Simulation and Comparison of SPWM and SVPWM FED Three Phase Induction Motor

IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013

ISSN 2320 – 6020

Simulation and Comparison of SPWM and SVPWM FED Three Phase Induction Motor Desh Deepak Gautam and V. K. Giri ABSTRACT: Generally, a voltage source inverter (VSI) is used to supply a three-phase induction motor with variable frequency and variable voltage for variable speed applications. For the control of output voltage of inverter a suitable pulse width modulation (PWM) technique is employed.PWM generation can be obtained by using different techniques such as Triangle comparison based PWM (TCPWM) and Space Vector based PWM (SVPWM). In TCPWM methods such as sine PWM, a three phase reference modulating signals is compared with a triangular carrier wave for the generation of the PWM signals. In SVPWM method, a revolving reference voltage vector is used as a voltage reference instead of using three phase modulating waves. The output voltage of the inverter can be controlled by using the magnitude of the reference signal. In present scenario, Space Vector Modulation (SVM) technique is becoming more popular technique for the speed control of three phase induction motor, synchronous motor etc.

KEYWORDS: Sine Pulse Width Modulation (SPWM), Space Vector Pulse Width Modulation (SVPWM), Voltage Source Inverter (VSI).

INTRODUCTION DC-AC converters are known as inverters which are electronic devices which are used to produce AC power from low voltage DC energy (from a battery or solar panel). So that they can be used when AC power tools are to be used but the AC power is not available. AC drives are more prevalent than dc drives. The output voltage may be fixed or variable at a fixed or variable frequency. The variable output voltage can be obtained by varying the input dc voltage and maintaining the gain of the inverter constant. On the other hand, if the dc input voltage is fixed, a variable output voltage can be obtained by varying the gain of the inverter which is normally accomplished by PWM control within the inverter. AC drives needs high power variable voltage and variable frequency supply. The research in Pulse width modulation schemes has been intensive in the last couple of decades. The PWM techniques have been used to achieve variable voltage and variable frequency in ac-dc and dc-ac converters. The PWM techniques are widely used in different applications such as variable speed drives (VSD), static frequency changers (SFC), un-interruptible power supplies (UPS) etc. The main problems are being faced by the power electronic design engineers while the reduction of harmonic content in inverter circuits. The classical square wave inverter used in low or medium power applications suffers from a serious disadvantage such as lower order harmonics in the output voltage.

Desh Deepak Gautam and V. K. Giri Department of Electrical Engineering M.M.M. Engineering College Gorakhpur 273010 (UP) India Email: dd.kanha@gmail.com1 and girivkmmm@gmail.com2

One of the solutions to enhance the harmonic free environment in high power converters is to use PWM control techniques. The objective of PWM techniques is to fabricate a sinusoidal AC output whose magnitude and frequency both can be restricted (4-7).

SINE PULSE WIDTH MODULATION (SPWM) Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a modulation technique that modulates the width of the pulse, specifically the pulse duration by comparing it to a reference signal. When a sine wave is used as a reference to compare with the carrier wave, the technique is known as sine pulse width modulation. In other words it can be said that SPWM is a method of pulse width modulation used in voltage source inverters. Instead of maintaining the width of each pulses the same as in the case of other TCPWM, the width of each pulse is varied in proportion to the amplitude of a sine wave evaluated at the center of the same pulse. This sinusoidal pulse width modulation (SPWM) is commonly used in industrial application. To change the effective output voltage of the inverter, the widths of all pulses are decreased or increased while maintaining the sinusoidal proportionality. In TCPWM methods such as sine-triangle PWM, three phase reference modulating signals are compared against a common triangular carrier to generate PWM pulses for the three phases.

The frequency of the carrier signal is very high as compared to the modulating signal. The magnitude and frequencies of the fundamental component in the line side are controlled by changing the magnitude and frequency of the modulating signal. It is simple and linear between 0% and 78.5% of six step voltage values, which results in poor voltage utilization.

The voltage range has to be extended and harmonics has to be reduce (3, 4).

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IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013

ISSN 2320 – 6020

SPACE VECTOR PULSE WIDTH MODULATION (SVPWM) The Space Vector Modulation (SVM) technique was originally developed as vector approach to pulse-width modulation (PWM) for three-phase inverters. This technique confines space vectors to be applied according to region where the output voltage vector is located. The determination of switching instants may be achieved using space vector modulation technique based on the representation of switching vectors in α-β plane. The Space Vector Modulation increases the output capability of Sinusoidal PWM (SPWM) without distorting output voltage waveform; and prevents unnecessary switching. In SVPWM methods, the voltage reference is provided by using a revolving reference vector. In this case magnitude and frequency of the fundamental component in the line side are controlled by the magnitude and frequency respectively, of the reference voltage vector.

The Space vector modulation utilizes DC bus voltage more efficiently and generates less harmonic distortion in a three phase voltage source inverter.[4]It is a more sophisticated technique for generating sine wave which provides a higher voltage to the motor with lower total harmonic distortion. The main aim of any modulation technique is to obtain variable output having a maximum fundamental component with minimum harmonics. The Space Vector PWM (SVPWM) method is an advanced; computation intensive PWM method and possibly the best techniques for variable frequency drive application (5-8).

Fig 1: Response at 8 Nm Load Torque using SPWM

SIMULATION RESULTS AND DISCUSSIONS The simulation result of three phase induction motor fed with SPWM has been shown in this section. The performance of a 5HP, three phase induction motor under the proposed scheme was experimentally studied. From results shown in figure 1 it can been seen that motor line current is nearly sinusoidal in nature, which fully satisfies the design requirement. With the increase of switching frequency the motor line current can be made more sinusoidal. At an increased load a small distortion is observed in the PWM output voltage waveform. This distortion can be filtered out using appropriate filter. The output response at 8 Nm load torque has been shown in figure 1, figure 2(a-c) and figure 3. It may be observed that the speed keeps on increasing to some level. The torque, rotor current and stator current are almost sinusoidal. SPWM output voltage and current can also be seen which are almost sinusoidal.

Fig 2: (a) Modulation index, (b) Motor Input Voltage, (c) Average Output Voltage of SPWM

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IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013

ISSN 2320 – 6020

Fig 3: Three Phase Output Voltage and Current of SPWM Inverter The output response at 11 Nm load torque has been shown in figure 4 and figure 5. From these figures it can be observed that the motor speed decreases from around 1700rpm with time and becomes constant at around 1300rpm. It may also be observed that frequency is constant after a small time around 1sec. The electromagnetic torque initially contains some instability but after some time it reached to steady state and fluctuates at around 4Nm. Figure 5 shows stator voltage and current. It can be seen easily that stator voltage is pulse width modulated and the current is almost sinusoidal. The output voltage of rectifier can be seen DC with some ripples. Figure 6 shows the output voltage of rectifier and filter used for converting single phase supply to DC supply for feeding to the PWM inverter. We can direct feed the PWM inverter by using a DC source.

Fig 5: SVPWM Output Voltage and Current

Fig 6: Output Voltage of Rectifier and Filter CONCLUSIONS

Fig 4: Response at 11 Nm Load Torque using SVPWM

In this paper first comparative analysis of Space Vector PWM with conventional SPWM for a there level Inverter has been carried out. The simulation study reveals that SVPWM gives 15% enhanced fundamental output with better quality i.e.

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IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013 lesser THD compared to SPWM. It may be concluded that Space Vector Modulation Technique may become the most popular and important PWM technique for Three Phase Voltage Source Inverters for the control of AC Induction, Brushless DC, Switched Reluctance and Permanent Magnet Synchronous Motors.

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