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Tech Insight
Application of Variable Frequency Drives in
Irrigation Pump Systems
The application of variable frequency drives (VFD) in turf
irrigation systems has become a hot topic as more pump station
manufacturers are offering VFD systems. When and how to apply
a VFD can make the difference in whether or not an irrigation
system performs as designed. This Tech Insight will cover
the benefits and concerns of VFD systems, the best irrigation
applications for using a VFD, and design considerations in
specifying a VFD based pump station.
Hoover Pumping Systems has been manufacturing irrigation
pump stations for the Florida market for over 15 years. Its
founder, Brent Hoover reacted to the shortcomings in pump
station design by building systems for use on Hoover projects.
The company's reputation is built on designing pump systems
that perform dependably and with a minimum of service and
maintenance. To date there are over 4000 "Hoovers"
installed in Florida and the Caribbean, both conventional
and VFD systems. Having experienced the good and the bad with
both technologies, we would like to offer this objective analysis
so that the designer can make the right decision for the specific
irrigation application.
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What
is a VFD? |
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A VFD or drive is an electronic device that changes the frequency
of the AC power going to a motor, thereby varying the motor
speed. When attached to a pump, the change in motor speed
affects the flow and pressure of the pump discharge.
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VFD
Benefits |
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Proponents of using VFDs in turf irrigation point to energy
savings as a major feature, although any device that reduces
the load on the motor can generate reduced energy usage. For
instance, a control valve that throttles the discharge flow
also lowers the motor load, although less than would a VFD.
Any calculation of energy savings requires assumptions about
the reduction in motor load and hours per year that the pump
is actually running. These values can be hard to predict during
the design phase. As an example, savings of $250 per year
can be realized from a 7.5 HP pump with standard efficiency
motor, operating 2600 hours per year, and a power cost of
$.08 per kWh, if the VFD lowers motor load by 25%. The energy
savings using a control valve would be considerably less.
A VFD can be configured to ramp the motor up and down in a
controlled manner. This can reduce water hammer and therefore
extend the life of the irrigation piping. In a properly designed
conventional pump system, a control valve and a hydro-pneumatic
pressure tank provide many of the same benefits.
The ramp up-ramp down feature of the VFD greatly limits the
motor inrush current that can be problematic for the utility.
Many power companies therefore require that motors over a
certain size be used in conjunction with a VFD.
A perceived benefit of a VFD is the elimination of the pressure
control valve. When using turbines or submersible pumps that
can produce very high pressure, a pressure relief control
valve must be specified in conjunction with the VFD. These
devices are essentially control valves and also need to be
maintained for proper operation. Over the years the control
valve has earned the reputation as a device that requires
frequent tweaking, ongoing maintenance, and yearly rebuilding.
New developments in control valve design and proper engineering
have minimized many of these problems and many valves operate
properly for years between rebuilds. These new valves have
fewer parts and are easier to calibrate and maintain.
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VFD
Concerns |
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In the Florida market, environmental factors such as heat,
humidity and lightning must be accounted for when utilizing
a VFD in a turf irrigation application. Most VFDs are rated
to operate in ambient temperatures up to 104° F. With
temperatures above that, the output of the VFD should be derated
and the life of the VFD can be severely reduced. In Florida,
regardless of whether the VFD is mounted by itself inside
the pump station enclosure or inside a large industrial enclosure,
temperatures will often exceed this threshold. The designer
should be aware of the need for auxiliary cooling. When the
drive is mounted in an industrial enclosure, an air conditioner
or air/water heat exchanger can provide the necessary cooling.
Some VFD manufacturers produce drives in a NEMA 4 package
that can be mounted outside. These should include a fan that
cools the drive when it is running and must be shielded from
direct sunlight.
As VFD prices have become more competitive, manufacturers
have cut back on some important features that are required
for turf irrigation applications. One area of concern is surge
/ lightning protection. The printed circuit boards inside
the VFD may be sensitive to surges, which can cause intermittent
problems or catastrophic failure. A well engineered VFD system
should include surge protection in addition to that supplied
with the VFD.
A VFD is a complex device that presents some challenges in
the areas of service and support. Troubleshooting and drive
configuration requires a highly trained drive specialist.
Generally these skilled professionals are available from the
pump station manufacturer, the producer of the VFD or local
VFD/Pump service companies. Outside of the warranty period,
this service can be very costly, as many VFD problems can
be difficult to trace. With a limited number of skilled personnel,
these service providers cannot always react quickly with an
on-site visit. Troubleshooting by telephone is frequently
ineffective.
VFD manufacturers tend to be reluctant to honor their warranty
if it can be shown that the problem was caused by excessive
temperature, power surges or lightning. Most VFD manufacturers
do not have a "loaner program". Therefore a drive
that needs to be evaluated must go back to the factory for
analysis, leaving the pump station down for an extended period.
