. The power range of VSDs start from a few hundred watts and end at tens of megawatts . The power conversion systems can be classified according to the type of the input and output power AC to DC ( rectifier ) DC to AC ( inverter ) DC to DC ( DC-to-DC converter ) AC to AC ( AC-to-AC converter ) 1.21 DC to AC(Inverters) DC to AC converters produce an AC output waveform from a DC source. Applications include adjustable speed drives (ASD), uninterruptible power supplies (UPS), Flexible AC transmission systems (FACTS), voltage compensators, and photovoltaic inverters. Topologies for these converters can be separated into two distinct categories: voltage source inverters and current source inverters. Voltage source inverters (VSIs) are named so because the independently controlled output is a voltage waveform. Similarly, current source inverters (CSIs) are distinct in that the controlled AC output is a current waveform. Dept. Of EEE, SSCE, Bangalore Page 12
Voice Controlled Home Automation Current source inverters are used to produce an AC output current from a DC current supply. This type of inverter is practical for three-phase applications in which high-quality voltage waveforms are required. A relatively new class of inverters, called multilevel inverters, has gained widespread interest. Normal operation of CSIs and VSIs can be classified as two-level inverters, due to the fact that power switches connect to either the positive or to the negative DC bus. If more than two voltage levels were available to the inverter output terminals, the AC output could better approximate a sine wave. It is for this reason that multilevel inverters, although more complex and costly, offer higher performance. Each inverter type differs in the DC links used, and in whether or not they require freewheeling diodes . Either can be made to operate in square-wave or pulse-width modulation (PWM) mode, depending on its intended usage. Square-wave mode offers simplicity, while PWM can be implemented several different ways and produces higher quality waveforms. Voltage Source Inverters (VSI) feed the output inverter section from an approximately constant-voltage source. 1.22 AC/AC CONVERTERS: ON/OFF Control: Typically used for heating loads or speed control of motors, this control method involves turning the switch on for an integral cycle and turning the switch off form integral cycles. Because turning the switches on and off causes undesirable harmonics to be created, the switches are turned on and off during zero-voltage and zero-current conditions (zero-crossing), effectively reducing the distortion. Phase-Angle Control: Various circuits exist to implement a phase-angle control on different waveforms, such as half-wave or full-wave voltage control. The power electronic components that are typically used are diodes, SCRs, and Triacs. With the use of these components, the user can delay the firing angle in a wave which will only cause part of the wave to be in output.
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