What is a VFD and what does it do in an HVAC systemSolutionA

What is a VFD and what does it do in an HVAC system?

Solution

A 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.

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.

The 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.