Choosing the Right Variable Frequency Drive

Choosing the Right Variable Frequency Drive

At one point or another, many people will have to decide on and purchase a variable frequency drive for their new equipment. Although the decision process may seem daunting with various models and feature sets to choose, there are really only a few questions that need to be answered in order to select the appropriate unit for your use. Take a few minutes and ask and answer the right questions, because choosing the wrong VFD model can be costly mistake.

  1. What is the available input power of your equipment, both in voltage and number of phases? Not only knowing the voltage input, but also the phase input power (1ф-Single Phase or 3ф-Three Phase) of your motor will determine VFD compatibility with your machinery.
  2. In addition to Horsepower, what is the Full Load Amperage (FLA) of the motor you intend to run with this VFD? FLA can be found on the nameplate of the motor you intend to run with your VFD. FLA information allows you to pick a drive that most closely matches your motor. If you select a VFD solely on the HP rating, you may undersize or oversize your drive. All VFD manufacturers will have both the HP and Current Rating for their drives clearly listed in their selection documentation.
  3. What is the application? Find out what type of machine the motor and VFD will be operating and determine whether the application is that of Variable Torque (VT) or Constant Torque (CT). VT applications include machinery that has low-torque requirements at slower speeds and high-torque requirements at faster speeds. CT applications include machines that require steady torque levels regardless of speed changes.
  4. What is the application environment? Stock standalone VFD’s are rated for specific atmospheres and carry ratings for Chassis (IP20), NEMA1 (IP21), NEMA12 (IP52) & NEMA4 (IP56) or NEMA4X (IP56-Corrosion Resistant). Chassis VFDs are unprotected drives and are suitable only for mounting in another enclosure. NEMA1 VFDs are protected from only large debris and provide no protection from dust or water in any form. NEMA12 VFDs are protected from dust and water dripping from less than a 15° angle to the top of the VFD. NEMA4 VFDs are protected from dust, splashing water and water sprayed on them at low pressure from a moderate distance. NEMA4X VFDs are protected from dust, splashing water, water sprayed on them at low pressure from a moderate distance and corrosive chemical components in the atmosphere. Additional NEMA ratings for various other application environments can be found here.
  5. What are the interface requirements? In all cases, there will be some type of input required from the user to make the VFD start, stop and determine the speed at which to operate. This input can come from either interaction with a digital keypad or via inputs from other buttons, switches or relays. Determine how you want to interface with the VFD and select one with the appropriate interface options.

Estimated Reading Time: 2 minutes

Greg Finley
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