Introduction to VFDs
Aka AC Drives, Adj Freq Drives, etc
Agenda
• What is a VFD ? Why use a VFD ?
• Applications: Variable Torque
• Common Questions
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What is a VFD ?
• VFD = Variable Frequency Drive
– Varies the frequency AND voltage applied to the motor
• SSRVS = Solid State Reduced Voltage Starter
– When starting, applies a reduced voltage to the motor
– Frequency is fixed
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Drive Basics
• An AC drive changes the speed of a motor by
changing the speed of the rotating magnetic field in a
motor that turns the motor shaft.
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The Internals of a AC Drive
DC Bus
Rectifier
Inverter
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Pulse Width Modulated Waveform
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Pulse Width Modulated Waveform (con’t)
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Pulse Width Modulated Waveform (con’t)
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How is a drive like a starter?
Disconnect
Fuse Protection
Controller
O.L. Protection
Drive
Controller
and
Overload
Motor
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How is a drive not like a starter?
starter
VFD
100 % Speed
0 % Speed
Time
•
•
•
•
Accelerate a load
Run at a selectable speed setting
Change speed while running
Stop the load
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A Typical Drive System
Drive
Motor
Machine
Westar Power
3
BAS/DCS/
PLC Control
Drives are just
one piece of
the process.
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Why AC Drives?
• Energy Savings !!!!
• Process Speed Control
• Reduced Mechanical Shock
• Reduced Maintenance Cost
• Provides Soft Start
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Energy Savings:
Variable Torque Applications
Pumps
Fans
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Energy Savings
Few Applications Require 100% Pump and
Fans to Run Continuously
• Systems Designed for Worst Case
– Emergency Conditions Require Higher Volumes
• Equipment is Sized Up to Next Rating
– Multiplying Safety Margins
• HVAC System Demand Changes
– Weekend and Night Time Occupancies are Lower
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Reduced Flow Accomplished By
• Change Motor and/or Equipment
– Mechanical VAV Boxes
– Fan belts
– Motor Base Speed
– Pump Impeller
– Inlet Guide Vanes
– Pump Valves
– Blade Pitch
• Variable Frequency Drive (VFD)
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9
A Common Analogy
• Controlling a system with a valve or diffuser is like
driving a car with your foot to the floor on the gas,
while controlling the speed with a brake
• VFD Control is the accelerator for your system
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Energy Savings
Why Do People Use Drives?
• 68% of all electrical energy is used by
centrifugal pumps and fans
• VFDs can run centrifugal pumps and fans
more efficiently than mechanical systems
• The energy that can be saved is ENORMOUS!
• Most applications pay for themselves in less
than two years
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The Affinity Laws
I FLOW is Proportional to the Motor Speed
II PRESSURE is Proportional to the Motor Speed
SQUARED
III POWER is Proportional to the Motor Speed
CUBED
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Affinity Laws for Centrifugal Loads
Speed
100%
90%
80%
70%
60%
50%
40%
30%
Volume
Pressure/
Head
100%
90%
80%
70%
60%
50%
40%
30%
100%
81%
64%
49%
36%
25%
16%
9%
Horsepower
Required
100%
73%
51%
34%
22%
13%
6%
3%
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Considerations for VFD Selection
•
•
•
•
Reduced Flow is Required
Power Costs
Hours of Operations
System Compatibility
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Duty Cycle
% Total Operating Time
100
80
60
40
20
0
10
20
30
40
50
60
70
80
90
100
% Flow Volume
Source: Electric Power Research Institute
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6
The Most Efficient Choice is…
Less Than 100% Flow
Accomplished by VFD
Equals
BIG ENERGY
SAVINGS!!
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Pump Systems
Other Benefits
•
•
•
•
•
•
Eliminates Valves
Longer Pump Seal Life and Reduced Impeller Wear
Eliminates Power Surge During Start-up
More Flexible Operation
Less Vibration and Noise
Helps Prevent “Water Hammer” and Cavitation
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Summary
• VFD’s Can Produce Significant Energy
Savings
• Selecting the right VFD and analyzing the
entire system will assure successful
operation.
• Other things to consider are…
– Available Power
– Communication Requirements
– By-pass Required
– Environmental Requirements
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