Variable Frequency Drive (VFD)…
What Is It, And Why Use It In HVAC?

Abbie Stancato At MicroMetl CorporationA Variable Frequency Drive (VFD) controls the speed of a motor, specifically a three phase motor. To better understand a VFD, let’s examine the motors they control.

Most commercial HVAC systems require powerful motors, and often operate on a three phase power (The standard method for electric power distribution systems because of its efficiency). A three phase motor is cheaper to operate than single phase motors because it uses less current (less current often results in reduced wire size and cost), especially where large motor requirements are necessary.

Most three phase motors are connected to HVAC fans and operate at a fixed speed. The motor is either coupled directly to the fan, or will operate the fan using a belt & pulleys (the combination of pulleys control the fixed speed of the fan).

Let’s assume you have an application requiring a two speed HVAC fan. Seems simple enough to install a two speed motor. However, you couldn’t possibly think simplicity would be the correct answer.

May I briefly digress; I recently completed a video which explains How To Read And Use An Air Duct Calculator.  In the video I explain the airflow issues an HVAC unit will encounter with duct-work, and other air distribution parts which comprise a HVAC system. Plenums; duct angles & reducers; curb adapters; concentrics; dampers sections and the like, all introduce resistance (static pressure), and affect the amount of air, or output of air from a HVAC unit.

That stated, let’s assume you perfectly balanced the HVAC fan airflow throughout the building at the low speed of your two speed motor.  Quickly after initiating the second fixed motor speed, you will discover the results are not as predicted. Why? Because the output of air from the fan is directly related to the static pressure (resistance) of the HVAC distribution system. The system’s static pressure will change to the increased air velocity affecting the air volume output of the fan.  If you’re getting too much air, you may be forced to introduce static pressure into your system on speed two. Should the airflow be too low, you must either change fan size or introduce inline duct fans or the like. Either way, it increases the project costs and aggravates the building owner.

Variable Frequency Drive VFD At MicroMetl.jpgThe solution is a Variable Frequency Drive (VFD). A VFD controls the speed of a three phase AC motor, while resulting in energy savings! It saves energy because the motor does not require as much current at lower speeds. Although it resembles the functionality of a multi-speed motor, it is very different.

A Variable Frequency Drive (VFD) can be set to specific speeds. This allows a motor to obtain multiple speeds, which can be adjusted or tweaked for precision. Additionally, they can be operated via controls to constantly vary the speed of a fan. An example is the MicroMetl Modulating Power Exhaust. As noted in a previous article; when connected to a pressure transducer, as building pressure varies, the pressure control will fluctuate and vary the speed of the VFD allowing the power exhaust to quickly adjust to varying pressures. A very different application is the MicroMetl Energy Recovery Units (ERV). It uses the integration of VFD’s to offer a constant air volume to a HVAC rooftop unit while monitoring changes in system static pressure from items such as dirty air filters, a dirty ERV wheel, system damper positions and the like. Therefore as the static pressure within the system changes, controls will alter the speed of the ERV fans as to maintain a consistent air volume output.

Other Advantages…

  • A VFD eliminates high in-rush current – When a standard motor is turned on, it draws a very high amount of current for a very short duration of time (acting as a short circuit for a brief amount of time). This spike of current draw can cause some circuits to overload. Additionally, the selection of over-current protection devices such as fuses and circuit breakers is made more complicated when high inrush currents occur. A Variable Frequency Drive is designed to slowly start, therefore preventing inrush current; reducing the stress on the components of the motor; thus extending the motor life.
  • The VFD can also be integrated into a 120 volt, or 220 volt single phase system by connecting single phase lines into the VFD, and connecting the output to a three phase motor.
  • Many manufacturers are integrating the drives directly into the fans as seen more often with backward incline fans.


  • Variable Frequency Drives require specific fuse types and can cause Harmonic Distortion. Harmonic Distortion affects other equipment in a building by basically transmitting noise back through the power wires supplying the equipment. This is easily cured with Line Reactors or Isolation Transformers. The Line Reactor is the most common and economical component used to isolate Harmonic Distortion.
  • Additionally, no one size fits all! You must acquire a VFD to specifically match the size and voltage of the motor (by Horse Power), as well as the incoming voltage to the VFD. You can undersize the motor to a VFD (match a 5 HP VFD to a 3 HP motor), but never oversize. For the sake of economics, always properly size the VFD to the Horse Power of the motor. Additionally, it is recommended you match only one motor to one VFD. You can operate two motors from a single VFD if properly sized (Example; One 5HP VFD to Operate Two 2HP Motors). However, if one of the motors thermally shuts down for any reason, once it resets and restarts, it will error out the VFD, and may damage it.

Not All Motors Are Equal

Without getting too complicated; a VFD affects the speed of a motor by changing the frequency of the power sent to a motor. A standard motor is not appropriately designed to overcome the negative effects including additional heat, audible noise, vibration, bearing problems, and insulation breakdown. A motor operating at very low speed with a VFD will cause the motor to generate heat. If you’re constantly controlling the motor at very low speeds, you should consider using a small motor.

Wrapping It Up!

Other common names for a Variable Frequency Drive are; Adjustable Speed Drive, Adjustable Frequency Drive, AC Drive, Microdrive, and Inverter. A VFD is a great solution when a flexible fan speed solution is required. The integration of Variable Frequency Drives in HVAC units are becoming required by code. The programming of some VFD’s are complex and can be overwhelming. I suggest you understand its basic functionality, but leave the programming to a skilled electrical worker.