Considerations for Selecting
VFD’s
Quentin Nesbitt, PE
Agricultural Engineer
Idaho Power
Company
Introduction
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What is a VFD
Applications of VFD
Technical Considerations
Variable Frequency Drives
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Other common terms – VSD - ASD - Variable
Frequency Panel
Solid state electronic device
Normal utility 60 Hz power no longer just 60 Hz
What does it look like?
What I want you to Know After
This Presentation
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Cost of Pressure (psi)
Higher awareness of energy wasting
practices
Good applications for VFD’s
Formulas
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HP = Flow X Head /3960 X Efficiency
Cost $=BHP/Motor Eff.*.746*hrs*$/kWh
Considerations
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Hp and pump curve shape with time at
different conditions determine energy
operating cost
Ask specific questions on system
characteristics
Where is the power, is it only single phase
- Line extension costs
- Single phase options-Utility running current and starting current limits
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Consider other options – multiple
pumps,valves
Potentially Ideal Applications
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Widely varied flow Requirements
Less total head needed as flow requirements
are dropped (Extreme friction loss or major
draw-down changes in a well)
Single Well serving a very large acreage
Long operating hours at less than full flow –
Varied crops, multiple systems on one pump
Steep pump curves
Alternatives
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Multiple Pumps
Valveing – automatic, manual, Pressure
regulating
Utilize more than is needed – more flow or
more pressure or both
Advantages /Disadvantages
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Ease of daily management (+)
Energy savings (+) – sometimes less than expected
Initial cost $ (-)
High Tech device where service is harder to come by
(-)
Power quality considerations – Harmonics (IEEE 519
standard) – Filtering usually required & adds to cost (-)
Usually the drive is 97-98% Eff.(heat loss in panel all
the time) (-)
Energy Savings
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Normally no Demand Savings – Sometime during the
month probably going to run at maximum
Energy Savings Calculations are not hard, but they do
require the Pump Curve and some knowledge of the
specific System
Design head is usually constant (although multiple set
points are advantageous)
Pump efficiency does not stay constant
Application example
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450 Hp Well
490 acres
3 Pivots, 2@130 ac, 1@100 ac,
150 ac of handlines and wheelines including
some corners
Analysis with: 450 Hp w/VSD, 450 Hp alone,
300 Hp &125 Hp Booster w/VSD on Booster
Cost estimates
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Cost per horsepower seems to range between $90 and
$200 per Horsepower
Medium voltage drives are considered to be 2300 volts
- seem to be on higher end even though large
horsepower
Low voltage is 480 volts and lower.
Single phase drives that convert to three phase maybe
a option for single phase customers
200 Hp 6 pulse filter $8000, LR-$500
Check with Idaho Power and know and understand
harmonic standards-(IEEE519)
Cost (cont.)
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Hp
40
100
200
300
600
$
$4,500
$20,000
$32,000
$38,000
$52,000
Summary
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Check with Utility know or understand your risk
get information to determine IEEE 519
requirements
Owner’s that I know are very pleased
There are applications where there are
cheaper alternatives