Manjari Mehrotra, Dr. A.K Pandey

Transcription

1 ISO 91:28 Certified Volume 2, Issue 5, November 212 Harmonics Analysis of VSI Fed Induction Motor Drive Abstract- This paper mainly focus on harmonics analysis of three phase AC supply available from three phase inverter which is fed to induction motor drive. Ideally the output of inverter should be sinusoidal but due to harmonics the output having distorted waveforms. Harmonics analysis has been done by using the MATLAB /simulink/fft. Index terms- Duty Cycle, Modulation-Index, Pulse Width Modulation. I. INTRODUCTION Nonconventional energy resources are very suitable solution for the modern requirement of energy demand. For the future energy demands and gives quality pollution free supply to the growing and today s environment conscious population, the present world attention is to go in the natural, clean and renewable energy sources. Most renewable forms of energy, other than Geothermal and Tidal power ultimately come from sun. Renewable energy resources may be used directly, such as solar ovens, Geothermal heating or indirectly by transforming to other more convenient forms of energy such as electricity generation through wind turbines or photo voltaic cells. Now a day about 6% of total load is motor load & 9% of this is induction motor load. AC motors are preferred on DC motor for industrial application. This is because construction of DC motor is very complicated due to use of commutator and brushes. Also commutation process involves losses due to arcing. Hence induction motor drive gives satisfactory performance as compared to DC motor drive. Energy comes from photovoltaic cell is dc before converting it in ac firstly dc is step up by Boost converter. MOSFET is used as switching devices in boost converter and it has very high switching frequency of range 2 KHz.. To get pure DC voltage in output of boost converter a DC π- filter is used. II. PULSE-WIDTH-MODULATED INVERTERS DC to AC converters is known as inverter. The function of an inverter is to change a DC input voltage to a symmetric to its AC output voltage of desired magnitude and frequency. The output voltage could be fixed or variable at a fixed or variable frequency. A variable output voltage can be obtained by varying the input DC voltage and maintaining the gain of the inverter constant. If the DC input voltage is fixed and it is not controllable, a variable output voltage can be obtained by varying the gain of the inverter, which is normally accomplished by pulse width modulation control within the inverter. The inverter gain may be defined as the ratio of the AC output voltage to DC input voltage [6]. Manjari Mehrotra, Dr. A.K Pandey 36 The output voltage waveforms of ideal inverters should be sinusoidal. However, the waveforms of practical inverter are non-sinusoidal and contain certain harmonics. For low and medium power application, square wave or quasi square wave voltages may be acceptable; and for high power application, low distorted sinusoidal waveforms are required. With the availability of high speed power semiconductors devices, the harmonics contents of output voltage can be minimized or reduced significantly by switching techniques. Pulse width modulation techniques are used in this paper for the modulation purpose. Out of 12 and 18 conduction mode later one is used due to the more utilization of switches in it. In this conduction mode each thyristor conducts for 18 and same branch thyristors will conduct after 12. With the help of such techniques harmonics are reduced in three phase voltage source inverter. Different PWM techniques are as under [11]. (i) Single-pulse modulation. (ii) Multiple-pulse modulation. (iii) Sinusoidal pulse modulation. In PWM inverters, forced commutation is essential. The three PWM techniques listed above differ from each in the harmonic content in their respective output voltage. Thus, choice of a particular PWM technique depends upon the harmonic content in the inverter output voltage [8]. In industrial applications, PWM inverter is supplied from a diode bridge rectifier and an LC filters. A. Single Pulse Width Modulations In single pulse-width-modulation control, there is only one pulse per half cycle and width of the pulse is varied to control the inverter output voltage. The gating signals are generated by comparing a rectangular reference signal of amplitude, A r, with a triangular carrier wave of amplitude, A c. The frequency of the reference signal determines the fundamental frequency of output voltage. By varying A r, from to A c, the pulse width, δ can be varied from to 18 degree. The ratio of A r to A c is the control variable defined as the amplitude modulation index. The amplitude modulation index or simply modulation index. M= (1) The rms output voltage can be found from V (2) = (3) The Fourier series of output voltage yields

