Stemming irrigation costs

Alternative systems can cut a farm’s electrical expenses

By Thomas Kirk

Stemming irrigation costs

Farmers know how expensive irrigation systems can be to run — especially when using a diesel generator. Unfortunately, for farmers only served by a single-phase line (which most modern motors are incompatible with), a diesel generator system seems to be the only practical answer. Stringing out a new three-phase line is typically too costly. However, there are alternative solutions such as Written-Pole motors, phase-converters, and variable frequency drives (VFDs) that can also decrease demand for variable loads.

Written-Pole motors are expressly made for operating on single-phase lines. A Written-Pole motor requires low starting current, using only 25 to 30 percent of that required by an equivalent single- or three-phase induction motor. As a result, a Written-Pole motor can operate on a single-phase line without causing voltage flicker that interferes with other customers. They are highly efficient (92 to 95 percent), and range in sizes from 15 to 100 hp.

In addition to irrigation, their agricultural applications include milling, grain drying, and salt water disposal. For larger horsepower uses, a Written-Pole motor/generator set may be an option. These sets — known as 1-to-3 systems — work well on steady loads, such as refrigeration.

Another alternative is a phase converter — essentially a step-up transformer. But one size doesn't fit all. A phase converter must be designed for a specific motor running at a constant load for a given application. It is the most economical way to get a three-phase motor. A phase converter for a 75-hp motor used for irrigation costs about $12,500.

VFDs are another way to reduce farm operation costs. VFDs vary the voltage and frequency supplied to motors, thus varying their speed. VFDs are useful for two reasons: they can convert single-phase power to three-phase power, and because they can vary a motor's speed, they can save the end-user energy for operations with a variable load.

For example, an irrigation system requires a maximum of 10 hp during peak watering times but only 5 hp most other times. It is inefficient to run the irrigation motor at 10 hp at all times. In addition, a VFD can vary the motor's speed according to the actual work required, saving both energy and demand on the system. This is important as the water lowers during the growing season.

When replacing a diesel-generator, the payback period for VFDs is around two to three years. Farmers should work with a reputable dealer and installer when converting to a VFD. Most installations will require the use of a filter to ensure that neighbors are not impacted by any harmonics generated by the VFD.

After identifying a potential business case for VFDs, phase converters, or Written-Pole motors, a more detailed cost benefit analysis should be performed. When installed in the appropriate situation, both the member and the electric cooperative will benefit from these technologies.

Before purchasing a Written-Pole motor, perform a motor starting and load flow analyses for any system that requires a large horsepower motor on a single-phase line.

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About the Author

Thomas Kirk is a technical research analyst specializing in energy efficiency and renewable energy for the Cooperative Research Network (CRN), a service of the National Rural Electric Cooperative Association.

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