18-08-2012, 12:14 PM
NOVEL SPACE VECTOR BASED GENERALIZED DISCONTINUOUS PWM ALGORITHM FOR INDUCTION MOTOR DRIVES
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ABSTRACT
This paper presents a novel space-vector based Generalized Discontinuous pulse width modulation (GDPWM) algorithm without angle estimation based on the concept of imaginary switching times. The conventional space vector pulse width modulation (CSVPWM) method with equal division of zero state vectors V0 and V7 is modified. A constant-variable μ and modulation phase angle δ ranging from 0 to 1 are used to generate infinite number of modulating waveforms. Also, the classical triangle comparison approach is replaced by space vector approach. To avoid the complex coordinate transformations, trigonometric calculations and sector identification involved in CSVPWM, the concept of imaginary switching times is utilized. The imaginary switching times proportional to the reference phase voltages are used to calculate the actual switching times for reduced complexity and memory required. The proposed PWM technique results in reduced current ripple over the CSVPWM at high modulation indices and low switching losses at all modulation indices. To validate the proposed method, simulation is carried out on V/f controlled induction motor and the no-load current waveforms at different modulation indices and frequencies are presented and %THDs is compared.
INTRODUCTION
Any PWM strategy for power electronic conversion is based on the factors such as waveform quality and switching losses. In recent years several PWM methods gained importance among which CSVPWM is the most popular because of the numerous advantages [1, 2]. It is capable of producing highest available fundamental output voltage with low harmonic distortion in the output current and is suitable for digital implementation. In this method the reference voltage vector is synthesized by time averaging two active states and two zero states in every sampling period, TS. However, CSVPWM suffers from the drawbacks like computational burden and inferior performance at high modulation indices. Moreover, as the CSVPWM is a continuous PWM (CPWM) method the switching losses of the inverter are also high.
CSVPWM ALGORITHM
With a three-phase voltage source inverter (VSI) there are eight possible switching states. The two states, from which no power gets transferred from source to load are termed as null vectors or zero states. The other six states called as active states. The active states can be represented by space vectors as given in (1) and divides the space vector plane into six equal sectors as shown in Figure-1 [6, 8, 12].
PROPOSED GDPWM ALGORITHM
As the CSVPWM is a CPWM technique, switching losses of the inverter are high. Where as in DPWM methods during each sampling period, each of the phases ceases the modulation and the associated phase is clamped to the positive dc bus or negative dc bus. Hence, the switching losses of the associated inverter leg during the period of clamping are eliminated. The performance of the PWM methods depends upon the modulation index. In the lower modulation range, the CPWM methods are superior to DPWM methods, while in the higher modulation range the DPWM methods are superior to CPWM methods. However at all the operating modulation indices, CPWM method has higher switching losses than DPWM methods.