Variable Frequency Drives and
Irrigation Pumps
Stefanie Aschmann
Bozeman, MT
11/30/11
1
Variable Frequency Drives
 Overview of draft “Variable Frequency Drives for
Irrigation Pumping” TechNote
ftp://ftp.wcc.nrcs.usda.gov/wntsc/Idaho%20pump%20Training/
Variable Frequency Drives
 Introduction
 VSD vs. VFD
 How VFD’s work
 Applications
 Operation/Design Considerations
 VFD Tool Demonstration
Variable Frequency Drives
 Pumping water for irrigation can be a major
expense for irrigated farms.
 In 2008 more than 570,000 pumps were used for
irrigation
 $15.5 billion dollars spent on energy
 Efficiency is a big issue
 “For the most part, the enormous cost of
inefficient pump operation and valve throttling
goes unnoticed”
There are several ways to vary the speed of
a pump. Not all save energy.
 Electrical (variable speed motor using
variable frequency drive, or VFD – most
common)
 Mechanical (diesel and gasoline engines)
 Hydraulic (hydraulic coupling)
 Variable speed pulley arrangements
 Changeable gearbox
 Magnetic coupler
Variable Frequency Drives are a subset of
Variable Speed Drives
Variable Frequency Drives
 Variable Frequency Drives are used exclusively
on devices that have electric motors.
 The frequency used in the term refers to the
frequency of the electrical current.
 In the US, electrical grid power has a
frequency of 60 Hz.
Think about your ride
 Tachometers (or rev
counters) on
automobiles, aircraft,
and other vehicles
show the rate of
rotation of the
engine's crankshaft.
Variable Frequency Drives
 Pump speed (e.g. 1780 rpm, or 1175 rpm)
refers to the rotational speed of the impeller.
 The motor shaft is connected to the impeller;
the impeller adds energy to the water.
 Slowing the rotation of the impeller reduces
the energy that is transferred to the water and
thereby the power requirement of the pump.
Slide
10
What does changing the
frequency do?
 The speed of an alternating current (AC)
motor depends on three principal variables:
 The fixed number of winding sets (known as
poles) built into the motor, which
determines the motor's base speed.
 The frequency of the AC line voltage.
 The third variable is the amount of torque
loading on the motor, which causes slip.
What does changing the
frequency do?
 It allows us to change the rotational speed
of the pump’s impeller.
 Changing the speed of the impeller has the
same effect as changing the diameter of the
impeller:
 It shifts the pump curve (usually down)
Why Variable Frequency Drives?
 Pumps often need to operate over a range of
flow rates and pressure
 Pump is designed to meet the greatest output
for both flow and pressure
 At other flows/pressure, the pump will be
inefficient
Pump Curve
Getting to Where We Want to Be
Often the pump
operator will
change the
system curve to
get to where
they want to be.
Original Operating Point
New Operating Point
Original Operating Point
New Operating Point
Variable Frequency Drives
Variable Frequency Drives
Applications
 Constant Pressure//Variable Flow
 Constant Flow//Variable Pressure
 Variable flow//Variable Pressure
Variable Frequency Drives
• Constant Pressure//Variable Flow Example
• 3 center pivots running off one pump.
• Each pivot requires the same pressure.
• But the operator wants to be able to run 1, 2 or 3
pivots at the same time.
The VFD maintains the pressure by shifting the pump
curve, thereby increasing or decreasing the flow.
Variable Frequency Drives
 Applications
 Constant Pressure//Variable Flow
 Constant Flow//Variable Pressure
 Variable flow//Variable Pressure
Variable Frequency Drives
Constant Flow//Variable Pressure Example
• A well that experiences drawdown over the
course of the irrigation season
• Want constant flow rate throughout the
season.
Variable Frequency Drives
Variable Frequency Drives
Variable Pressure/Variable Flow Example
• Multiple systems of wheel lines and pivots
Variable Frequency Drives
VFD Operation Considerations
• Installing a VFD for power savings

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Motor load below 60%, electric motor becomes inefficient
VFD itself is about 97% efficient
Soft Start Option
Single to 3 Phase Conversion
Automation
Open delta current balancing
What are VFD’s potential problems?
Additional losses – heat, motor loading, etc.
 VFD systems may create electric power system
harmonics (harmonic currents are caused by
certain types of equipment that draw current
intermittently while running ).
 Operation of pumps at critical shaft speeds, far
from design (BEP), can result in deflection & shaft
failure
 Pump bearings and seals may wear faster at
higher or lower shaft speeds

Slide 27
VFD potential problems
(continued)
 Too much capacity that can burn out the motor.
 Explosion proof motors must be approved to operate
over the entire operating range.
 At the lower rpms the cooling fan is not rotating fast
enough.
 Mechanical seals must be designed to operate over the
entire speed range.
 High shaft speeds lead to additional NPSH requirement
to prevent cavitation.
Slide 28
Sizing
 In the design process, the size of the VFD needs to
account for
 all inherent drive inefficiencies
 the motor load including service factor.
 Drive size may also be increased
 to minimize, voltage distortion and interference
with other electrical equipment.
 Almost double is needed when converting single
to three phase
 Care should also be taken to not select a VFD too
large as the VFD output might exceed motor
Slide 29
specifications and cause motor failure.
Motor load
Slide 30
Motor insulation class
Filters
 Line filters may be required for VFD’s to
regulate voltage. Imbalance in voltage
generates more heat and loss of efficiency in
the VFD and motor.
 The EMI and RFI generated by the installation
should be measured. If the interference
exceeds limits defined by the current IEEE 519,
electric utility may require that filters be
installed.
Environmental Control
 An increase in temperature will see a dramatic
drop in VFD efficiency and may require
installation of a cooling mechanism.
 External Heat Sinks
 Self-Contained Cooling Systems
Environmental Control
 VFD’s cannot tolerate dust or dampness;
 Should be installed in enclosures that meet
NEMA 4 standards (dust & water tight).
 Adequate sunshades or pump houses are
required for all installations.
Environmental Control
 Other factors that may affect VFD efficiency
are.
 Radio frequency or stray high frequency
signals.
 Line voltage variation greater than ±10%.
 Line frequency variation greater than ±2Hz.
 Altitude greater than 3,300 feet (1000
meters)
VFD Tool Demonstration
Case No. 3 (see handout)