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Properly sizing your Variable Frequency Drive (VFD)

First, let’s discover what a Variable Frequency Drive, or VFD, is; a VFD is a type of computerized motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor.

Next let’s briefly explain how they work as it might sound simple but in reality, it is quite complicated; the VFD converts the AC power to DC and then inverts it back into a modified three-phase AC output. They can be and are designed for many applications and are great for maintaining pressure and flow, dissolved oxygen levels and are typically seen in most waste water applications.

Here are some basics to remember:

  • VFDs do not produce horsepower (hp)
  • VFDs do not produce torque
  • VFDs supply current that, with the correct voltage and frequency, makes the motor run at the desired speed

Finally let’s discuss sizing; VFD sizing should always be based on the application’s motor current and voltage rating, not the horse power (HP). Motors can be rated for multiple input voltages, but it is very important to know the input voltage for the application because VFDs are not rated for multiple voltages and only operate on the voltage for which they are designed, such as 100-120V, 200-240V, 380-480V or 525-600/690V.

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