variable frequency drives

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Introduction to AC Drive Theory
by James M. Shumberg

Variable frequency drives are sometimes called AC drives (the author's preference), adjustable frequency drives, VFDs, or inverters. VFDs provide us a reliable means of adjusting or changing the frequency of alternating current applied to the motor and thereby changing the motor speed.
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About AC Induction Motors

Most industrial AC induction motors, manufactured in the US, are designed to operate with a current that alternates in the direction of flow 60 times per second (HZ). If this frequency of alternation is changed, the speed of the motor changes. By controlling the AC frequency with a variable frequency drive, you control speed. A proper voltage to frequency ratio (V/HZ) must be supplied. For most industrial motors manufactured in the US this ratio is 460 volts to 60HZ or 7.6:1. I.E.: to achieve 10% speed, you should supply 10% frequency (or 6 HZ). To properly apply electric power at 6HZ the output voltage of the variable frequency drive must be 45.6 volts (6HZ x 7.6). 
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