Variable Frequency Drive​

What Is a Variable Frequency Drive?

A variable frequency drive (VFD) is an electronic device used to control the speed of electric motors by adjusting the frequency and voltage supplied to them. VFDs can be installed on motors that drive fans, pumps, and compressors, allowing these components to operate at variable speeds to match system demand rather than running at full speed all the time.

How Does a VFD Work?

Here is the basic process for variable frequency drives:

1. The rectifier converts incoming AC (alternating current) power into DC (direct current) power.
2. The DC bus filters and stores the DC power using capacitors.
3. The inverter switches the DC power back to AC power at the desired frequency and voltage using transistors. This adjusted AC power controls the motor speed.

By controlling the frequency of the power supplied to a motor, a VFD can precisely control the motor's speed and torque. When using VFDs in HVAC, airflow, water flow, or compressor output can be adjusted in real time to meet the actual heating or cooling needs of the building.

What Are the Advantages and Disadvantages of VFDs?

Advantages of Variable Frequency Drives in HVAC

• Energy savings: VFDs allow motors to run at optimal speeds, reducing energy consumption, especially in applications with varying load requirements, which are common in commercial buildings. Improving energy efficiency of your HVAC system can support your organization's sustainability efforts.
• Reduced wear and tear: Motors and mechanical components experience less stress with fewer start/stop cycles, leading to longer equipment life and reduced maintenance costs.
• Soft starting: VFDs provide a "soft start" capability, gradually increasing motor speed and reducing inrush current, which can prevent voltage dips and stress on the electrical grid.
• Improved comfort: They enable more precise control of temperature, humidity, and airflow, improving the comfort level of the building's occupants.
• Noise reduction: Lower speeds enabled by VFDs result in quieter HVAC operation, which benefits noise-sensitive environments.

Disadvantages of Variable Frequency Drives in HVAC

• Initial costs: The initial cost of purchasing and installing a VFD can be higher than traditional motor starters, but the long-term savings can be worth the investment.
• Harmonic distortion: VFDs can generate harmonic distortion in the power supply, which may require additional filtering equipment.
• Complexity: Proper installation, programming, and maintenance of VFDs require expertise. It's important to use qualified technicians to avoid operational issues and ensure optimal performance.

Resources for Variable Frequency Drives in HVAC

• Case study: Renton School District
• Trane TR200 Series

Variable frequency drives are used in HVAC applications to help save money, reduce unnecessary wear and tear on equipment and reduce operational costs. They are especially valuable in places where loads vary significantly throughout the day or year, like office buildings, hospitals, shopping centers, educational institutions, and manufacturing facilities. To determine whether a VFD would be beneficial in your HVAC system, contact your local Trane Commercial representative.