Proper Application of VFDs


Variable frequency drives (VFDs) are an integral part of process design and plant operations. The advantages of having a variable-speed and reversing-capable motor include greater control of process conditions such as flow rate, pressure, conveyor speed, etc. Along with this process advantage, there are also energy savings from not running equipment at full speed when not needed. Since the cost and size of these motor starters have continued to shrink over the years, we see them being applied and retrofitted into more and more applications.

Here are a few considerations to review when applying VFDs in both new installs as well as retrofit applications:

  1. Motor selection. One consequence of variable speed is the generation of harmonics by the VFDs’ internal electronics. These harmonics can cause high-voltage spikes on the motor leads and end up at the motors. If the motor is not VFD rated, which includes higher-voltage insulation in the motor, then the motor will get “pinholes” in the insulation and eventually fail. Also, if the motor is in a hazardous-rated gas or dust area, then it is required to be a VFD-rated motor due to the possibility of the motor getting hotter than allowed by the hazardous area requirements. Overheating can be caused by harmonics as well as the cooling fan on the motor running slower when the motor is operating at a reduced speed.
  2. Motor cabling selection. One of the best ways to mitigate harmonics generated in VFD electronics is to make sure the cabling on the load side of the VFD out to the motor is using a VFD-rated cable (a shielded cable with an increased ground size). It serves two purposes: 1) It contains the harmonics inside the cable so that it doesn’t affect other sensitive electronics in the area such as level transmitters, temperature transmitters, etc., and 2) it conducts those harmonics back to the VFD where they can be properly dealt with and mitigated.
  3. Grounding. Most issues arising from harmonics are due to improper grounding of the VFD, the VFD cable, and the motor. Having a good quality and adequately grounded system according to the manufacturer’s instructions is critical in dealing with these harmonics.
  4. Long motor lengths. High-voltage spikes can cause more issues as motor lengths get longer. As motor leads get to 300-500’ in length, additional considerations for Load Reactors, dV/dT filters, or other mitigation strategies need to be considered to overcome issues like reflected wave (voltage spike) issues and cable charging currents that can cause VFDs to fault out.

With proper design and installation, VFDs are a great way to increase the efficiency and control of your system. If you have questions about or issues with your VFDs, please contact us for an evaluation.

Brent Kooiman, Senior Project Engineer

This article was originally published in the Current Connections Fall 2020 issue.


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