Motion Control Systems Integrator
VFD - Variable Frequency Drives

Variable Frequency Drives for Industrial Applications

Prior to the 1980s, variable-speed industrial applications required DC drives and motors because AC motors could not run at variable speeds without impractical, expensive drives and controllers. Subsequently, motor-controller manufacturers have been developing and distributing practical, economical variable-frequency drives (VFDs) for standard, induction AC motors. Controlling the output frequency of a VFD varies the motor speed: A low frequency drives the motor at a slow speed. A high frequency drives the motor at a fast speed.

What is a VFD?
A VFD is a motor controller that converts three-phase, 60Hz AC to variable-frequency AC that drives an induction motor at a frequency-proportional speed. A VFD consists of three sections:

  • A rectifier, usually a full-wave bridge, that converts the AC input to rippled DC.
  • A DC bus that flattens the DC, removing almost all ripple so that the DC is clean. Too much ripple can cause harmonics that waste energy in a motor and can cause it to overheat.
  • An inverter that converts the clean DC to a variable-frequency AC output, which is positive and negative sets of variable-width, constant-voltage pulses. In the motor windings, these pulses integrate as sinusoidal current waves.

VFD Benefits
VFD benefits include:

  • Lower energy costs because VFDs contain modern, extremely-efficient electronic components.
  • Lower maintenance costs because VFD applications require less maintenance. For example, AC motors are brushless, but DC motors contain brushes that need replacement and are difficult to maintain. Also, there are usually no mechanical transmissions to maintain because VFDs vary motor speeds electronically.
  • Longer motor life. Motors run cooler and last longer because VFDs efficiently limit motor-winding current at startup and during acceleration.
  • VFDs provide smoother acceleration and deceleration, which places less stress      on the driven application equipment.

Industrial VFD Applications
VFDs provide greatest efficiency increases in variable-load applications that require motors to run at less than maximum design speeds. Industrial VFD applications include:

  • Conveyer systems
  • Pumps
  • Fans
  • Compressors

Industrial variable-frequency drives provide accurate and efficient speed control in motor-driven industrial applications. VFD standards are still evolving because associated technologies are relatively new. Therefore, industrial companies should carefully analyze their VFD requirements, consult with a drive specialist, and then purchase the best VFD drivers and AC motors.