Variable Frequency Drives (VFDs) in Municipal Projects

Variable frequency drives offer energy and cost savings.

Credit: iStock/ikonacolor

What kind of difference can a variable frequency drive—better known as a VFD—make for a municipality? A significant one. Just look at the town of Mooresville in North Carolina, where a VFD developed by Eaton has not only boosted the energy efficiency of the municipal water treatment plant, but has also extended the life expectancy of a pair of new 800-horsepower pumps at the facility.

And Mooresville is not alone. Plenty of municipalities and the owners of commercial buildings are turning to VFDs to more efficiently operate the motors that power their facilities. By using these drives, which vary the frequency and voltage that is funneled toward electric motors, facility owners can reduce their energy costs and boost the lifespans of their motors.

Eric Houg, marketing manager with adjustable speed drivers for the motors and drives division of Toshiba International Corporation, says that the growing demand for VFDs is easy to explain: These drives are simply the most efficient and cost-effective way to control a three-phase induction motor.

At a time in which municipalities and building owners continue to watch their bottom lines closely, the cost savings that VFDs can bring is essential.

Investing in VFD technology might cost owners extra money upfront, but will result in significant savings in the long run. Manufacturers say that some users can hit the payback period just six months after investing in VFD technology.

“For a building owner, there is a very large amount of energy savings that you can achieve with these VFDs,” says Houg. “Anytime you can slow down a fan mechanically or electrically, you can get an energy savings out of that.”

How much owners can save varies. But when a motor is running at its nominal load, end-users can expect to reduce the amount of energy it consumes by 3% when using a VFD. That’s not a huge savings, but the savings jump significantly when motors are not running at that nominal load.

When a motor is running a 75% load, it will consume about 25% less energy when equipped with a VFD. A motor running a 50% load will consume on average 75% less energy when using a VFD.

Those figures are significantly more impressive, and could convince owners to incur the upfront costs needed to purchase VFDs. It won’t take long for them to recoup their upfront investment if they can enjoy energy savings of 50% to 75% on their motors.

“With a VFD, you can slow down the fans of these motors,” says Houg. “You then get a lesser air flow and that big energy savings.”

Once end-users pay for a VFD system, they can expect a short payback period, Houg says. The energy savings that VFDs generate each year, means that some users will recoup their costs in just six months, while the vast majority will have saved enough to cover their upfront costs in just two years.

That’s because end-users can expect to save from 35% to 50% on energy costs after installing a VFD.

“We are seeing a steady increase in the number of users who are exploring and then investing in variable frequency drives,” says Houg. “The economic argument is simply so strong for these drives. Once you show users the numbers, once you show them how quickly they can pay back their
initial investment, it’s a pretty easy argument to make.”

Saving Energy, Boosting Motor Life in North Carolina
Public utility officials in Mooreseville are already experiencing the financial benefits of a new VFD system, developed by Eaton, that was installed in their water treatment plant in July 2015. The SC9000 EP series of drives has helped increase the energy efficiency, with a notable reduction in the use of power at the plant.

Barry McKinnon, public utilities director with the Town of Mooresville, says that the municipality had a financial responsibility to investigate VFDs. Not looking at a way to save on energy costs would have been a disservice to the community’s residents, he says.

“As a representative responsible to the citizens of Mooresville, we wanted to find a way to implement the equipment we had on hand,” says McKinnon. “Eaton’s variable-frequency drive solution has helped us ensure water treatment services for our customers.”

Mooresville built its new water treatment plant in 2008. The plant was initially rated to pump 12 million gallons a day (mgd) with the ability to grow to 36 mgd if needed. Demand, though, did not rise as quickly as town officials had expected, meaning that the plant’s two new 800-horsepower pumps sat idle for several years.

Today, water flow from the town’s old water plant and the new one merge into the municipality’s main pipes. But when the town’s new 800-horsepower pumps did run, the pumps at the older plant could not create enough pressure to overcome the increased pressure in the main system of pipes. That dramatically decreased the efficiency of Mooresville’s old water treatment plant.

This left town officials with a challenge: If the 800-horsepower pumps did not run on a regular basis, the odds were high that they would eventually rust out and become unusable. Public utility officials, then, needed to find a way to run the 800-horespower pumps in a way that would not negatively impact the operations or efficiency of the town’s older water plant.

This is where Eaton came in. The town of Mooresville hired the company to design and install a VFD that would keep all of the public utility’s pumps running efficiently, preventing the newer, higher-power pumps from overpowering the older water facility. This would allow the town to use both the new and old treatment plants to handle increased capacity and provide all-important redundancy, a necessity in water treatment.

Eaton, along with an electrical engineer, delivered a VFD system that controlled the water flow, allowing it to run at a lower speed. This made it possible for the public utility to use both the new and old water treatment facilities.

Eaton also designed a compact, integrated control gear system that close-couples the new VFD with the plant’s existing AMPGARD medium-voltage motor control. Because Eaton engineers integrated the VFD and motor control with a bus connection, it was able to install the new drive without requiring the public utility to move expensive cables and equipment. The VFD system remained in one room, with no modifications, and tied back to the plant’s existing communications system.

Mooresville public officials say that the drive has performed as advertised, increasing energy efficiency at the plant with no noticeable drop in power usage. The VFD system also controls the pressure from the 800-horsepower pumps, allowing the new and old plants to work in tandem. This, of course, has provided protection to the new pumps. They are no longer at risk of rusting away because of disuse.

Credit: Eaton Pumps with VFD controller

Credit: Eaton
Pumps with VFD controller

Jerry Wang, project engineer with Eaton, says that Mooresville, like other municipalities, is benefitting from the main money-saving feature of VFD systems: This technology gives utilities and building owners the ability to change the speed of their motors. For buildings or municipal systems that don’t always need to operate their motors at full power, this can provide a significant cost savings.

“Look at a water treatment plant. The demand for water is not always the same,” says Wang. “It’s like at a Super Bowl party. The first commercial hits, and most of the people run to the washroom at around the same time. You need higher water supply at that moment. The old way to do that meant having a large motor trying to pump the water into the city. You’d have that motor running full speed the whole time. If you are constantly doing that, if your pumps are constantly running at full speed, you are wasting a lot of energy. With a VFD you don’t have to do that. You can control the speed of your motors.”

As Wang says, with a VFD system, you don’t have to pump anything you don’t need at a given moment. For example, a VFD system could regulate the speed of the motors powering the air-conditioning system at a commercial building. Over time, the energy savings add up.

Then there are the savings that come with boosting the lifespan of motors. Houg explains that VFDs reduce the stress on motors.

“That’s another important savings tool that sometimes gets lost,” says Houg. “You can expect a longer lifespan on your motors when you are regulating how often they are operating at full speed. It can be expensive to repair or replace those motors. With VFDs, you won’t have to do this as often.”

End-users can save money, too, by taking advantage of the rebates and financial incentives that states and municipalities are offering to building owners and municipalities that install VFDs.

“These incentives, along with the short payback period, do make a difference,” says Houg. “It allows users to justify spending that extra money for a VFD. If they know that they can qualify for a rebate, and they can pay back their upfront costs in a short period of time, that really is a persuasive argument.”BE_bug_web


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