume a considerable amount of energy (dissipated heat losses).Whereas the output of such circuits is a simple PWM waveform. An advantage of using dedicated analog circuit for PWM generation is that the resolution of the output PWM waveform is very large.
However the high resolution of the PWM signal will introduce additional problems like absence (or reduction) of power electronics device's dead time  and significant increase in power electronics device switching losses .
A microcontroller can also be used to generate a PWM waveform for three-phase inverter control [4-6]. For PWM waveform generation a lookup table that corresponds to the desired or similar to the desired PWM can be pre- programmed in the microcontroller . While using a microcontroller the resolution of the PWM waveform is compromised . The resolution of the PWM waveform depends on factors such as the microcontroller clocking frequency software array handling capacity of the microcontroller and bit size i.e. 8bit 16bit or 32bit.
Nevertheless since the PWM waveform has to be pre- programmed therefore a PWM waveform of high resolution is required as a reference. The PWM waveform lookup table programmed in the microcontroller will be an estimate of the original high resolution PWM waveform. In addition lookup table provides elimination of selective harmonics [9-10].
This research paper proposes an estimation technique to construct a lookup table for a three-phase inverter to drive a three-phase induction motor. The resulting line-to-neutral line-to-line and common-mode voltages will be examined. Due to microcontroller limitations resolution of the estimated PWM waveform is kept to 6. The three-phase inverter along with the microcontroller software is particularly suitable for commercial and household applications.
Four popular types of reference signals will compared to triangular carrier signal to generate four separate PWM
waveforms. The popular types of reference signals are
sinusoidal third-harmonic injection sinusoidal trapezoidal and triangular waveforms. The peak voltage of all four reference signals are adjusted to 1volt. Whereas the frequency of carrier signal is kept 60 times of the reference
signal frequency and the peak carrier-signal voltage is 1volt. To create the PWM signal since the reference and carrier signals both are periodic therefore the comparison will be done from 0 to p. For the remaining time i.e. p to 2p the PWM will be the compliment (opposite). Fig.1 displaysa sinusoidal reference signal the triangular carrier signalvSIN represents the regular (high resolution) PWM waveform and vSIN-E represents the estimated PWM waveform. The estimated PWM waveform vSIN-E is created by visual
inspection. The resolution is kept to 6. Fig.2 displays
athird-harmonic injection sinusoidal reference signal the triangular carrier signal vINJ represents the regularPWM waveform and vINJ-E represents the estimated PWM waveform. Fig.3 displays trapezoidal reference signal the triangular carrier signal vTRA represents the regularPWM waveform and vTRA-E represents the estimated PWM waveform. Fig.4 displays triangular reference signal the triangular carrier signal vTRI represents the regularPWM waveform and vTRI-E represents the estimated PWM waveform. The signals vSIN-E vINJ-EvTRA-E and vTRI-E are for one phase suppose vAN of the three-phase inverter. The lookup table for the remainingtwo phasesi.e. vBN and vCN are implemented by phase shifting the lookup table of vAN by
120 and 240 respectively. With the resolution of 6 the
lookup table will consist of 60 elements to represent the three-phase estimated PWM waveform from 0 to 2p.
3. RESULTS AND DISCUSSION
Three-phase inverter hardware was built and the estimated PWM waveforms for sinusoidal third-harmonic injection sinusoidal trapezoidal and triangular lookup tables were
separately embedded in separate microcontroller of
individual three-phase inverter. Fig. 5 displays the conventional three-phase inverter-motor drive system. The switches S1 to S6 are the power electronic devices which make the inverter power circuit. The inverter is adjusted to produce a PWM frequency of 35Hz. The load connected to the inverter is a 90W three-phase squirrel cage type induction motor. The motor is tested under no-load condition; the frame of the motor is grounded out. The DC voltagesource VDCis adjusted to 100V. Two series connected capacitors C1 and C2are connected across the voltage source. The common point of the two capacitors i.e. point N is connected to the ground.The three phase output terminals of the inverter are labeled as A B and C. The line-to-neutral voltages of the three-phase system are vAN vBN and vCN
whereas the line-to-line voltages are labeled as vAB vBC and vCA. Three resistors RCOMare connected in wye-connection and a terminal is obtained from the neutral point of the resistor network to observe the common-mode voltage vCOM.
Fig. 6 7 8 and 9 display theexperimental results for
voltages appearing across the inverter output terminal when the estimated sinusoidal third-harmonic injection sinusoidal trapezoidal and triangular respectively lookup tables based microcontrollers were used. The 120 phase shift between the line-to-neutral and line-to-line voltages can be observed. The motor operated successfully in all four cases. However smooth operation is observed in the case of third-harmonic injection sinusoidal and trapezoidal lookup tables were tested. In the case of triangular lookup table the motor produces the lowest speed at no-load condition. In addition of sinusoidal and third-harmonic injection sinusoidal lookup table the common mode voltage had a peak-to-peak voltage of 100V. Whereas in the case of trapezoidal and triangular lookup tables the common mode voltage had a peak-to-peak voltage 50V.Fig. 10 11 12 13 displays the FFT of line-to- neutral line-to-line and common mode voltage for the estimated schemes of sinusoidal third-harmonic injection sinusoidal trapezoidal and triangular respectively. It is observed that the line-to-neutral and line-to-line have highest fundamental harmonic for estimated trapezoidal scheme and lowest fundamental harmonic existed in the estimated triangular scheme. The harmonic contents for common- mode voltage are minimum for trapezoidal scheme and maximum for third-harmonic injection sinusoidal scheme.
In this paper the use of a low-cost microcontroller i.e.
89S51 to generate the PWM pulses for a three-phase inverter is described. The main feature of the system is its simplicity; which is primarily due to the minimum hardware
required. The generation of lookup table to be programmed in the microcontroller has been explained. Estimated lookup tables for popular PWM schemes specificallysinusoidal third harmonic injected sinusoidal trapezoidal and triangular methodshave been realized. Several characteristic results are presented and they are found to agree well with other established works.Additional work on this subject ought tofocus on output waveform feature to approach the reference signals mentioned herein while at the same time reduce power electronic devices losses. It may be achieved by using a processor with bigger memory and higher bit handling capacity.
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