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ined with feedback devices such as tachometers, encoders and resolversin a closed-loop system, are continuing to replace DC drives in demandingapplications.Encoders can be added to inverter-dutythree-phase motors foruse in closed-loop vector drivesystems.By far the most popular AC drive today is the pulse width modulated type.Though originally developed for smaller-horsepower applications, PWM isnow used in drives of hundreds or even thousands of horsepower – aswell as remaining the staple technology in the vast majority of small integraland fractional horsepower “micro” and “sub-micro” AC drives.Pulse width modulated refers to the inverter’s ability to vary the outputvoltage to the motor by altering the width and polarity of voltage pulses.The voltage and frequency are sythesized using this stream of voltage pulses.This is accomplished through microprocessor commands to a series ofpower semiconductors that serve as on-off switches. Today, these switchesare usually IGBTs, or isolated gate bipolar transistors. A big advantageto these devices is their fast switching speed resulting in higher pulse orcarrier frequency, which minimizes motor noise.“Sub-micro” drives provide awide array of features in a verysmall package.-58-