Save With VFDs

Use of Varying Frequency to Control Irrigation Pumps Gains Fans

Jim Patrico
By  Jim Patrico , Progressive Farmer Senior Editor
B.J. Wilkerson said variable-frequency drives save water and have reduced his irrigation costs by $8 to $12 per acre. (Progressive Farmer photo by Mark Wallheiser)

B.J. Wilkerson has his hands full overseeing 2,000 acres of irrigated corn and peanut production in addition to running a 700-head cow herd near Trenton, Florida. With 33 center-pivot sprinklers pumping groundwater for the crops, anything he can do to save time and money is sure to get his attention.

That's why during the past five years, Wilkerson has been installing variable-frequency drives (VFDs) on his Valley-based irrigation systems, particularly on farms with one well and more than one pivot. Wilkerson said the technology not only saves water but also has saved him at least $8 per acre in operating costs to run the sprinklers.

"We installed our first VFD five years ago on a pair of new systems we were running off one well," Wilkerson explained. "Our dealer said the VFD would allow running both pivots or only one at a time rather than having to run both systems simultaneously to match the output of the pump and well.

"We had run both sprinklers together one season on corn and knew how much that had cost. After a season running the systems with VFD, the best we could estimate comparing pivot trips and power consumed, we were saving $8 to $12 per acre over the previous year," he said. "Since then, we've installed six more."

NEW PUMP POPULARITY

VFD motor controllers match motor output to variable loads. But, they have become increasingly popular with irrigators across the U.S., said Tyler Fields, national sales manager for Valley Water Management. "Growers in the western U.S., where they have to pump irrigation water up and down hills, have been taking advantage of VFDs for 15 years," he said. "But, with rising production costs and serious efforts at boosting irrigation efficiency, we're now seeing it adopted in the High Plains and the eastern part of the country."

VFD systems use rectifiers and rapid-switching transistors to convert AC line current into a synthesized sine wave of pulsating DC (direct current), which allows nearly instantaneous computer manipulation of frequency (typically from 45 to 60 hertz) to control motor speed and torque. The result is a pump operating in its "efficiency sweet spot," thus saving electricity and maintaining the most efficient line pressure in the sprinkler system, Tyler explained.

Veteran irrigation researcher and Texas AgriLife engineer Leon New said success with VFDs depends upon having properly sized pumps to take advantage of the efficiency offered by variable motor speed.

"VFDs can reduce the pump speed and replace typical squeeze valves," he explained. "Very few pumps I've tested in the High Plains were not squeezed, and some squeeze valves were barely open. That indicates too much pump for the water the well can deliver, an expensive inefficiency."

For regions such as the South Plains of the Texas and Oklahoma Panhandles with sketchy groundwater supplies, New suggests sizing pump bowls for more water than a well can produce to ensure the pump will be operating within the manufacturer's multispeed design parameters at available flow rates. VFDs can then be set to maintain the required pressure, which can reduce pump costs by slowing the pump.

ONLY WHAT IS NEEDED

Tyler said one VFD system at the Sunbelt Ag Expo farm, near Moultrie, Georgia, consists of a six-tower pivot with a corner arm and a 250 hp pump.

"When that system is running full bore, it requires 245 hp, but when you shut down the corner arm, the power demand drops to 140 hp. With the VFD, we reduce our pump by 100 hp and give the system only what it requires, and nothing more."

Zimmatic dealer Greg Sweatt, of Whitewater Irrigation, Cochise, Arizona, said his experience shows slowing pumps by 10% usually results in an energy savings of about 15%. But, that requires a pump with a wide efficiency range to make up for the speeds it may run.

"Our growers like the efficiency and the power savings, but in some cases, being able to control the water conditions in the well is more important," he explained. "By using well depth monitoring and computer software, we use VFDs to slow pumps down when wells approach their water-level limits. This prevents them from sucking air and cavitating."

Wilkerson said VFDs allow irrigators to maintain optimum pressure in their sprinkler systems regardless of rapid changes in flow demand.

"We really like to use VFDs where pivots are surrounded on three to four sides by public roads," he explained. "That way, we can turn off end guns and not water the right-of-way, and still maintain constant pressure in the system. In addition to using less electricity, that saves a lot of water, too."

The fourth-generation Florida farmer said he figures a six- to seven-year return on investment (ROI) for what he spends on VFDs, particularly on smaller fields. On 140- and 200-acre pivots, he estimates the payback ranges from three to four years.

Wilkerson said conservation of resources and inputs is at the forefront in all his planning.

"I have kids who may want to farm someday, and if that comes to fruition, we'll have to have the resources to make it possible," he explained. "I try to do everything I can to save water and electricity, and do more with less. It may not seem like much, but if you can save 40,000 to 50,000 gallons or more per crop over a lifetime you've done something."

UTILITY ASSISTANCE

Fields said in many areas, local utilities offer rebate incentives for installing VFDs, which helps them better manage demand -- especially during peak pumping periods. Such rebates can significantly reduce time to a positive ROI.

He offers the following example based on assumptions from a central-Nebraska project in which a VFD installation on a 75-hp center pivot with a corner cost $9,650.

The system required 46,518 kilowatt hours (kWh) per season before the VFD installation, running 900 hours. With the VFD, it requires only 30,058 kWh. Based on a $2 demand charge and a $0.08-per-kWh base rate, the resulting savings of 16,460 kWh amounts to an annual savings of $1,490.

"Figure in a $3,200 utility rebate, and that gives you 4.32 years to a positive ROI," he said.

Jim Patrico