Variable-frequency drives help gas-fired generator achieve surprising efficiency
Anyone who has dealt with installing three-phase electrical power from a main line to remote areas such as irrigation well sites has suffered sticker shock. Just to get service, bids range from $50,000 to $100,000.
That was the situation Tom and Ellis Moore of Sunray, Texas, faced several years ago as they expanded their 12,000-acre grain and cotton operation in the Northern Texas Panhandle. In the Moores’ case, the estimate for a mile of line for three-phase power was $100,000, Tom recalls. "That’s when we went looking for alternatives."
Today, Moore Farms is generating its own electricity with natural-gas-fired engines, which is significantly cheaper than the power company’s per-kilowatt rate and improves overall fuel-use efficiency on the wells and sprinklers powered with the generators.
The success they had with generator sets prompted the Moores, two other partners and manager Richard Hall from Dumas, Texas, to launch AgH2O, a company that provides generators and variable-frequency drive (VFD) control panels to other irrigators who want to avoid the high cost of commercial three-phase power.
Cleaner energy. The reasons to switch are numerous, but overall fuel econ-omy compared with a natural gas engine on each well is key.
"In our testing, we had the gas company come out and monitor fuel consumption of two of our wells, each powered with a 350 Chevy V8," Tom says. "Then we had them monitor a similar well powered with a generator set—a 90-hp Caterpillar 3304TA driving a 164-kw three-phase generator. The results showed the generator set would use $900 less natural gas per month than the Chevy engines."
Common wisdom would bet against such efficiency from burning natural gas to turn a generator for electricity to run a motor pumping water. Tom says much of the savings come from the use of VFD, which allows softer starts and stops of pump motors, and through computer controls that supply optimum frequency and voltage to meet the well motor’s pumping load. (See "The Nitty Gritty on Variable-Frequency Drives" on page 58.)
The Moores were astounded at the generator set’s ability to provide relatively low-cost kilowatts, when compared with the going commercial power rates from their utility company.
"On $4.50 (per thousand cubic feet) natural gas, we can generate a kilowatt of power for just over 4¢, compared with nearly 11¢ per kilowatt delivered by the utility," Tom says. "That’s only a fuel cost comparison. It doesn’t include depreciation, parts, oil and maintenance, but fuel costs add up quickly when you consider the upfront cost of three-phase installation."
In addition to the monetary benefits, the Moores appreciate the flexibility that comes with a generator system.
"If we get a lightning storm and the power is knocked out, it’s not the farmers who get service restored first by the utility company," Tom says. "If we get a gen set knocked out, we have the flexibility of pulling another one in on a trailer and being up and running again in a few hours."
The brains behind the efficiency of generators powering well-pump motors and center pivots lies within the variable-frequency drive (VFD) control panels. VFD circuitry provides soft starts and stops for pump motors, then spoon-feeds them with optimum current under load.
Moore says other benefits include "cleaner" power and fewer intermittent power losses, common with public utilities.
"When you use three-phase power from the utility company, one or more ‘legs’ of that power are supplying homes and businesses on the grid with 120-volt and 220-volt AC power, so you always have some current and voltage imbalance to reduce your motor efficiency. With our gen sets, you have none of that," Moore explains.
Also, there’s the maintenance angle. One of AgH2O’s generator sets can run three to five sprinklers and wells—all on one engine running at about 85% capacity. "We’ve found the best performance for engine life, maintenance costs and overall upfront investment costs is the 85% load rating on our Cat engines," Moore says. "That’s higher than Caterpillar recommends, but they are assuming pumps will be started instantly from a dead stop, which draws tremendous current loads. We, however, can use the VFD to take up to 6 minutes to bring a motor and pump on line."
On the three-sprinkler system, the 315-hp 3406TA engine starts and warms for six minutes before the VFD panel brings the 100-hp electric motor on line to start pumping the 400' well directly below the generator set to water the circle and produce cooling water for the engine. Once it is running, the VFD cues a 75-hp electric motor and pump to start watering another circle a quarter mile away, and subsequently signals an additional 50-hp submersible pump to start another pivot in yet another adjacent field. The three wells theoretically pull 168 kw, and the generator set is rated at 296 kw to provide extra capacity.
"We’re testing cellphone technology right now so we can control individual gen sets on the farm," Moore says. "That way, if it rains on one, we can shut it down while continuing to water the fields that didn’t receive rain."
What’s It Cost?
The price tag for pumping water with a generator powered by natural gas depends on various factors, including the type of pump, size of motor, depth of well, etc. AgH2O has worked on projects ranging from $90,000 to $175,000, with a minimum of three wells and a generator set tied to four wells, says manager Richard Hall.
"The neat part about generators is they have residual value," Hall explains. "If you have three-phase installed, you’ve spent a similar amount of money as one of our systems cost, and all you have is the electrical service. With a gen set, you can sell it, move it around or expand it."
The Nitty Gritty on Variable-Frequency Drives
When you combine an electronic rectifier with inverter circuitry, you get variable-frequency drives (VFD), which alternate currents of various frequencies and voltages.
Most electric motors used for pumping irrigation water are three-phase induction units designed to operate on a traditional 60-hertz frequency. A motor that is run by VFD can be started and stopped more efficiently with slower frequencies and lower voltages. Then, once it is running under load, power can be adjusted for the optimum current draw compared to a given load.
One of the advantages of using a VFD on an electric well motor is the elimination of the tremendous current draw that happens when starting the motor from a dead stop (locked-rotor status). Also, with VFD controls, the motor and pump can be slowed to a stop more evenly by using the motor to brake itself against out-of-phase frequencies.
The use of VFD technology can account for significant improvements in power consumption when compared with three-phase installations operating solely at 60-hertz and 480-volt AC power.
For more information on AgH2O and the generator set technology using natural-gas-powered engines, visit www.FarmJournal.com/VFD_irrigation