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Mechanical / HVACaka: VFDaka: adjustable frequency driveaka: variable speed driveaka: AFD

Variable Frequency Drive

In Plain English

An electronic controller that slows down motors on fans and pumps to save energy when full speed is not needed.

Definition

An electronic device that controls the speed of an AC electric motor by varying the frequency and voltage of the power supplied to it. Variable frequency drives (VFDs) are used on HVAC fans, pumps, and compressors to match equipment output to actual system demand, dramatically reducing energy consumption. Reducing a fan to 50 percent speed cuts power consumption to approximately 12.5 percent due to the cube law relationship between speed and power.

Why It Matters in Bidding

VFDs are a meaningful cost adder on motor-driven HVAC and pumping equipment, and they appear frequently in energy-code-driven specs and value-engineering proposals. Estimators must price not just the drive but its enclosure rating, harmonic filtering, line reactors, and the electrical and controls integration, since these accessories often exceed the drive's base cost. VFDs also intersect the electrical scope, so the bid must clearly assign who furnishes and wires them.

Example

Pricing a chilled-water plant retrofit, the estimator includes VFDs on three pump motors, adds NEMA-rated enclosures and line reactors, and coordinates with the electrical sub on who terminates the power and BAS control wiring.

Related Terms

Variable Air VolumeBuilding Automation SystemAir HandlerChiller

Frequently Asked Questions

Estimators must add the enclosure with the correct NEMA rating, line reactors or harmonic filters where required, bypass options, mounting, and the labor to wire power and controls. Integration with the building automation system and any required electrical inspection coordination also carry cost. The accessories and integration often rival the base drive price.
It varies by project. Sometimes the drive is furnished with the mechanical equipment and wired by the electrical sub; other times electrical furnishes and installs it. Estimators must read the division of responsibilities in the specs and clarify it in the bid, because an unassigned VFD is a frequent source of scope-gap change orders.
VFDs are often proposed as a VE or energy-code measure because reducing fan or pump speed cuts power consumption sharply at part load. Estimators may need to present the added first cost against projected energy savings in a payback analysis, helping the owner decide whether the drive justifies its premium over a fixed-speed starter.

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