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What is a Medium Voltage Drive (VFD)?


2020-medium voltage or VFD v2-2Medium voltage drives (also called VFDs or variable frequency drive) are adjustable speed drives used to control the torque and speed of AC motors. The use of VFDs in industrial powertrains is increasing, in part due to a reduction in their cost and size. If you are in the market for a VFD or you need to replace one, there are certain basics that you need to know.

Basis for VFDs

The equation representing the relationship between power, voltage, and currents is extremely simple: P = VI, or power = voltage x current. From this simple relationship we can see that at a given power level, the voltage and current are inversely proportional. In other words, the higher the supply voltage, the lower the current draw will be.

For AC motors used in high-power applications, operating at a low voltage causes the motor to draw a very high current, resulting in higher energy usage, lower efficiency, and higher cost -- and none of that is good. However, based on the power equation we just discussed, we can see that the current draw can be reduced by increasing the voltage supplied -- and that is the basis for medium voltage drives.

What Is a Medium Voltage Drive

In the context of drives, medium voltage is classified as 2300 or 4160 VAC (3300 or 6600 VAC in Europe and most of the world). That said, most medium voltage drives fall into one of these categories current source inverter (CSI) or voltage source inverter (VSI).

  • CSI medium voltage drives receive a constant current and then output a three-phase AC current that can be adjusted
  • VSI medium voltage drives also output an adjustable three-phase AC voltage but instead of constant current their input is a constant voltage

VSI medium voltage drives also output an adjustable three-phase AC voltage but instead of constant current their input is a constant voltage

There is one thing they both have in common: both the adjustable AC voltage and current include variable frequency, which is why medium voltage drives are often called medium voltage Variable Frequency Drives.

Where Medium Voltage Drives are Used

There are many different industries that depend on medium voltage drives, including wastewater plants, petrochemical, mining, food and drug, and general manufacturing. VFDs are part of the powertrain for conveyor belts, mixers, pumps, compressors, and blowers. That said, you'll most often see VFDs used when the power requirements are 500 - 20,000 hp. Within this range, the difference in current draw between low voltage and medium voltage is significant.

Benefits of Medium Voltage Drives

Here are some of the benefits of medium voltage drives:

  • Savings in costs related to capital and operation when compared to other equivalent solutions
  • Substantial energy savings made possible because VFDs only use as much energy as the load actually requires
  • Peak energy charges avoided because VFDs have a soft start where speed is gradually increased through varying output voltage, current and frequency
  • Greater operational flexibility and control accuracy when compared with equivalent fixed speed operation and low voltage VFDs
  • Smaller physical footprint than more traditional solutions

Getting the Right Medium Voltage Drive for You Application

In order to get the right medium voltage drive, here are some key considerations:

  • What is the application? The type of drive you choose will depend heavily on what you will be using it with and for.
  • What size do you need? This information can usually be determined by looking at the face plate of the motor it will be driving. Look for voltage, full load amp rating, how many phases the motor has, and it's horsepower.
  • How easy is it to safely to maintain your panel? If your answer is not easy, then the life of your VFD will be shortened and there will be dangerous safety issues.

You should also have a good idea of how you will be controlling and monitoring your panel (VFDs work in conjunction with an AC motor and a VFD controller with an operator interface) and make sure that you can keep it clean, cool, and dry at its intended location.

Also note that if you don't have a good quality power supply for the VFD then you may have issues with its performance. And, last but not least, before making a final decision, take time to inform yourself about the certifications and warranties involved and determine if there is any optional equipment you need.


Medium-voltage drives are an integral part of many modern powertrains in a variety of industries that probably include yours. VFDs offer several benefits over more traditional approaches such as fixed speed motors and are relatively easy to use. If you have any questions about whether a VFD is right for your drive train or even medium voltage rewinds, you can contact us here at HECO and put our experience with medium voltage drives to work for you.

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