Are your AC motors burning more power and money than necessary? Learn how faster AND slower wins the energy race.

Key Takeaways:

  • Variable-frequency drives can significantly lower energy consumption.
  • They can provide other benefits, such as tax incentives and improved health.
  • Variable-frequency drives require expert installation and maintenance.
  • These drives also require users to be vigilant to avoid electrical problems.

Keeping your office utility bills as low as possible isn’t easy. We previously covered some best electrical practices that included the basics, such as turning off lights not in use, and more time-intensive tasks like conducting an electrical audit. This time we’re taking a deep dive from number five on that list by covering how variable frequency drives work.

These units go by a few different names – VFDs, variable-speed drives, and AC drives – and they essentially have one job: to provide the speed many businesses need to optimize their energy expenditures and save money. This guide will quickly explain how a variable frequency drive works and cover the benefits and potential drawbacks of installing one.

What does it vary, exactly?

Variable frequency drives change the voltage and frequency (see, it’s not just a clever name) supplied to AC electric motors, which typically power compressors, fans, or pumps, which HVAC systems rely on. Varying the frequency changes the motor’s speed while adjusting the voltage alters its torque, or the amount of force it generates while rotating.

Controlling those two factors is the first benefit of a variable frequency drive because it allows for a more gradual and smoother activation/deactivation of the motor. Businesses have different power needs depending on the size of their operations, which is where “variable” becomes important. A good unit allows the business to program start and stop periods for the motor, ranging from seconds to minutes, creating a power up and cool down balance that suits them.

More benefits of a variable frequency drive

Having the power to decide how fast or slow a motor goes makes a business more energy-efficient and reduces utility bills. Installing a variable frequency drive could even make commercial-property owners eligible for tax incentives, based on the site’s square footage, due to reduced CO₂ emissions.

There are further qualifications required (it is the IRS), but don’t lose hope. An article in Pumps & Systems stated that a 20% reduction in motor speed could cut energy use by 50% – exactly the amount the government seeks when compared to other buildings meeting minimum energy-efficiency requirements set by ASHRAE Standard 90.1. This standard governs how well new buildings or additions to existing buildings and systems comply with energy efficiency standards.

Today’s variable frequency drives are technologically impressive. Many of these easy-to-install models have self-diagnostic tools that help users catch problems quickly, and advanced overload protection prevents against power surges, which can put equipment out of commission.

Avoid pushing a motor pedal-to-the-metal to prevent early burnout and hardware damage, which reduces maintenance and replacement costs. A healthy motor also helps preserve anything it’s powering. For example, consider a traditional air-conditioning system.

How secondary systems benefit from variable frequency drives

Temperatures fluctuate throughout the day, creating shifting demands on an air-conditioning system. It also means there will be varying burdens put on the air conditioning’s pump speed and pressure. Traditional air-conditioner designs consider the motor, which is working on hyperdrive all the time regardless of other factors.

A variable frequency drive improves on the traditional design because it can dial motor speed back or ramp it up as required so the motor isn’t put under constant and excessive strain. This creates another subtle but powerful benefit, which might not be noticed immediately: quieter working conditions.

Pumps or fans running hard all day can generate a lot of noise, especially if they require maintenance. Adding a variable frequency drive to balance energy use could contribute to a more productive and healthy workplace. However,  a badly fitted or poorly monitored variable frequency drive can become a business liability.

Potential disadvantages of a variable frequency drive

A variable frequency drive doesn’t allow businesses to just forget they’re there. It has to be operated smartly and vigilantly to maintain a decent degree of operational efficiency.

Every motor has a designated frequency which is a kind of sweet spot. Going too high or too low outside that zone harms efficiency rather than optimizing it. An example is a motor frequency set too low on an AC motor using a fan. Insufficient fan speed will probably cause the motor to overheat, therefore hampering performance and leading to faster burnout.

Frequency shifts are also directly connected to a motor’s resistance (a counteracting force on current flow) and Volts to Hertz (V/Hz) ratio. This can lead to two problems. Increasing resistance combined with constant voltage decreases current and lowers torque. Conversely, decreasing resistance coupled with constant voltage boosts torque and could cause burnout. A well-managed variable requency drive stays in the safe zone by maintaining current draw levels and balancing the V/Hz ratio generating safer levels of torque.

Other potential downsides are cooked circuits, weird telephone sounds, and flickering office lights. This could raise concerns that your business is haunted, but it’s more likely that you’re using more than one variable frequency drive which can lead to an increase in harmonic distortion and cause these spooky effects. These and the other drawbacks can be overcome by choosing a reliable power partner to install and maintain your drive(s).

Speak to our electrical experts with any questions

Every dollar you save on energy helps you invest in beating the competition. The UES team has decades of electrical experience helping Florida businesses design and maintain excellent electrical systems. Just drop us a line on our contact page to get more info on sizing the right variable frequency drive or to get a free quote on any commercial or industrial project!