2 ISO 91:28 Certified Volume 2, Issue 5, November 212 (4) Due to the symmetry of the output voltage along x- axis, the even harmonics (for n=2, 4,6 ) are absent. The Figure 1 shows the single pulse modulation. Fig 1 Single Pulse Width Modulation B. Multi-Pulse Width Modulation The harmonic content can be reduced by using several pulses in each half-cycle of output voltage. The frequency of reference signals sets the output frequency f o, and the carrier frequency f c determines the no of pulses per half cycle p. The modulation index controls the output voltage. This type of modulation is also known as uniform pulse-width modulation (UPWM). The no of pulses per half cycle is found from: (5) (6) = (7) C. Sinusoidal Pulse-Width Modulation Instead of maintaining the width of all pulse the same as in the case of multiple pulse modulation, the width of each pulse is varied in proportion to the amplitude of a sine wave evaluated at the centre of the same pulse. The distortion factor and lower order harmonics are reduced significantly [1]. The reference signal with a triangular carrier wave of frequency,. This type of modulation is commonly used in industrial applications and abbreviated as SPWM. The frequency of difference signal determines the inverter output frequency, and it peak amplitude, control the modulation index, M, and then in turn the rms output voltage the no. of pulse per half cycle depends on the frequency. The arm output voltage can be varied by varying the modulation index M. it can be observed that the area of each pulse corresponds approximately to the area under the area under the sine wave between the adjacent midpoints of off periods on the gating signals. If is the width of m th pulse can be extended to find the rms output voltage = (8) It can also be applied to determine the Fourier coefficient of output voltage as = [sin n ( + ) sin n ( + + )] (9) Where modulation ratio. is defined as the frequency Fig 2 Multi- Pulse Width Modulation If δ is the width of each pulse, the rms output voltage can be found from: 37 Fig 3 Sinusoidal Pulse Width Modulation The distortion factor is significantly reduced compare to that of multiple-pulse modulation. This type of modulation eliminated all harmonics less than or equal to 2p-1 for p=5, the lowest order harmonics is ninth. The

3 output voltage without filter ISO 91:28 Certified output voltage of an inverter contains harmonics. The PWM pushes the harmonics into a high frequency range around the switching frequency and is multiple, that is around harmonics and so on. The frequency at which the voltage harmonics occur can be related by = ( ±ĸ) (1) Where the nth harmonics equal the k th sideband of j times the frequency-modulation ratio n =j ±k =2jp±k for j=1, 2, 3.. k=1, 2, 3. The peak fundamental output voltage for PWM and SPWM control can be found approximately from =δ for δ 1. (11) Volume 2, Issue 5, November 212 IV. SIMULATION RESULTS The simulink model of inverter consists of supply, PWM inverter and filters. The line and phase voltages of inverter can be obtained from V-I measurement block from there filtered value of inverter line voltage is obtained. The simulink block diagram is designed to obtain the different waveforms. For the satisfactory performance of a drive output should be ripple free so that the input taken from the inverter is analyzed for harmonics. Firstly, the harmonics are reduced by pulse width modulation (PWM) technique. After that the ac output of inverters having ripples are fed to the low pass filter. With the use of filters ripple are reduced in the output. Fig 4 Simulink Block Diagram Of VSI Fed Induction Motor Drive The average output voltage of boost converter is found approximately same to the rated value of output voltage of boost converter. The output of boost converter is not pure dc but it contains some ripple which is observed at the scope. For getting a smooth dc a pie (π) filter is used at the output side of boost converter. Due to this filter output voltage of boost converter becomes approximately smooth and very less ripple is present in output voltage Time(sec) Fig 5 Boost Converter Output Voltage before Filter Figure 6 shows the boost converter output with filter. The X-axis shows the time (sec) and Y axis shows the magnitude of output voltage of boost converter. After filteration output voltage of 4V is reached as same of 38

