UPGRADING SYSTEMS
a pump that is about 14% oversized (17 feet/120 feet). One might assume that installing a VFD and operating the pump at a lower speed would result in an immediate 14% reduction in energy consumption.
However, that is not the whole story — inherent VFD system inefficiencies of about 5% need to be factored in. Therefore, actual energy savings from simply reducing the speed of a reasonably oversized pump are closer to 9%.
More substantial energy savings are possible when the target flow rate, discharge pressure or other hydraulic conditions are expected to change. It can be difficult to predict actual VFD-related energy savings, especially when the magnitude and duration of changes to pump operating conditions are not precisely known. However, using the information in table 2 can provide a rough estimate of potential energy savings.
Table 2. Potential energy savings (percent), compared to the magnitude and duration of reductions in irrigation system flow rate or pump discharge pressure
Percent of time that flow or pressure is reduced 510
20 40 50 5 2334 4.5 Percent
reduction in flow or pressure
60 80
10 334678 10 15 3458 9.5 20
11
25 357 12 30 458 14 35 469 16 40 46 10
18 1See Example 1 | 2See Example 2
Note: The table values account for the typical 5% VFD system efficiency loss (reducing energy savings) and a conservative 7% increase in efficiency from reducing the speed of a reasonably oversized pump. The table values also assume no change in bowl/impeller and motor efficiencies or annual operating hours.
The irrigation system is split into multiple sets of different acreages, where only part of a field is irrigated at once. Perhaps Set A has only 70% of the area of Set B. Assuming the application rate per acre is the same and the required pump discharge pressure is similar for both sets, Set A is irrigated with a 30% reduction in pump flow rate for 50% of the time. The potential energy savings is about 17%.
The aquifer level for a well pump changes throughout an irrigation season. Perhaps a well pump is sized for 250 feet of TDH to provide enough pressure in all conditions. The aquifer is expected to fluctuate by ±50 feet from the average TDH of 200 feet. The annual aquifer variations provide for an average 20% reduction (50 feet/250 feet) in pump lift 100% percent of the time, which roughly equates to a 22% reduction in energy consumption.
26 Irrigation TODAY | Fall 2019
irrigationtoday.org 17 14 22 5 67
12 17
3 4 6 10 12 14 18 222 14.5 171 19.5 22
27
20 26 32 23 26
30 34
37 42
100
Variable frequency drives can be used to adjust the speed of alternating current motors, and it is becoming more common to see VFDs on new and retrofitted irrigation systems.
Installing VFDs can be a great investment for irrigation system owners in the right conditions, but there are also drawbacks to be aware of. For example, VFDs are more sensitive to incoming power quality and will typically shut off temporarily (known as tripping) if certain power quality thresholds are exceeded. VFDs can also impart stress on pump motors, potentially causing premature failure unless special attention is given during the design process.
Discussing these points with a local irrigation dealer would be beneficial when considering upgrading to or retrofitting with this type of equipment. Additional VFD design specifications are publicly available at
www.itrc.org/reports/pdf/ vfdspecsfarmreport.pdf.
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