10 Things to Consider When Selecting a VFD

Follow These Guidelines and Build a Reliable VFD System

By Jim Montague

Implementing variable-frequency drives (VFDs) can pay back big benefits, and adopting them is easier than it might seem at first. You just have to follow a few basic guidelines, which are highlighted in "Building a Reliable VFD System" by Brian Shuman, Belden's senior product development engineer. The whitepaper is located at

  1. What are the torque demands of the loads or processes in your planned system? Will any of the loads be hard to start? VFDs have limited over-current capacity, so hard-to-start loads may require an over-sized unit to cover higher current demands.
  2. How many motors will the drive control? If it's more than one, will they start sequentially or simultaneously? Calculate the total peak currents of all motor loads under the worst operating conditions your planned system will see. Size the VFD according to this maximum current requirement.
  3. Will your applications require a quick start or an emergency stop of the load? If so, high currents will be demanded of the VFD. Over-sizing the drive may be necessary.
  4. Is motor overheating a potential concern for any of your planned VFD applications? It may be, for reduced-speed, constant-torque applications.
  5. What range of motor sizes will your process or processes require the VFD to handle? Remember, smaller motors aren't as efficient as larger ones, so improvements due to the VFD will likely be apparent. However, since large motors use much more power, even small increases in efficiency can produce appreciable savings over the life of the motor.
  6. Will the VFD system be operating in an environment containing volatiles, airborne particulates or high ambient temperatures? For volatiles, make sure all materials used are resistant to chemicals and are properly grounded. For particulates, ensure proper sealing. For high temperature, allow for needed cooling.
  7. Do you need equipment or drive protection features that will ensure continuity in processing? Be aware that you can have a drive that trips instantly in an over-current condition, or one that maintains constant motor torque and reduces motor speed to maintain current required
  8. Do you want or need a lot of diagnostic capability in your unit? How critical is it to get a handle on load downtimes and be able to do a detailed fault analysis?
  9. Are power factor correction capacitors present on the motor loads your VFD system will have to handle? When these are switched, they typically generate power disturbances, and VFDs can be negatively impacted. Isolation transformers or line reactors may be necessary.
  10. Is it possible the power source for your planned system will occasionally be switched while the VFD is operating? This might happen, for example, when loads are switched to stand-by generators during a power outage. Some drives can handle a brief power outage, while others can't.