Introduction to Variable Frequency Drives (VFDs)

What are Variable Frequency Drives?

They are power conversion devices which convert fixed voltage and frequency to a variable voltage and frequency, controlling the speed of both elements that are used for power systems. Unlike, Tesla’s invention of the 3-phase AC motor, VFDs allow AC motors to run at different speeds. VFDs are also known as AC drives, adjustable frequency drives, or variable-speed drives. AC power can be converted to DC power-however, AC power is used for most appliances.

The Different Types

The three most common types of VFDs are: current source inversion (CSI), pulse-width modulation (PWM), and voltage source inversion (VSI). CSI VFDs are mostly used for industrial processing and power generation. They produce regenerative power. PWM VFDs encode messages into a pulsing system and control the rotation of electrical motors. They are cost-efficient and are often used in the industrial production because of their strong, energy-saving power source. VSI VFDs are often the least preferred because they have very little power. Many VSI VFDs are combined with PWM VFDs outputs for better power conversion.


VFDs are the main cause of polluted power systems. Harmonics, also known as fundamental frequency, is a theory that was developed by Allen Forte. He believed that any wave shape can be structured by sound waves. Harmonic frequency is harmful to power systems. Three-phase reactors act as a harmonic filter, reducing the harmonic pollution from high-frequency noise signals.

The Benefits

VFDs are not the only way to convert power to equipment, but they preserve the most energy out of all of the other power conversion methods. They control the amount of voltage speed and electric current received by electrical equipment. They can reduce the speed of voltage by up to 20 percent and energy costs by up to 50 percent. VFDs prevent equipment problems like motor burnout, excess torque, wear on motor and pumps, shaft damage, and powerline disturbances. They can control motor temperature without the use of different controllers.

VFDs are used to power anything from solar panels and agricultural equipment to computer hardware. One is used per appliance. The type used depends on how much power is required by the equipment.

Where to Get Them

They can be purchased through a wholesaler or distributor. There are companies that allow customers to build their own drive. If a business is operating several pieces of industrial equipment, they may want to contact a distributor for special pricing.

“Reduce costs, save energy, and preserve the life of industrial equipment and appliances by using variable frequency drives.”

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