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ANALYSIS AND COMPARISON OF SPACE VECTOR & SINUSOIDAL PWM CONTROLLED TWO LEVEL INVERTER FED 3-( INDUCTION MOTOR

C. Harinatha Reddy K. Santhosh Kumar R. Durga Rao

Associate Professor PhD Student Associate Professor

V.I.T.S Deshmukhi IIIT, Gachbowli V.I.T.S Deshmukhi

Hyderabad Hyderabad Hyderabad

Under the Guidance of Dr. G. Tulsi Ramdas Director for Academic Planning , JNTU, Hyderabad

and Dr. T. Brahmanada Reddy, Associate Professor, GPREC, Kurnool.

ABSTRACT

The current generation of variable speed drives is typically based around the real-time digital generation of pulse-width modulated (PWM) waveforms using either microprocessors or application-specific integrated circuits in many ways. PWM generation techniques for getting the control of inverters have the major advantages that component drift and tolerance problems associated with earlier analog implementations. Of all the various PWM techniques Sinusoidal and Space vector PWM techniques are more prominent. The implementation of these techniques through MATLAB Simulation is discussed in this paper.The Space Vector technique is essentially based around the decomposition of a reference voltage vector into voltage vectors realizable on a six pulse inverter although, as it will be shown, space vector modulation is actually a special case for the Sinusoidal PWM technique. A Two level Inverter is being used for the Space control of a 3-phase Induction Motor. The Two control strategies, namely, Sinusoidal Pulse Width Modulation and Space Vector Modulation are to be employed for the V/F control in order to have the control on Speed and Torque. A comparison is presented on the performance of a 3-phase Induction Motor, between Sinusoidal PWM technique and Space vector PWM technique. These aspects of space vector modulation are discussed in this paper to illustrate their practical engineering significance. The MATLAB Simulator is employed for the Simulation and Analysis work.

Keywords PWM Techniques, Sinusoidal PWM, Space Vector PWM Induction motor,Speed control, Torque

1 INTRODUCTION

The current generation of variable speed drives is typically based around the real-time digital generation of pulse-width modulated (PWM) waveforms using either microprocessors or application-specific integrated circuits. Digital PWM generation techniques have the major advantages that component drift and tolerance problems associated with earlier analog implementation are eliminated while real-time waveform generation provides essentially instantaneous control of the amplitude and phase of the resulting PWM waveform, as is required in more complex motor control strategies. While a microprocessor generation technique allows considerable flexibility for optimizing the resulting PWM waveform, the upper switching frequency of the inverter is often constrained by the computation time required to complete the PWM switching times in the microprocessor.

To date, the prevalent real-time digital PWM waveform generation technique has been the regularly

sampled, asymmetrical triangulation method. In this method the PWM switching times are defined by the intersection points of a high frequency symmetrical triangle (carrier) waveform with a sampled reference (sinusoid) waveform. Recently however, a new PWM generation technique viz. space vector modulation has gained favor. The space vector technique is essentially based around the decomposition of a reference voltage vector into voltage vectors realizable on a six pulse inverter although, as it will be shown, space vector modulation is actually a special case of the triangulation technique. Compared to the traditional triangulation technique, space vector PWM waveforms are shown to have advantages with reference to ease of generation, and reduced motor losses. These aspects of space vector modulation are discussed to illustrate their practical engineering significance. Here mainly the technique is stressed on 3-phase two level inverter and the load is taken as 3-phase induction motor. The simulation of both Sinusoidal and Space vector PWM techniques and their results are discussed in this paper.

2. Need for the Inverters and their control

DC to AC converters are known as inverters. The function of an inverter is to change a dc input voltage to a symmetric 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. On the other hand , if the dc input voltage is fixed & 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 (PWM) control with in the inverter. Inverters are widely used in industrial applications like variable-speed ac motor drives , stand by power supplies and uninterruptible power supplies. The input may be a battery , fuel cell solar cell or other dc source. Inverters can be broadly classified in to two types as Single phase inverters and Three Phase Inverters. These inverters generally use various PWM control signals for producing an ac output voltage. The various methods for the control of output voltage of inverters can be classified as:

a) DC bus voltage control method (External control of dc input voltage).

b) PWM techniques (Internal control of the inverter).

The schematic of DC bus voltage control method is as shown in figure2, in which the rectifier provides the dc supply to the inverter. The inverter is used to control the

fundamental voltage magnitude and the frequency of the Ac output voltage. AC loads may require constant or adjustable voltage at their input terminals, when inverters feed such loads, it is essential that the output voltage of the inverter be so controlled as to fulfill the requirements of the load.

Fig 2

In many industrial applications, to control the output voltage of the inverters is often necessary to cope with the variations of dc input voltage, to regulate voltage of inverters, and to satisfy the constant volts and frequency control requirement. There are various methods to vary the inverter gain. The most efficient method of controlling the gain is to incorporate PWM control with in the inverters. Among the various PWM controls Sinusoidal Pulse width and Space vector Pulse width control are mostly preferred. In this paper both Sinusoidal and Space vector PWM controls are simulated using Matlab Simulink by considering the Two level Inverter fed 3-phase Induction motor for different cases such as No-load and Load conditions.

Sinusoidal PWM Technique for Two Level Inverter

The general principle of SPWM states that whenever a triangular carrier wave of frequency fc is compared with the fundamental frequency f of the sinusoidal modulating wave then the points of intersection determine the switching points of power devices. Sinusoidal Pulse Width Modulation (SPWM) is one of the most popular schemes devised[2] for the control of a two-level inverter. In SPWM, a modulating sine wave corresponding to the fundamental frequency of the output voltage is compared with a triangular carrier wave of high frequency, which corresponds to the switching frequency of the devices. Each leg of the two-level inverter is controlled by the corresponding modulating wave. The modulating waves for the individual legs are displaced by 120° with respect to each other as shown in the top trace of Fig 4.2.

Fig 4.1 A typical Two-level inverter

Fig 4.2 Modulating and carrier signals(TOP) and pole voltage VAO (BOTTOM) showing two voltage levels

Thus, the inverter employed in the system shown in Fig.4.1 is a two-level inverter because any pole voltage e.g. assumes one of the two possible values namely 0 (when S4 is turned on) or (when S1 is turned on) as shown in Fig. 4.2 or vdc. The ratio of the peak value of the modulating signal and the peak value of the carrier signal is defined as the amplitude modulation ratio (also called modulation index) and is denoted as ma .The ratio of the frequencies of the carrier wave and the modulating wave is defined as the frequency modulation ratio and is denoted as mf . In the range of linear modulation, 0 < ma ................
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