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Variable Frequency Drives (VFD)

Variable frequency drives (VFD) are a type of industrial equipment which vary frequency and voltage input to AC motors, thereby controlling speed and torque. Systems including variable frequency drives generally consist of three primary components: an AC (alternating current) electric motor, a controller which serves as an electrical current conversion system, and an operator interface designed for use by a human operator to control the motor speed, which usually includes an alphanumeric display and cable-connected keypad. Often, the operator interface also includes a serial communications port to enable computerized configuration and control as well as input and output (I/O) terminals for connecting further devices. The entire VFD operation is regulated by an embedded microprocessor programmed via proprietary firmware.

Applications of variable frequency drives may be abstractly classified into four ‘quadrants’ based upon the direction (forward or reverse) of the motor’s speed and torque. This classification provides a simple means of conceptualizing the different operations of VFDs by considering the motion of the AC motor. Quadrant I operations entail forward acceleration with positive speed and torque, and Quadrant II operations involve forward deceleration, or braking, with positive speed and negative torque. Quadrants I and II thus form a pair of possible states of forward motion–one in which the ‘load’ of the motor is accelerating forward and one in which it is decelerating, but still moving in a forward direction. Similarly, Quadrants III and IV describe possible states of reverse motion. Quadrant III is defined as uniform reverse acceleration with negative speed and torque, and Quadrant IV describes reverse deceleration with negative speed and positive torque. This quadrant schema accounts for all the possible motion variables, enabling complete description of how a given VFD is used.

VFDs start and stop motors gradually. During a starting sequence, a low frequency and voltage are applied, and then increased at a controlled rate for acceleration. Oppositely, the frequency and voltage are decreased gradually at a controlled rate during the stopping sequence, and the motor is shut off when the frequency approaches zero.

Main benefits of VFD systems in the context of industrial control panels include significant savings of energy. By operating motor load applications at variable rather than fixed speed, VFDs lead to savings of energy cost in addition to giving operators greater control over performance.


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