HVAC or Heating Ventilation and Cooling systems consume on average 35% of the overall electricity in any building that they are installed in.
To reduce this power use and fall in line with new, stricter code requirements over energy usage, variable frequency drives or VFDs are installed. VFDs reduce the electrical consumption of the HVAC plant.
This guide will explain what VFDs are, their use in HVAC, how they can save you money, and where they are used.
Variable Frequency Drive Principle
VFDs alter the frequency of an AC current before it reaches the motor that it is powering. This adjusts the rotational speed of the motor and reduces the amount of power required.
Benefits of HVAC VFD
HVAC systems are designed to cater to worst-case scenarios, such as the hottest day of the year, and so are rarely required to operate at 100% of their capacity. In fact, these scenarios may only occur 1% of the year. A variable frequency drive can reduce the HVAC capacity to less than 100% by changing the rotational speed of an HVAC motor and the installation benefits from –
Energy Saving Benefit
For building owners and operators, this can be the most important aspect of a VFD. VFDs can offer significant savings on energy costs, which means lower costs and emissions.
Reduced start up current
An HVAC motor has a running current and a much higher startup current. Startup currents can incur ‘surge’ electrical costs and generate overheating in the motor. VFDs can control this starting current within its permitted range.
Reduced disturbances
This tight control on power delivery also protects other devices connected to the electrical distribution system that might otherwise be vulnerable to damage from line voltage fluctuations.
Controlled acceleration
A standard domestic type HVAC system often switches on and off abruptly as demanded by the thermostat. Where HVAC installations are more complex, switching off the system is not possible. VFD control these installations tightly by modulating the capacity of the HVAC system to give a smooth response.
Extended motor life
Simply put, a motor with a VFD installed is less prone to wear and tear than the same motor operating at 100%.
Different Types of VFD Circuits Explained
There are 3 main types of VFD drives.
CSI
Current source inversion (CSI) drives are used in high power industrial installations where a quick response is not required. They are the least common type of VFD.
Pros
- Regenerative power capability – they can transmit power back from the motor to the power supply
- Generate a clean waveform
Cons
- Cause cogging, which is a kind of rotational movement at low speeds
- Require expensive and large inductors
VSI
Voltage source inversion (VSI) drives are more common than CSI type and offer a more dynamic control. They are still widely used in the industry as an alternative to PWM drives.
Pros
- Generates a steady voltage input by storing and releasing power
Cons
- Cause cogging, which is a kind of rotational movement at low speeds
- Can derate (reduce effectiveness) quite rapidly at high and low temperatures.
- Have a poor power factor
PWM
Pulse-width modulation (PWM) drives are the most common type used in the industry. PWM drives form the basis of the component description in the following section.
Pros
- Highest efficiencies
- Creates an accurate frequency
- No cogging
- Cheaper than VSI and CSI
Cons
- Can require additional hardware
3 Major Components of VFD
A VFD has 3 main components that allow the unit to output the frequency required for the application:
Rectifier section or converter
The incoming current is either a single or 3-phase Alternating Current (AC). This is converted or rectified to a Direct Current (DC). A rectifier is made up of pairs of diodes that only allow electricity to flow in one direction.
DC intermediate section
The intermediate, or filter, section of the VFD consists of a capacitor that alternatively stores and releases electrons depending on whether there is a surplus or a deficit after the conversion. This smooths out the DC current from the rectifier into a steady signal.
Inverter Section
At the inverter section, the DC is changed back to AC, but with the frequency that the motor requires. An inverter is a series of switches that can turn on and off very quickly with specific timing.
Applications of VFD in HVAC
VFD function in HVAC is to adjust the rotational speed of motors –
Fans
Fans are often the number one HVAC energy consumer in a building. When installed with a VFD, you can see dramatic reductions in energy costs when they are operating below 100%. Consider the following example from energy.gov.
A motor driving a fan requires 16.4 kilowatts (kW) of power at 100% capacity. To deliver 50% capacity, according to fan law, it requires only 1/8th of that. While it’s important to note that both VFDs and the motor they are driving are less efficient at operating capacities less than 100%, this still means an 83% reduction in power usage for this example once these are taken into account.
Pumps
Pumps with variable speed frequency motors benefit from the ‘pump law.’ So, if you reduce the flow rate by 50% for a given pump, you need only 1/8th of the power required than when it operates at 100%. And if it’s reduced by 25%, then a quarter of the power will be required. The final saving, as above, will depend on the efficiencies of the system but can be substantial.
Compressors
Compressors can be a bit more complicated than fans or pumps when it comes to applying VFDs. A VFD HVAC compressor forms part of a refrigeration system such as a chiller or a heat pump and drive the process. They offer significant energy savings (50% and higher) on refrigeration system operation by modulating the work done by the compressor in response to the cooling (or heating) demand of the building.
Using a VFD on single-phase vs. three-phase motor
Single phase power supply is for small electrical loads. While varying frequency drives for 3-phase motors are far more common, 3-phase power is not always available. Single phase VFDs are;
- Often not as suitable for retrofitting to existing motors
- Less efficient than 3-phased VFD
- More costly than 3-phase VFD
Where a 3-phase supply is unavailable or impractical, then the installation of a suitable 1-phase motor and VFD can be a good solution.
How to improve VFD performance
While VFDs can offer great improvements in the performance of the HVAC system, their own performance can be affected by:
- The ambient temperature
While VFDs are rated for a wide range of temperatures (approx. 14°F – 104°F), they can function erratically or not at all when subjected to these temperatures for extended periods. - Ventilation
VFDs are typically 95-97% efficient, meaning that there is some power loss in the frequency conversion process. This power loss takes the form of heat generated in the electrical components and must be ventilated. - VFD Size
The VFD must be matched to the size of the motor that it is controlling. The motor can be undersized when compared to the VFD but not the other way around. A larger motor will try to draw a larger current through the VFD than what it is rated for, damaging the VFD. - An efficient control system
The VFD will only reduce motor speed when it knows when to. The sensors that communicate information to the VFD need to be calibrated accurately at the installation stage, and the control system programmed correctly.
People Also Ask (FAQ)
Is it possible to build my own VFD?
VFDs are modular by design, made up of electrical components that are readily available to purchase. With the right tools and knowledge, it is possible to build your own VFD. That said, VFDs can be cheap to purchase (depending on your project size), so getting a prebuilt warrantied unit is always the safer option.
Can VFD be installed DIY?
As above, it is certainly possible, however as with any work involving electrical installation, it is highly recommended that a professional electrician carries out the work.
How is VFD efficiency calculated?
Manufacturers typically advise efficiencies of 96-98% for a VFD. The 2-4% power loss is in the form of heat generated by the conversion process, and a small portion is used to power the operation of the VFD itself. VFD efficiency is 
where Ein = Electrical input and Pout= Electrical output.
Which is better, VFD or VSD?
The answer here is that it depends on the application. Firstly, they differ in that VFDs control by varying the frequency to an AC motor while VSDs work by varying the voltage to an AC or DC motor. Generally, VSDs are better for smaller applications and, as a simpler component, are cheaper.
Conclusion
VFDs are an integral part of HVAC systems, helping organizations to address high energy costs and emissions. With smaller HVAC systems in homes also using similar forms of this hardware, it can only benefit peoples’ pockets in the short term and the environment in the long run.
