What are AC variable frequency drives?

In an HVAC system, adding a Variable Frequency Drive to different motors can deliver several benefits. What are the advantages, and are VFDs something you should consider adding to your facility’s system?

In an HVAC system, a Variable Frequency Drive (VFD) is an electrical device that’s added to increase or decrease the revolutions per minute (RPM) of the alternating current (AC) motor used to power the system’s fans, pumps, compressors, or other components. To achieve this, the drive modulates the electrical power frequency delivered to the motor. The result is that system airflow can be varied with far greater precision.

This is significant because, in general, heating, ventilation, and air-conditioning (HVAC) systems are inherently designed to operate only at peak load despite such loads being required relatively infrequently, usually for only short periods, across a year.  This means the system is by nature inefficient, a design shortcoming that deploying a VFD corrects

It's worth saying that there’s nothing controversial in this assertion. Variable frequency drives are long-proven tools for better managing commercial HVAC systems. Their impact on performance is undisputed, not only in terms of reduced consumption but also reduced stress on the motors themselves. Given that lower energy costs and extended motor life are highly desirable outcomes, it’s easy to see why if you’re not using one already, it’s worth considering doing so. Let’s dig deeper into the details and answer the question “why?”.

How HVAC fans work

We need to start with an understanding of how HVAC systems work. As we’ve already noted, commercial HVAC fans, pumps, and compressors (the motor-driven elements of the system) are designed to perform only at peak loads. This is unnecessary because a building’s heating and ventilation requirements are variable. As a result, when only reduced airflow is needed, the system will rely on throttling to deliver the required outcome. However, when it does this, drive motors still operate a full speed. When this happens, in simple terms the system still works just as hard, even though output is reduced.

That makes little sense, so why is it the case? Principally, because there are three fundamental issues in HVAC system design.

Designed for peak loads

First, HVAC systems have historically been sized only for peak load conditions. So, almost all facilities have HVACs working harder than they need to deliver the heating and ventilation outputs/performance required.

Oversized by design

Secondly, and it’s another historically embedded issue, HVAC systems are routinely oversized. What this means is that not only are they designed from first principles for peak load conditions but, they have no mechanism for anticipating or accounting for unexpected or unpredicted loads. The result is that system performance doesn’t match performance requirements at any given moment. It’s an “all-in, all of the time” approach which may mitigate failure to meet requirements at a specific time but does so only by over-delivering all of the time.

The induction motor “problem”

Finally, HVAC systems are usually powered by induction motors. That’s because these motors can perform in demanding conditions cost-effectively so on the face of it, they make sense. However, in mechanical terms they’re sub-optimal. In an induction motor, there’s no built-in mechanism to slow down the rate of the motor and get a reduction in power required.

Take these three issues together and you can see why a variable frequency drive provides a needed counterweight. The VFD will convert the variable line current in an AC motor into a direct current and then modulate the direct current as required so that the voltage and frequency being used match the load placed on the HVAC system. Put more simply, the VFD means the HVAC motor can work harder when demands are high while using less power when demands decrease. The facility manager gains control over the motor’s operating speed without compromising flow rate and system performance.

Do you need a variable frequency drive?

We could leave it here and say the argument for VFDs is made. The realities outlined above make a compelling case for deploying a variable frequency drive if you’re not already using them. But let’s summarize the advantages just to hammer home the point. The benefits of deploying a VFD are both mechanical and commercial, as follows:

1. As with any mechanical system reliant on moving parts, wear and tear is a constant issue. Components in a motor will erode more quickly over time based on greater demands put on the parts. Obviously, a motor designed to continually output to meet peak loads will require more maintenance and part replacement than one that isn’t so specified, and at times it may break down altogether. This issue is mitigated by the addition of a Variable Frequency Drive.
2. There’s also unnecessary wear on tear on your budget (not just your motor!), which is an inevitable by-product of a motor running at full speed when it doesn’t have to. Reducing motor speed by a quarter has been shown to reduce energy consumption by over 50% so if, using a VFD, you can halve motor speed then the knock-on effect on your systems energy consumption (and your costs) will be dramatic. Energy savings will be significant. 
3. Many businesses face significant seasonal differences in electrical use and consumption. Peaks vary according to times of year and energy charges often disproportionately reflect this. If a VFD is deployed to make currents reflect actual need, then many unnecessary charges will be avoided.

While the headline advantages of VFDs are listed above, there are many other benefits. These include better ventilation because building airflow is more closely matched to requirements and the fact that air quality indoors may also be improved due to greater system control over temperature, humidity, pressure, and carbon dioxide (CO₂) levels in buildings with a VFD in place.

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