Drives, motors and switchgear
M
any irrigators are turning to variable speed drives (VSDs)
to reduce electricity costs which have skyrocketed over the
past three years. This is to be expected because electricity
is a significant ‘raw material’ in irrigation farming.
Irrigation design contributes to performance
Understanding pump design is an important starting point when
designing for the required head pressure. The safety factor is built
into the irrigation design to ensure that the pump supplies sufficient
pressure at the highest delivery point furthest away from the pump.
Approximately 95% of the time the pump runs at full speed and the
pressure is regulated with a mechanical device. This configuration
wastes energy.
Most pumping systems are designed for the maximum flow rate
requirement at a fixed maximum head position. This is also known
as the constant speed pumping requirement. However, few irrigation
systems operate at the designed constant speed pumping require-
ment because of the continuous change in static and friction head
pressures in both block or centre pivot irrigation. This results in ir-
rigation systems wasting energy. The required power of a centrifugal
pump varies in proportion to the cube of the speed. Therefore a small
reduction in speed will result in a large energy reduction.
How canVSDs bring about this
substantial reduction in energy?
VSDs improve the process of an application by controlling the speed
of the motor. This ensures that the pump runs no faster than it needs
to and does not require mechanical throttling devices or methods.
If one considers the energy efficiency of the whole pumping sys-
tem, irrigators can make significant savings by increasing component
efficiency through improving the method of varying the flow rate.
Typical conditions where energy is wasted in irrigation
systems
• Irrigating when not needed. Only irrigate to meet crop water
requirements for maximum production
• When irrigating different sizes of plantations with the same pump,
the electric motor runs at full speed regardless of the exact flow
rate requirement. This wastes energy against the rows of regula-
tors and flow control valves such as for bigger and smaller pivots
or blocks
• At different pressure head requirements irrigating plantations with
the same pump, allowing the electric motor to run at full speed
regardless of the pressure head requirement. Again, this wastes
energy against regulators and choke valves
• Irrigating plantations that are located on an incline with a mov-
ing pivot sprinkler system. The electric motor runs at full speed
regardless of the pressure head position, wasting energy
• The irrigation system is in an ‘over pressured’ state with the pump
running at full speed regardless of the pressure head requirement.
This wastes energy against regulators such as pivots running at
a pressure greater than required
Common misconceptions about VSDs
in irrigation systems
A common misconception is that no energy saving benefit results
from fitting VSDs on open pipe systems such as transferring water to
dams. Moving water at reduced flow rates over longer periods with
a VSD can bring about huge savings. The demand and total energy
reduction outweigh the addition of energy linked to the extended
pumping period.
A 'Soft Starter' device could be used in the place of a VSD. Not so
because the Soft starter is a voltage ramping device which provides
a softening effect on the mechanical components during the few
second starting process. Energy saving benefits do not result from
using a soft starter on motor driven systems as the process cannot
be changed. The soft starter device is not effective as variable speed
control even on fine-tuned fixed pumping systems.
VSD technology is said to be expensive but the payback period
for using VSDs typically is very short just from the energy savings
alone. Irrigators should view the higher initial cost layout against
Cut electricity bills on
irrigation systems
By F Steyn, ABB South Africa
Valuable information on how variable speed drives can improve your bottom line, the different quality of product that is
available and how to ‘right-size’ with pumping systems.
T
ake note
• A soft starter ramps up the speed of the machine whereas a VSD
allows speed control.
• In pumping and irrigation applications, VSDs can play a significant
role in optimising flow rates, and saving energy.
• In a typical irrigation application, VSDs can result in up to 40%energy
savings.
Electricity+Control
July ‘13
8