Should the VFD fail outside the warranty period, it must be
replaced quickly to maintain proper irrigation levels. The
VFD may not be available locally, as distributors tend to
rely on factory stock. Once shipped from the factory, the
VFD Specialist must configure the drive for the specific application,
adding additional expense and time for the replacement.
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Ideal
VFD Applications |
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Taking these issues into account, there are some excellent
opportunities to apply a VFD in a turf irrigation pump system:
- Booster pumps for potable water or irrigation are good
applications for VFDs because the incoming pressure and
discharge demand can vary greatly based on time of day and
other variables.
- Some conventional pump stations utilize jockey or pressure
maintenance pumps to keep the mainline pressurized, offsetting
the weeping of field valves. The jockey pump plays a vital
role by reducing wear and tear on the main pump when low
flows are required. For example, The Hydraulic Institute
Standards recommend that pumps in general should not run
below 25-30% of their Best Efficiency Ratio primarily due
to damaging heat build up. A VFD on the main pump in lieu
of a jockey pump may enable the pump to better operate at
low flows.
- Any pump station with dramatically varying flowrates
could be a good candidate for a VFD. Over a wide flow range,
the VFD can be a better regulating device than a control
valve. However the irrigation designer should strive, whenever
possible, to achieve more evenly balanced flow requirements
across the irrigation zones.
- A VFD will also be effective in controlling a pump with
a steep pump curve (where TDH drops substantially with increased
flow). If the pump must often be operated at less than the
maximum flow rate, a VFD system provides better energy efficiency
and system control than a conventional pump system.
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Specifying
a VFD Pump Station |
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Once it has been determined that a VFD is appropriate for
the application, the designer should consider the following
issues to insure that the system will perform as intended
and in a reliable manner:
- Is the system designed to maintain constant pressure?
The pump station should be designed to maintain a constant
discharge pressure. In a well designed VFD system, a pressure
transducer provides a control feedback. The VFD, when properly
tuned, maintains constant pressure, in effect matching pump
output to actual demand in the field. If a feedback signal
from a pressure transducer is absent, most of the advantages
of a VFD are lost.
- How is the temperature limitation of the VFD being handled?
If the VFD is to be installed in an industrial enclosure,
an air conditioner or air/water heat exchanger should be
specified. If the VFD comes in a self-contained NEMA 4 package,
it can be mounted under the pump enclosure or on an open
skid. In that case, the VFD should include a cooling fan
and fins to dissipate the heat and also be shielded from
direct sunlight.
- What electrical protection is being offered? The pump
station manufacturer must include a separate device for
disconnect, ground-fault/short-circuit protection, and surge
/ lightning protection as required by the National Electrical
Code, the VFD manufacturer, and best design practices.
- Does the pump station include a VFD for each motor? If
the pump station includes dual main pumps, the pump station
should include an individual drive for each motor. Many
manufacturers include only one VFD and let the second and
subsequent pumps start manually. This compromise is done
to reduce cost, but except at maximum flowrate, the pressure
and flow contribution of the pumps will be unequal, thus
negating most of the benefits of VFD control.
- What pump control logic is provided? The pump station
should be designed to not only maintain a constant discharge
pressure, but also to retire the pump on a "no-flow"
signal. Without this logic, the system will cycle between
high and low operating set points and the life of the motor
and piping will be affected. Some VFD systems are designed
to merely ramp the motor up and down and provide little
pump control or the additional benefits inherent in a packaged
pump station. To provide the "no-flow" retirement
as well as "loss of prime" and "low pressure"
shut down sequences the pump station should include a programmable
logic controller (PLC) or an integrated logic card within
the VFD.
- What is the history of the pump station manufacturer?
The pump station manufacturer should be a single company
with experience in the turf irrigation industry and references
for VFD stations in a variety of applications. The manufacturer
should submit complete and detailed specifications, CAD
drawings, system operation procedures for the system, and
have detailed Operation and Maintenance manuals available.
- Who will service and maintain the system? The manufacturer
should have full-time VFD Specialists on staff who are company
employees (not subcontractors). The pump station manufacturer
should not rely solely on subcontractors, the VFD manufacturer's
technical staff or local VFD service companies as this will
impact cost and timeliness of troubleshooting and repair.
Unlike with conventional irrigation pump stations, the addition
of the VFD insures that most customers will be tied to the
pump station manufacturer for service and support for the
life of the station.
There is no doubt that the use of VFDs has value in many
turf irrigation applications. It is important that the designer
carefully think about whether the specific situation warrants
the use of a VFD, and if the added expense is justified. If
so, the designer should develop detailed specifications similar
to those supplied for a conventional system, taking into consideration
the special criteria that a VFD entails.
For more information about VFD pump stations, please contact
George Newman or Kevin Cavaioli at Hoover Pumping Systems,
Pompano Beach Florida. 954-971-7350.
Email: sales@hooverpumping.com
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