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Inverters are circuits specifically designed to change dc voltage to ac voltage. Systems such as variable-frequency motor drives and uninterruptible power supplies (UPS) convert ac power to dc and then convert the dc back to ac. This may sound like an odd way to provide an ac output voltage if ac voltage is the original supply, but in the case of the variable-frequency motor drive, the frequency of the supply voltage will be 50 or 60 Hz and the output ac voltage needs the possibility of frequencies between 1-120 Hz. In the case of the UPS, the ac supply voltage needs to be changed to dc so it can be stored in a battery for later use if the power supply is interrupted. Since the voltage is changed to dc and is stored in a battery, it must be changed back to ac to be usable. In the UPS, the output frequency will be a constant 60 Hz.
The earliest use of converting dc voltage to ac voltage was in circuits specifically designed for providing variable-frequency ac voltage for single- and three-phase ac motors. Today inverter circuits similar to the original ones are used to provide three-phase voltage from single-phase voltage sources, to provide three-phase voltage with variable-frequency and variable voltage from a fixed three-phase power source, and to provide isolation by using batteries as buffers or storage. Modern industrial circuits use one of three types of inverters: variable-voltage input (VVI), pulse-width modulation (PWM), and current-source input (CSI).
It's extremely important to realize that inverter circuits may be found in equipment as one part of the total system or they may be a stand-alone circuit. For instance, in an ac variable-frequency motor drive the inverter is only a part of the total circuit, and its job is to change dc voltage back to ac voltage.
At this point one may not fully understand why all the equipment and circuits are needed to change voltages and frequencies in industrial applications, but keep in mind that in factories today, a large variety of expensive equipment exists that may come from different parts of the world. Therefore, requirements for voltages and frequencies may not match what is available in the factory. In some instances, this equipment is one-of-a-kind and must be installed and used as is, which requires circuits that can change voltage and frequency easily.
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