Table of Contents
Variable frequency drives are one of the most effective energy-saving tools in modern HVAC. By matching motor speed to demand instead of running flat out, they can sharply reduce the energy a fan or pump consumes.
But the same drives that lower your energy bill also introduce a problem most facility teams never see coming: they slowly destroy the bearings in the motors they control.
This article explains what a VFD is, how it works in an HVAC system, why it saves energy, and the hidden cost it carries for your motors, along with how to prevent it.
What Is a VFD in HVAC?
A variable frequency drive (VFD) is an electronic controller that adjusts the speed of an AC motor by varying the frequency and voltage supplied to it. In an HVAC system, that motor is usually driving a fan, pump, or compressor. You may also see VFDs called variable speed drives, adjustable frequency drives, or inverters, but in HVAC the term almost always refers to this kind of motor speed control.
Instead of a motor running at one fixed speed and being throttled mechanically with dampers or valves, a VFD lets the motor run only as fast as the current heating or cooling load requires. That simple change, matching output to demand, is the foundation of everything a VFD does for an HVAC system.
How a VFD Works
A VFD does its job in three stages.
First, a rectifier converts the incoming AC power to DC. Next, a DC bus smooths and holds that power. Finally, an inverter switches the DC back into AC at whatever frequency the drive selects, using a technique called pulse width modulation (PWM).
Because an AC motor’s speed is tied to the frequency of its power supply, changing that frequency changes the speed. Lower the frequency and the motor slows down; raise it and the motor speeds up. This is what allows a single HVAC motor to ramp smoothly from a trickle to full output and hold any point in between.
Why HVAC Systems Use VFDs
The main reason is energy. Fans and pumps follow what are known as the affinity laws: the power they draw rises with the cube of their speed.
In practical terms, a small reduction in speed produces a large reduction in energy use. A fan running at 80 percent speed can draw roughly half the power of the same fan at full speed. Because most HVAC equipment spends the majority of its life serving partial loads, running motors only as fast as needed adds up to substantial savings over a year.
VFDs bring other benefits as well. They provide a soft start, ramping a motor up gradually rather than hitting it with full inrush current, which reduces stress at startup. They give finer control over temperature, airflow, and pressure. And by avoiding constant full-speed operation, they can extend the service life of belts, couplings, and other driven components.
Where VFDs Are Used in HVAC
Almost anywhere there is a motor whose output needs to vary, including:
- Air handling units and fan-coil units
- Supply, return, and exhaust fans, including fan-wall arrays
- Chilled-water and condenser-water pumps
- Cooling tower fans
- Chiller compressors
In a large commercial building, data center, hospital, or industrial plant, that can mean dozens or hundreds of VFD-driven motors running around the clock.
The Hidden Cost: How VFDs Damage Motor Bearings
Here is what the energy-savings story usually leaves out. The same fast switching that makes a VFD efficient also creates a voltage on the motor shaft.
When a drive switches its output on and off thousands of times per second, it produces high-frequency effects that induce a voltage between the motor shaft and the motor frame. This is called shaft voltage. When that voltage climbs high enough to overcome the thin film of lubricant in a bearing, it discharges through the bearing on its way to ground, arcing across the rolling elements and raceways.
Each discharge is tiny, but they happen constantly. Over time, the accumulated discharges erode the bearing surface through a process much like electrical discharge machining (EDM). The first visible sign is a frosted, pitted look on the raceway. As it progresses, it develops into a washboard pattern of fine ridges known as fluting, which produces noise, vibration, and eventually bearing failure. A bearing damaged this way can fail in a matter of months, well short of its expected service life.
We cover the full mechanism, from shaft voltage to fluting, in our guide to bearing damage due to electrical discharge.
Why This Is the Facility Owner’s Problem
It is easy to assume that bearing protection is handled by whoever supplied or installed the HVAC equipment. Often it is not. The drive, the motor, and the mechanical installation may each come from a different source, and none of them necessarily owns the long-term reliability of the bearing.
The facility that owns and operates the system is the one that absorbs the cost when a motor fails: the emergency service call, the lost comfort or process stability, and the downtime while a replacement is sourced and installed. That is why specifying shaft protection is a decision best made by the facility itself, not left to assumption.
If you own VFD-driven HVAC equipment, it is worth confirming that your motors are protected and making that protection a requirement in what you buy and what your contractors install.
The Fix: Shaft Grounding Systems
The way to stop electrical bearing damage is to give the shaft voltage a path to ground that bypasses the bearing. That is exactly what a shaft grounding device does. Mounted to the motor, it keeps continuous contact with the shaft and channels the current safely to the frame, so it never discharges through the bearing.
SGS shaft grounding systems are built for this. The CR Series is a sealed, axially mounted brush that fits standard HVAC motors and can be serviced in under a minute while the motor runs, so protecting a unit never means taking it offline. With 40 years of shaft grounding experience behind them, SGS systems are made in the USA and designed to hold a reliable path to ground for the life of the motor.
Shaft grounding is not only an HVAC concern. The same VFD-induced bearing damage affects motors across many sectors, from data centers to semiconductor plants to heavy industry. You can see the full range on our industries page.
Frequently Asked Questions
Do all VFD-driven HVAC motors need shaft grounding?
Any motor running on a VFD is exposed to shaft voltage and electrical bearing damage, so shaft grounding is recommended wherever a drive is used. The larger the motor and the more continuously it runs, the more important that protection becomes.
Doesn’t a VFD’s soft start protect the motor bearings?
No. Soft start reduces the stress of starting the motor, but it does nothing about the high-frequency shaft voltage the drive produces while running. That shaft voltage is what damages the bearings, and only a path to ground, such as shaft grounding, prevents it.
Can shaft grounding be added to HVAC motors already in service?
Yes. Shaft grounding can be retrofitted to motors already installed and running. The CR Series mounts externally on the motor shaft and installs in the field, so existing air handlers, pumps, and fans can be protected without replacing the motor.
Keep the Savings, Skip the Repairs
VFDs are here to stay in HVAC, and for good reason: the energy savings are real. But those savings come with a cost to the motors that most facilities only discover after a bearing fails. Understanding shaft voltage, and protecting against it with shaft grounding, is what lets you keep the efficiency without paying for it in motor repairs.
If you want help protecting your HVAC motors, get in touch with SGS.
