VARIABLE SPEED PUMP CONTROL:
HOW AND WHY OF VARIABLE SPEED PUMPING
MICHIGAN WEA
2015 Energy Seminar
October 20, 2015
Presented by Gary Patterson
Water/Wastewater Technical Sales Director
DSI/Dynamatic
© 2011 Drive Source International, Inc.
Share of motor
electrical consumption
Cooling Compressors
Air
Compressors
Conveyors
7% 2%
35%
18%
Fans
Other
16%
22%
Pumps
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Energy Consumption by Pump Type
Positive Displacement 27%
Rotodynamic 73%
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Typical Centrifugal Pump
Performance Curve
Function of pump design, and rotating speed
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System Curve
Function of site conditions: Hydraulic lift, piping
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Pump Operating Point
“Adding comfort margins to the calculated system curve to ensure a
sufficiently large pump will generally result in installing an oversized
pump, which will operate at an excessive flow rate or in a throttled
condition, which increases energy usage and reduces pump life.”
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Performance curves at various
pump speeds
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Benefits of variable speed
pumping
•
•
•
•
•
•
•
•
•
Finite control of flow as conditions vary
Pump mechanical wear reduced (bowl, impeller)
Bearing life increased
Seal life increased
Reduced noise
Reduced vibration
Omit throttling valve (or reduce wear)
Eliminate water hammer
Reduced cycling on/off
• Reduced energy consumption
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Pump Affinity Laws
100%
• Pump brake
horsepower varies
approximately as
the cube of speed.
HP a RPM 3
90%
Pump load
80%
Percent horsepower
70%
60%
• Normally speed
reduction limited to
70% or higher
50%
40%
30%
20%
10%
0%
0%
10%
20%
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30%
40%
50%
60%
Percent Speed
70%
80%
90%
100%
Variable Speed eddy current drive
vs. Throttled Discharge
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Example No. 1
Proposed Project Payback Spreadsheet
Client: Sabine River Authority
Nominal Rating
% Speed
700 hp 889 rpm induction motor
Assumed bhp at full speed:
650
horsepower
Assumed power cost:
0.08
$/kw-hr
Assumed project cost:
$190,000.00
Pump rpm Load hp Load kw
kw-hr/yr
Annual Cost Annual Savings Payback (yr)
94
835.66
540
403
3,528,878.40
$282,310.27
92
817.88
506
378
3,308,389.35
$264,671.15
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$57,571.00
$75,206.00
3.300
2.526
Example No. 2
Proposed Project Payback Spreadsheet
Client: Miami-Dade County
2000 hp 600 rpm synchronous motor
Nominal Rating
1900
horsepower
Assumed bhp at full speed:
0.08
$/kw-hr
Assumed power cost:
$250,000.00
Assumed project cost:
% Speed Pump rpm Load hp Load kw
kw-hr/yr
Annual Cost Annual Savings Payback (yr)
100
600
1,900
1,417
12,416,424.00
$993,313.92
95
570
1,629
1,215
10,645,531.53
$851,642.52
90
540
1,385
1,033
9,051,573.10
$724,125.85
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$141,671.40
$269,188.07
1.765
0.929
Available Motor Speeds
AC Squirrel Cage Induction Motors
• Synchronous Speed = (120 x 60)/ no. of poles (pairs
only)
• 2 Pole
3600 rpm
• 4 Pole
1800 rpm
• 6 Pole
1200 rpm
• 8 Pole
900 rpm
• 10 Pole
720 rpm
• 12 Pole
600 rpm
• 14 Pole
514 rpm
• …minus about 1% slip
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Variable Speed Options
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Variable frequency AC drives
Eddy Current Drives
Two-speed AC induction motors
Wound Rotor AC Induction motors
Fluid Couplings
Rare earth permanent magnet drives
Variable Pitch Sheaves
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Variable Frequency AC Drives
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•
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•
Fractional to Unlimited Horsepower
Low and Medium Voltage
Since 1970s
Wide variety of configurations
Improved over the years, but still occasionally
problematic
• Converts AC line power to DC, and then reconstructs
power to simulate sinusoidal AC power at adjustable
frequency and voltage
• Probably the most common solution
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Eddy Current Drives
•
•
•
•
•
•
•
•
•
Fractional to 10,000 hp
Suitable for low and medium voltage motors
Since 1930s
Horizontal, Vertical designs
Air cooled, water cooled available
Long-lived, very reliable
Simple to diagnose and repair
No conversion of electrical power
Transmits adjustable torque from a constant speed AC
motor
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Two Speed Induction Motor
•
•
•
•
Relatively Inexpensive
Two connections required (essentially two starters)
Limited to specific speeds and combinations
Adjacent pole combinations can be very practical
• 2 Pole
3600 rpm
• 4 Pole
1800 rpm
• 6 Pole
1200 rpm
• 8 Pole
900 rpm
• 10 Pole
720 rpm
• 12 Pole
600 rpm
• 14 Pole
514 rpm
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Wound Rotor Induction Motor
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•
•
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Available to thousands of HP
Available for low and medium voltage
Common in 1950s to 1970s
Generally expensive motor and control
Special starting requirements
Requires controlled resistance in rotor circuit, via slip
rings and brushes
– Liquid Rheostat
– Step Resistor Banks
– Slip Power Recovery System (electronic frequency and voltage
conversion)
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Wound Rotor Induction Motor
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Fluid Drives
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•
•
•
•
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Available 100 to several thousand horsepower
Hydrostatic unit interposed between motor and load
Driven at constant speed by AC induction motor
Slip loss characteristic proportional to percent speed
Long lasting
Can be high maintenance (oil leakage, oil contamination)
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Rare Earth Magnet Technology
Rare Earth Magnet ASD
● Introduced in 1999
● Common trade name is
“MagnaDrive™”
● Received recognition as a
competitor to Variable Frequency
Drives due to reduced energy
consumption
● Transfers torque across the air
gap using magnetic force
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Variable Pitch Belt Drive
• More “adjustable” than “variable”
• Limited to relatively small horsepower
• Often manually adjustable only
• Commonly called “Reeves” Drive
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Thank you!
© 2011 Drive Source International, Inc.