4 Stator Current (A) Output voltage with filter Rotor Current (A) ISO 91:28 Certified rated value. Both diagrams can be obtained from the above simulink blocks. Firstly, some transients occur. Volume 2, Issue 5, November Time(sec) Fig 6 Boost Converter Output Voltage after Filter Figure 7 shows the inverter output voltage. From the diagram it is clear that the waveform is very much close to the sinusoidal output. THD of this waveform is about 3.5% Fig 8 Rotor Current of Induction Motor Figure 9 shows the waveform of stator current. X-axis shows the time(sec) and Y-axis shows the stator current(a). At starting there are no of spikes which are due to the transients after that steady -state condition is achieved Fig 7 Inverter Filtered Output Voltage The initial part of waveform is very distorted due to transients but at steady state the output voltage and current is approximately same as the rated value. Figure8 shows the rotor current waveform. X-axis shows the time (sec) and the Y-axis shows the rotor current. Fig 9 Stator Current of Induction Motor The speed of motor is during transient period is less but during steady state it is approximately same to the rated speed of motor i.e. 147 rpm. The mechanical torque developed by motor is observed to be N-m which is approximately same to the rated mechanical torque at the rated output power. Figure 1 shows the motor torque waveform. X-axis shows the time (sec) and Y-axis shows the torque (N-m). 39

5 Speed (rpm) Torque (N-m) ISO 91:28 Certified Fig 1 Motor torque in N-m Volume 2, Issue 5, November 212 [3] T.Ohnishi and H.Okitsu, A novel PWM technique for three phase inverter/converter, International Power Electronics Conference, 1983, pp [4] K.Thorborg and H.Irie, Trapezoidal modulating signal for three-phase PWM inverter, IEEE Transactions on Industrial Electronics, Vol.IE3,No.2,1986,pp [5] K. Thorborg and A. Nystorm, Staircase PWM: an uncomplicated and efficient modulation technique for ac motor drives, IEEE Transactions on Power Electronics, Vol IA27, No.5, 1991, pp [6] M.H Ohsato, G. Kimura, and M. Shioya, Five-stepped PWM inverter used in photo-voltaic systems, IEEE Transactions on Industrial Electronics, Vol.38, October, 1991, pp [7] P.D. Ziogas, The delta modulation techniques in static PWM inverters, IEEE Transactions on Industry Applications, March/April 1981,pp [8] R.O CaCeras and I.Barbi, A boost dc-ac converter: Analysis, design and experimentation IEEE Transactions on Power Electronics, Vol 14, No.1, January 1999, pp [9] R.O CaCeras and I.Barbi, A boost dc-ac converter: Operation, analysis, control and experimentation, Industrial Electronics control and Instrumentation Conference, November 1995,pp [1] H.S Patel and R.G Hoft, Generalized techniques of harmonic elimination and voltage control in thyristor converter, IEEE Transactions on Industrial Applications Vol IA9, No ,pp ,Vol.IA1,No.5,1974,pp Fig 11 Motor speed in rpm Figure 11 shows the motor speed waveform. Due to the minimization of harmonics in the output voltage of inverter the settling time becomes to small, here it is.3 sec and the speed will be equal to the rated speed. X-axis shows the time (sec) and Y-axis shows the speed (rpm). V. CONCLUSION In this paper the model circuit has been simulated and various waveforms of output voltage and current have been analyzed. The practical value of voltage, current, speed and torque is same to the rated value of induction motor drive. The waveform of inverter output voltage is not pure sinusoidal because of presence of harmonics. The THD analyzed from the FFT analysis is 3.5%. These harmonics are generated in two manners. One from the power semiconductor device which are used as switching device in inverter and other from the nonlinear load (induction motor drive). Due to nonlinear load inter harmonics also come in inverter output voltage. REFERENCES [1] B.Biswas and P. Purkait Current Harmonic analysis of inverter fed induction motor drive system under fault condition IMECS29. [2] B.D. Bediford and R.G. Hoft, Principal of inverter circuits. New York: John Wiley and Sons