Variable frequency drives can improve system performance and cut farmer costs.
By David Stover
Using variable frequency drives (VFDs) in agricultural pumping operations is a proven and efficient method to preserve natural resources while enhancing farming operations.
In fact, using VFDs for irrigation is gaining wider acceptance across the country as U.S. government agencies continue to offer incentives to farmers who implement more efficient equipment and practices. In many states, farmers are receiving rebates for using VFDs in agriculture and irrigation settings, and some electric utilities offer farmers credits if they adopt VFD technology.
Water needs change, and irrigation systems don’t always require a constant flow rate or constant pressure. VFDs are electric controllers that vary the speed of the pump, allowing the pump to respond smoothly and efficiently to fluctuations in demand.
With their better overall hydraulic performance and lower lifecycle costs, vertical turbine pumps paired with VFDs can provide greater savings opportunities in irrigation systems and increase well life.
VFDs are often applied as an addition to existing systems to increase the overall system effectiveness. When a VFD is installed properly, pumps can work more efficiently, thereby extending product life, reducing energy consumption, and decreasing electrical system stress.
A soft start
A motor without a VFD operates at a constant speed. Upon startup, a constant-speed motor is subject to high torque and electrical surges that can reach up to 10 times the full current load. VFDs, on the other hand, have a soft-start capability that gradually ramps up a motor’s operating speed. Soft-start capabilities associated with VFDs greatly reduce the stress on the motor and related components so the pump system can last longer.
The soft-start and stop function of VFDs are also beneficial in reducing water hammer. Water hammer can be a serious problem in irrigation systems—severe enough to burst lines in some situations.
A quick energy change usually develops within a system when the pump is suddenly turned on or off. Adding a VFD can help eliminate the problem by softly starting and stopping the pump, effectively eliminating water hammer and ultimately preventing damage to the pump, piping, valves, and other pump components.
A VFD solves another common issue related to centrifugal pump selection: oversizing the pump. A common, yet often unnecessary, practice is to make pump selections account for extreme conditions or the maximum design load. However, pumps rarely operate at full load conditions, so the results of oversizing are often higher operating costs and reduced performance. With the high energy costs associated with pumping water, a VFD can improve profitability for farms and farmers.
VFDs monitor the performance of a system, ensuring optimal operation, all the while providing a level of protection for the motor. Many VFDs come equipped with overload protection for the motor, along with several pre-programmed functions to monitor electrical conditions that would be detrimental to the motor.
Installing a VFD on a single pump serving multiple irrigation lines provides the flexibility to change pressures for different pumping requirements. For instance, if one field is using a drip irrigation line and another field is using a sprinkler irrigation line, the VFD enables the pump to vary the flow of water while maintaining a constant operating pressure.
In other words, when cutting back on the flow, the pump will reduce in speed and the flow will reduce, but still keep as close as possible to the best efficiency point on the pump curve. As the system calls for a larger volume of water, the pump will supply more but still keep constant pressure. This eliminates the need for throttling devices such as pressure-reducing valves or flow-control valves.
Additionally, using VFDs to control irrigation system pumps helps reduce water waste in the form of evaporation or runoff. By slowing down or speeding up the pump depending on system needs, VFDs help regulate water pressure and maintain water flow even when multiple water sources are in use, thereby conserving water.
Monitoring from afar
VFDs for irrigation systems and agricultural applications are specifically designed to handle larger pump motors, such as those found in vertical turbine pumps. VFDs designed for agricultural applications also generally come equipped with advanced monitoring controls.
Advanced monitoring provides another means of managing water consumption and maximizing energy efficiency. They include remote monitoring capabilities that enable farmers to monitor and shut down irrigation pumps even when they’re not on-site, eliminating unnecessary pumping time and providing additional system protection.
Since irrigation systems are typically located outside, VFDs may require special protection from exposure to weather conditions, dust, heat, cold, and other environmental factors. Depending on the region and related weather conditions, different outdoor enclosures are available, including some designed to offer protection from direct sunlight and fluctuating temperatures. There are even enclosures available with built-in air conditioning units to offset the heat generated from the VFD itself.
VFDs are effective and proven tools for cutting operating costs and improving overall system performance. Now loaded with expanded capabilities, including modular design and advanced communication systems, today’s VFDs provide irrigators streamlined ways to achieve higher energy efficiencies, benefitting both the farmer’s bottom line and the greater goals of reducing reliance on electricity and conserving water.
David Stover is the market development manager for turbine products for Xylem Inc. He has 16 years of expertise in the field of vertical turbine products and 40 years of industry experience. Stover is based in Atlanta and can be contacted at email@example.com.