Author : Miss. Manjiree Eknath Kathar 1
Date of Publication :24th August 2017
Abstract: Most of the industrial dives use an induction motor because these motors are rugged, reliable, and relatively inexpensive. Induction motors are mainly used for constant speed applications because of unavailability of the variable-frequency supply voltage. But many applications need variable speed operations. Recently, power electronics and control systems have matured to allow these components to be used for motor control in place of mechanical gears. These electronics not only control the motor’s speed, but can improve the motor’s dynamic and steady state characteristics. Adjustable speed ac machine system is equipped with an adjustable frequency drive that is a power electronic device for speed control of an electric machine. It controls the speed of the electric machine by converting the fixed voltage and frequency to adjustable values on the machine side. High power induction motor drives using classical three - phase converters have the disadvantages of poor voltage and current qualities. To improve these values, the switching frequency has to be raised which causes additional switching losses. Another possibility is to put a motor input filter between the converter and motor, which causes additional weight. A further inconvenience is the limited voltage that can be applied to the induction motor determined by inconvenience is the limited voltage that can be applied to the induction motor determined by the blocking voltage of the semiconductor switches gives rise to the concept of multilevel inverter control which in turn controls the dynamic performance of motor. Recently many schemes have been developed to achieve multilevel voltage profile, particularly suitable for induction motor drive applications. The diode clamp method can be applied to higher level converters. As the number of level increases, the synthesized output waveform adds more steps, producing a staircase waveform. A zero harmonic distortion of the output wave can be obtained by an infinite number of levels. Unfortunately, the number of the achievable levels is quite limited not only due to voltage unbalance problems but also due to voltage clamping requirement, circuit layout and packaging constraints. In this paper, a three-phase diode clamped multilevel inverter fed induction motor is described. The diode clamped inverter provides multiple voltage levels from a series bank of capacitors. The voltage across the switches has only half of the dc bus voltage. These features effectively double the power rating of voltage source inverter for a given semiconductor device. The proposed inverter can reduce the harmonic contents by using multicarrier PWM technique. It generates motor currents of high quality. V/ƒ is an efficient method for speed control in open loop. In this scheme, the speed of induction machine is controlled by the adjustable magnitude of stator voltages and its frequency in such a way that the air gap flux is always maintained at the desired value at the steady-state. Here the speed of an induction motor is precisely controlled by using three level diode clamped multilevel inverter.
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