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The Effects of VFDs
on Centrifugal
Wastewater Pumps
Technical Series 2014
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Course Objectives
• Understanding of AC
Induction Motors
• VFDs- Are they Magic?
• Variable Speed and
Wastewater Pumping
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AC Induction
Motors
Theory & Design
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Basic Motor Vocabulary
• Poles- Number of magnetic poles (or
coils) 180-degrees apart in the stator
• Service Factor- Amount of overload
motor can handle typically expressed in
amps
• Synchronous Speed- Speed at which
magnetic field in the motor is rotating;
also the “no-load” speed of the motor.
Expressed in RPM’s
• Slip- Difference between Synchronous
Speed (Theoretical) and Full Loaded
Speed (Actual). Expressed in a
percentage
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Motors
• Rotational Speed of AC
induction motors depend on
two things:
Poles in the stator
Frequency of power (Hz)
• Poles in the motor are a
constant
• Speed varies directly with
Frequency
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Motors
• In the US power is 60hz or 60 cycles per
second
• This is just the standard in the early
days the cycles varied wildly. It was
Nikola Tesla the inventor of the AC
induction motor who standardized on
60Hz
• Motor speed is determined by number
or North & South Poles in the motor
(+/-)
Simple DC Motor
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Poles & Speed
• Motor speed is determined by number
or North & South Coils in the motor
(+/-)
• These always occur in twos 2-pole/4pole/6-pole
• Formula for figuring speed in RPM
120 x Frequency / Number of poles
 120 is constant
 120 x 60 = 7200
 7200/ 4 = 1800RPM
Two & Four Pole Three Phase Motors
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Motors
• What is my Speed?
– Difference between
Synchronous Speed and Full
Load Speed is described as Slip
– Slip is typically between 3% &
5%
– So;
Synchronous
• 1800RPM * 0.97 = 1746RPM
(1750RPM)
• 1800RPM *0.95 = 1710RPM
(1700RPM)
RPM
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VFDs Why we
use them
Review of VFD principal
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VFDs- How it works
• Converts AC to DC to AC
• Allows for modulation of the
Frequency
• Remember frequency
controls speed in AC motors
• Power is not true Sine wave
but “Block-wave”
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VFDs- How it works
• The block nature of the AC
power of a VFD creates
harmonics in the pump
causing vibration
• Lower Pulse (slower) drives
have higher harmonic
distortions
• Higher Pulse (faster) drives
have less harmonic
distortion
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High Pulse Drives
• 18-Pulse drives allow for smoother operation of rotating
equipment as by using insulated gate transistors to
synthesize a cleaner sine wave power curve
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High Pulse Drives
• 18-Pulse and higher drives
have drawbacks
• Voltage unbalance/pulses
cause induced AC voltage on
the motor shaft
• Leads to a continuous
discharge of voltage most
often across the bearing
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Why Use a VFD
• Pumps are often designed for
worst case scenario. (I&I or peak
demand)
• These extremes do not happen
often
• However pumps slide up and
down curve depending on
demand and system
• The ability of a VFD to modulate
speed allows for pump to run at
BEP across operating range
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Variable Speed
and Wastewater
Pumping
Bringing it all together
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Using a VFD with a submersible pump
• Pump COS
 100GPM at 100’TDH
 3500RPM
 4.04HP (at design)
 5.25Impeller Diameter
 Change System Pressure to 56’TDH
• What Happens?
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Pump & System Head Curve
• System head drops from 100’ to 56’
• At 3500RPM efficiency drops below 40% suction cavitation is a
problem
• With VFD at 2625RPM efficiency is constant at 62%
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Wastewater Pumps & VFDs
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Cost Savings on a VFD
• At Constant Speed- System would use $45,485.00.
• At Variable Speed- System would use $19,742.00.
• Variable Speed nets a $23,433.00 energy savings.
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PRESENTATION TITLE
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Wastewater Pumps & VFDs
• The ability to maintain efficiency
is useful for wastewater pumping
where constant flow & pressure
is not a concern
• Eliminates the need for a shaver
pump
• Keeps pump from either suction
or recirculation cavitation
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Wastewater Pumps & VFDs
• VFDs give the operator the ability
to run the pump/motor in
reverse. This can be beneficial to
clear a blockage in the volute
• Eliminates throttle valves in the
system
• Avoids water hammer and
pressure spikes by controlling
speed and ramp up & ramp
down
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Wastewater Pumps & VFDs
• Potential Pump Issues
Service factor for many motors
is 1.15 for constant speed but
drops to 1.0 on VFD
Decrease bearing life unless
shaft currents are mitigated
(Insulated bearing or SGR)
First Criticals can move around
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Wastewater Pumps & VFDs
• Pipeline Issues- depending
on how drive is program you
can see velocity issues
For example our test pump at
100GPM and 3500RPM has
plenty of flow to achieve 2FPS
in a 4” force main
However at 2625RPM we only
produce 75GPM when 80GPM
is needed for 2FPS
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Wastewater Pumps & VFDs
• Other Factors
More complex controls usually
utilizing a transducer
Higher noise in motor due to
chatter in the motor windings
produced by the block wave
form
Don’t be tempted to turn
pump so slow it can not move
solids
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The Take Away
Technical Series 2014
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The Take Away
• VFDs when properly applied
to collection systems can
yield substantial cost
savings and longer service
life
• Look for equipment that is
designed for use with VFDs
to ensure maximum benefit
• Weigh all options as well as
Total Cost of Ownership
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HYDROMATIC HPE
www.HydromaticHPE.com
•Information on Pumps
•Specifications
•CAD/DWG/PDF Drawings
•Downloads
•White Papers
•Distributor Locator
More content added each month
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