Reduce HVAC Energy Costs with Danfoss Variable Frequency Drives

In commercial and industrial facilities, electric motors are responsible for operating some of the most energy-intensive equipment in the building. Supply fans, return fans, cooling-tower fans, chilled-water pumps, condenser-water pumps, boiler pumps, process pumps, and compressors may run thousands of hours each year.

The problem is that many of these motors operate at full speed even when the system does not require full output.

A Danfoss variable frequency drive, commonly called a VFD, provides a more efficient approach. Rather than running a motor at 100% speed and mechanically restricting airflow or water flow, a VFD adjusts the motor’s speed to match actual system demand. This can reduce electricity consumption, improve system control, extend equipment life, and lower operating costs.

For qualifying businesses in Kentucky, installing VFDs may also create an opportunity to receive a cash incentive through the LG&E and KU Business Rebates program. The program supports energy-efficiency improvements in existing buildings, new facilities, and major renovations.

What Is a Variable Frequency Drive?

A VFD is an electronic controller that regulates the speed and torque of an AC motor by changing the frequency and voltage of the electrical power supplied to the motor.

Motor speed is directly related to electrical frequency. A standard induction motor connected directly to a 60-hertz power source will operate near its rated speed whenever it is energized. With a VFD, the frequency can be reduced when the system requires less airflow, pressure, or water flow.

For example, instead of operating a supply fan at full speed and using dampers to restrict airflow, the VFD can slow the fan itself. Similarly, a chilled-water pump can reduce its speed as control valves close and system differential pressure requirements decrease.

Common VFD applications include:

• Air-handling-unit supply and return fans

• Rooftop-unit fans

• Cooling-tower fans

• Chilled-water and condenser-water pumps

• Boiler-loop and hydronic-system pumps

• Domestic-water booster pumps

• Kitchen and laboratory exhaust systems

• Refrigeration compressors, pumps, and fans

• Industrial process fans, pumps, conveyors, and mixers

Danfoss AC drives are used throughout HVAC and water-boosting applications to improve control, increase energy-saving potential, protect equipment, reduce maintenance requirements, and improve system reliability.

Why Reducing Motor Speed Saves So Much Energy

The energy-saving potential of a VFD is especially significant in centrifugal fan and pump applications. This relationship is described by the affinity laws:

• Flow is proportional to motor speed

• Pressure is proportional to the square of motor speed

• Power is proportional to the cube of motor speed

The third relationship is especially important. Because power changes approximately with the cube of speed, a modest speed reduction can produce a much larger reduction in power consumption.

Consider a simplified example:

• At 100% speed, a fan requires approximately 100% power.

• At 80% speed, theoretical power drops to approximately 51%.

• At 70% speed, theoretical power drops to approximately 34%.

• At 50% speed, theoretical power drops to approximately 12.5%.

Actual field performance will vary based on static-pressure requirements, motor and drive efficiency, minimum ventilation requirements, system curves, and control sequences. Nevertheless, systems with long operating hours and frequently changing loads can offer substantial savings.

According to Danfoss, reducing the average speed of a pump or fan by 20% can produce energy savings of approximately 50% in applicable systems. Reducing average speed by 50% can increase potential savings to approximately 80%.

Why Choose Danfoss VFDs?

Danfoss VFDs are designed to provide more than basic motor-speed control. HVAC-specific models incorporate functions that can simplify installation, improve system performance, and reduce lifecycle costs.

HVAC-Focused Control

The VLT HVAC Drive family is engineered for operating fans, pumps, and compressors. Depending on the model and application, integrated functions can support:

• Closed-loop PID control

• Differential-pressure control

• Pump staging and alternation

• Fan or pump monitoring

• Automatic motor adaptation

• Automatic energy optimization

• Sleep mode and wake-up functions

• Broken-belt and dry-pump detection

• Fire or smoke override operation

• Building automation system communication

These capabilities can eliminate or reduce the need for separate control components while giving operators better visibility into equipment operation.

Automatic Energy Optimization

Danfoss drives can optimize the voltage supplied to a motor as its load changes. For the VLT HVAC Basic Drive FC 101, Danfoss states that its Automatic Energy Optimizer can provide an additional 3% to 5% energy savings under appropriate operating conditions. The drive is designed to control induction and permanent-magnet motors in fan, pump, and compressor applications.

Reduced Mechanical Stress

Across-the-line motor starting creates a rapid increase in current and torque. This can place considerable stress on motors, belts, bearings, shafts, couplings, and connected equipment.

A VFD provides controlled acceleration and deceleration. Soft starting reduces abrupt mechanical loading and can help:

• Extend belt and bearing life

• Reduce water hammer in pumping systems

• Minimize pressure surges

• Reduce stress on couplings and shafts

• Decrease maintenance interruptions

• Improve process stability

The maintenance savings can be especially valuable in facilities where equipment is difficult to access or where an unexpected shutdown would interrupt production, patient care, tenant comfort, or other critical operations.

Building Automation Integration

Modern Danfoss drives can communicate with building automation and industrial control systems through supported communication protocols and optional fieldbus configurations.

This integration allows facility teams to monitor operating information such as:

• Motor speed

• Frequency

• Current

• Power

• Energy consumption

• Run status

• Alarm conditions

• Pressure or temperature feedback

Instead of treating the VFD as a stand-alone motor controller, a facility can use it as part of a broader energy-management and fault-detection strategy.

Where Are the Best Opportunities?

