Variable-torque VFDs, shown installed on a McQuay Vision indoor air handler (second panel from right), are efficient, robust and reliable, helping contractors to save time and money.

Since the 1970s, variable-frequency drives (VFDs) have been used in indoor and outdoor air-handling units, rooftop units, self-contained air conditioning systems and other applied air HVAC systems to regulate fan motor speed and save energy.

In the early days, VFDs were not always reliable. In addition, they were expensive, bulky, prone to overheating, and their mean-time-between-failure (MTBF) ranged from 2 ½ days to 2 years. The early technology was based on analog controls and silicon controlled rectifier (SCR) power devices. To combat this poor reliability, manual bypass switches were routinely installed to provide fail safe operation. If the VFD failed, the manual bypass would override the VFD, ensuring that the fan motor would continue to operate, albeit at a constant high speed.

Fortunately, we’re light years away from the old, unreliable VFD technology. Today, VFDs (also known as adjustable-frequency drives or adjustable-speed drives) are sophisticated, microprocessor-based controls with a mean-time-between-failure of more than 20 years. These new VFDs are smaller, run cooler, are readily available, and their energy-saving performance in air-handling systems is well-known.

Simply put, VFDs regulate the voltage and frequency of the current supplied to the fan motor based on need. VFDs alter the frequency and voltage of the supply current, thereby allowing adjustable motor speed. For example, an HVAC system serving a classroom can be programmed to reduce fan speed when the room is unoccupied and increase it when the room is occupied. VFDs also reduce starting current demand and provide a soft-start capability that reduces the strain on the motor and bearings.

Today, VFD technology has advanced even further with an application-specific, variable-torque drive designed for fan and pump operation. This is in lieu of the general-purpose drive that handles both variable-torque and constant-torque applications. Variable-torque VFDs provide a more cost-effective, reliable solution for any non mission-critical fan application.

Manual bypass switches on VFDs are still often a boiler-plate specification for most projects - an increasingly unnecessary leftover from the earlier days of unreliable VFDs. Even today, however, some applications mandate bypass switches with good reason for redundancy: mission-critical applications such as hospitals, health care centers, and data centers are a few examples. But in non-mission-critical applications - schools, offices, commercial buildings, retrofits and renovations - contractors can save time and money by omitting bypass switches while still having the peace of mind that their projects will operate reliably, comfortably, and efficiently.


Factory-installed or field-retrofitted variable-torque VFDs are the most reliable choice for fan motor applications because they are essentially custom-designed for starting, stopping, and operating a fan, ensuring long life and low maintenance.

Better efficiency.Variable-torque VFDs are optimized for minimal harmonic distortion and improved efficiency for fan control in air-handling units, rooftop, and self-contained systems - compared to general-purpose VFD products, which can be used for a variety of synchronous or induction motor applications needing variable speed. Variable-torque VFDs add significantly less capacitance to your power system than general-purpose VFD products that support both variable and constant-torque loads. This eliminates power factor losses from excess capacitance and typically results in the same or a lower level of harmonic performance. Modern VFDs have advanced motor control logic and a robust, three-phase diode bridge design that ensures reliable operation and also reduces excess motor noise.

More robust components. One of the primary causes of premature VFD failure are voltage spikes that can damage the diode bridge and power circuits. Variable-torque VFDs are designed with a 1,600V, PIV (peak inverse voltage) rating, providing at least 33 percent more protection for 460V applications than the typical PIV ratings of most general-purpose VFD diode bridges. This also provides protection over the full life of the VFD.

Fewer electronic components reduce costs.Variable-torque VFDs can eliminate the added cost of mitigating the capacitance that exceeds the requirements of the HVAC system. In some cases, the added cost for accessory line reactors can be eliminated while still complying with IEEE 519 harmonic distortion requirements. The newer drives also more easily handle so-called “dirty power” for 460V applications, eliminating the extra cost of power conditioning.


Without the added control complication of bypass switches, VFDs have become more plug-and-play, making them easier to get up and running while reducing installed cost. Drives installed at the factory precisely match the amperage draw of the motor. It is essentially a custom fit. Plus they are factory-wired, configured, and tested.

All that a contractor has to do is bring the power and control wiring to the control panel, and the air handler is ready for operation. In addition, factory-installed components are properly sealed to meet a manufacturer’s leakage rate requirement. Field-installed VFDs can run the risk of leakage because not all contractors will ensure that panel penetrations are properly sealed.

VFDs provide one of the best energy-saving measures for air-handling systems and can cut energy use by 30 percent depending on several variables (see more information at

They also may be eligible for energy rebates from utility companies. The Energy Policy Act (EPAct) of 2002/2005 offers tax incentives for certain energy saving measures that can be monitored and measured. Depending on the application, VFDs may qualify for this rebate.

With their newer and more reliable technology, VFDs have become workhorse energy-savers. Their reliability ensures continuous HVAC operation with little maintenance. Variable-torque VFDs for fans offer a low-cost benefit and are relatively simple to remove and replace. They are also available in stock in most major cities. For remote locations, an extra drive in inventory is not cost prohibitive in the rare event of a failure or malfunction. That saves you time and reduces your costs - always a No. 1 goal in the business of mechanical contracting.

Publication Date:04/07/2008