By Mike Brandt, PE

The use of Variable Frequency Drives (VFDs) to control electric motors has increased over the last few decades. Not only can they provide control, but they can also increase efficiency and reduce costs through operational energy savings. We have used them on industrial projects to reduce the water hammer in a pumping application or to start a conveyor belt or production process line.

A simple Google search will show you lists of reasons why you should use VFDs, including their ability to:

  • Vary a motor’s speed to match the application, saving energy that can’t be done when using other motor starting methods.
  • Match the speed of the motor to the process with a single unit versus using a system that requires more energy to run and needs more equipment to reduce its speed.
  • Eliminate speed reduction equipment, such as gearboxes, belts, valving, etc., that require maintenance but also add inefficiencies that the motor must drive.
  • Adjustable torque limiting.
  • Reduce power line disturbances over across-the-line motor starting.
  • Operate in reverse operation.
  • And several others.

VFDs can also limit a motor’s starting in-rush current, reduce electrical peak demand charges, and ramp up the motor speed to a control setpoint.

Yet, with all the benefits of using a VFD to run an electric motor, there are some inherent disadvantages to consider that will help prevent premature motor failure. One problem we’ve experienced is motor bearing issues on industrial projects. However, after experiencing these issues firsthand, we’ve found two main mitigation techniques that can be put into place to minimize or eliminate these problems.