ANALYSIS OF CASCADED H-BRIDGE MULTILEVEL INVERTER WITH LEVEL SHIFTED PWM ON INDUCTION MOTOR

摘要

Multi-level converter technology has emerged as a very important alternative in the area of high-power medium voltage energy control. Two level inverters are those which creates a voltage or a current with levels either 0 or ±V dc. To achieve an eminence output voltage or a current waveform with a lowest amount of ripple content, they need high switching frequency. When working at high frequency, high power and high voltage applications these two level inverters have a few restrictions. The multi-level inverter is to produce a quasi sinusoidal voltage from many levels of dc voltages. The obtained output waveform has more steps, which creates a staircase wave form that reaches to a preferred wave form, as the number of levels increases. By the on and off the power switches a sequence of pulses can be produced by using SPWM or sinusoidal pulse width modulation technique which is widely used in power electronics. Sinusoidal pulse width modulation is widely used in many industrial applications and also used for so many years and it is characterized by constant amplitude pulses with dissimilar duty cycle for every period because of its circuit ease and strong control mechanism. To minimize the harmonic content and to control the output voltage of inverter the width of this pulses are modulated. Many PWM techniques are present to regulate the motor, but out of that Sinusoidal pulse width modulation or SPWM is the typically used scheme in motor control and inverter application. Finally a unipolar and bipolar SPWM voltage modulation technique is proposed because this method offers the benefit of successfully doubling the switching frequency of the inverter voltage, thus creating the output filter smaller, economical and trouble free to implement.