Not every motor is an equally strong candidate for variable-speed control. The best opportunities generally involve motors that:

1. Operate for a large number of hours each year

2. Serve loads that vary throughout the day or season

3. Currently use dampers, valves, or bypass lines to reduce output

4. Frequently cycle on and off

5. Are oversized for current system requirements

6. Serve critical equipment where improved control and monitoring have additional value

A fan that operates only a few hours per month may not generate enough savings to justify a VFD based on energy alone. A 50-horsepower air-handler fan operating throughout the year, however, may offer a significantly stronger return.

Cooling-tower fans are another common opportunity. Tower capacity changes with outdoor wet-bulb conditions and building cooling demand. A VFD can modulate fan speed to maintain condenser-water temperature rather than repeatedly cycling the fan at full speed.

Variable-flow chilled-water systems can also benefit. As two-way control valves close, VFD-controlled pumps can reduce speed while maintaining the required differential pressure. This prevents the system from consuming full pumping power when only a fraction of the design flow is required.

Putting the Savings into Perspective

Consider a hypothetical 40-horsepower fan motor operating 4,000 hours annually.

If the motor averages approximately 80% speed after the VFD is installed, the affinity laws indicate that fan power could theoretically fall to approximately 51% of full-speed power. The actual savings must be adjusted for drive losses, system static pressure, operating profile, and existing control method, but the financial impact can still be significant.

The project may create savings in several areas:

• Reduced annual kilowatt-hour consumption

• Lower peak electrical demand

• Reduced wear on belts, bearings, and motors

• Fewer hard starts and mechanical shocks

• Improved temperature, pressure, and airflow control

• Potential utility incentives

• Better operational data and diagnostics

A proper analysis should use measured or estimated operating hours, motor loading, speed profiles, utility rates, and the existing control strategy. Danfoss also provides energy-calculation tools for estimating energy savings, carbon reductions, and drive performance.

Lowering Project Costs Through LG&E and KU Business Rebates

Energy savings are only one part of the financial equation. Eligible commercial and industrial customers may be able to reduce the initial cost of a VFD project through the LG&E and KU Business Rebates program.

The program provides cash incentives for qualifying energy-efficient improvements, including equipment replacements and upgrades. Projects that do not fit a listed one-for-one incentive may be evaluated through a custom rebate application.

This distinction is important because VFD projects can vary considerably. Some involve adding a drive to a single constant-speed fan. Others include multiple drives, motors, sensors, controls, and building-automation-system modifications.

Who May Be Eligible?

According to the program’s published requirements:

• Commercial and industrial customers paying into the applicable Demand-Side Management, or DSM, mechanism may submit a Business Rebates application.

• An industrial customer that previously opted out may need to opt back into the DSM mechanism and remain enrolled for a minimum period.

• LG&E customers receiving gas service only are not eligible.

• Incentive requirements, funding, and rebate amounts are subject to change and availability.

Because incentive levels can be revised, businesses should verify current requirements before ordering equipment or beginning installation.

Prescriptive Versus Custom Incentives

Depending on the current program structure and the specifics of the project, a VFD installation may follow one of two general paths:

Prescriptive rebate:A predefined incentive may be available when the project meets listed equipment and application requirements.

Custom rebate:A custom application may be necessary when the project does not match a standard measure. The incentive may then depend on the project’s calculated or verified energy savings.

LG&E and KU state that projects not listed among the standard one-for-one replacement incentives may qualify for a custom rebate application. The utilities also offer engineering support to help customers evaluate efficiency projects and navigate the program.

Do Not Wait Until After Installation

Rebate planning should begin during project development, not after the VFD has already been purchased and installed.

A typical project may require documentation such as:

• Existing motor horsepower

• Existing control method

• Proposed VFD make and model

• Equipment specifications

• Estimated annual operating hours

• Baseline motor loading

• Proposed operating-speed profile

• Project cost and itemized invoice

• Proof of purchase and payment

• Installation or commissioning documentation

• Energy-savings calculations

• Photographs or a site inspection

Requirements will vary. The safest approach is to contact the rebate program early, confirm the applicable incentive path, and retain all supporting documentation.

A Smarter Approach to Motor-Driven Energy Use

A correctly selected, installed, and commissioned Danfoss VFD can deliver much more than adjustable motor speed. It can help a building respond dynamically to changing loads, reduce unnecessary electrical consumption, decrease mechanical wear, improve control, and provide operators with more useful system data.

When combined with a qualifying LG&E and KU rebate, the economics can become even more attractive. The utility incentive can help offset the project’s initial cost, while the VFD continues generating operational savings throughout its service life.

The result is a practical energy-efficiency measure that can support lower operating costs, improved comfort, more reliable equipment, and a stronger long-term facility strategy.

Ready to Evaluate Your VFD Opportunities?

Before replacing another motor, throttling a pump with a valve, or restricting fan output with a damper, consider whether variable-speed control could provide a better solution.

A professional VFD assessment can help your team:

• Identify high-value motors and applications

• Estimate annual energy and demand savings

• Select the correct Danfoss drive and enclosure

• Review harmonic, bypass, and communication requirements

• Develop an effective control sequence

• Estimate simple payback and lifecycle savings

• Identify potential LG&E and KU rebate opportunities

• Prepare the necessary application documentation

With the right engineering and rebate strategy, a Danfoss VFD project can start saving money before the long-term energy savings are even fully realized.

Rebate eligibility, requirements, funding, and incentive levels are subject to change. Always confirm current program requirements with LG&E or KU before purchasing equipment or beginning a project.