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Pump Applications Using VFDs
Are VFDs worth it for pump
applications?
Have they been oversold to
the market?
Why Are VFDs Specified for Pumps
 Process conditions are not
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fully developed
Variable process conditions
Poor pump selection
Future process upgrades
Energy efficiency-Reduced
operating cost
Prior art-Industry practice
Over-speeding a pump to
reduce pump frame size
 Electrical supply restraint
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Soft starting
Braking- Dynamic or hold
Unlimited number of starts
and stops
Waterhammer mitigationFatigue
Ignorance -Engineer having
no understanding of other
process control solutions
Pump Speed Control Solutions
Mechanical
 Cone & disc variator
 Cyclic variator
 Vee belt & pulleys
 Gearbox
 Internal combustion engine
 Scoop control fluid
couplings
 Hydraulic drive
Electrical
 Variable Frequency Drive
 Eddy current drive
 Two speed motor
 Direct Current drives
 Slip ring motors
 Multiple pole motors
 Relay pulsed motors
Process Solutions-Alternatives
 Pressure, temperature
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or flow control valves
Bypass valves
Larger suction tanks or
sumps
Holding tank
Pump for longer
periods
Stop/start controls
 Change pump impellor
diameter
 Alternate pump type
 Multiple pumps
 Different sized pumps
Pump Considerations
Pump Selection-The Issues
 Duty point(s)
 Casing pressure rating
 Static head (Hs)
 Efficiency
 Friction loss (Hf)
 Specific speed
 Dead head
 Moment of inertia
 Transients
 Curve shape
 Design factors
 Stability over range
- head
- flow
- NPSHa
 Best efficiency point
 1st Critical speed
System Design-Issues
 Software allows the
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analysis of systems
Excessive design
factors used
Pump suppliers design
factors
New vs. Old pipe
friction losses
Pipe wall /lining
tolerances
 Motor/VFD Efficiency
 Wire to Water kW
 The original Affinity
Laws are based on
systems with no static
head
 Affinity Laws overstate
energy savings
 Revise the 2nd Affinity
Law for Minimum Flow
Pump Curve #1
Pump Curve #2
Existing Pump Oversize?
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This is a common pump dilemma that VFDs are used
to solve but the VFD does NOT save the energy! The
credit goes to the reduced head/flow requirements.
VFD suppliers offer the retro-fit of a VFD to change
pump speed to meet reduced process conditions
Change of pump or impellor reduced diameter
achieves the necessary reduced flow, hence power
A flow control valve achieves the necessary reduced
flow and maintain the best efficiency point (BEP)
A multiple small pumps and motor could be cost
effective
Pump Curve #3
Pumps using VFDs- Considerations
 Energy savings with a VFD occurs for duties reduced
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to between 60% to 85% of the BEP.
Where duty is reduced to only 85% of BEP, a control
valve energy demand is less than that for the
combined VFD installation inefficiencies
Wire to water energy kW-hr per m3 delivered
should be the criteria used in assessing a VFD
application
VFDs offer little benefit for systems with more than
50% static head
VFDs are ideal for closed systems with varying
process duties-no static head
Electrical Design Considerations
What is a Variable Frequency Drive?
Legacy- < 600Hz
Today >20kHz
 BJTs (Bipolar Junction
 IGBT (Insulated Gate
Transistor)
 SCRs (Silicon Controlled
Rectifier)
 GTO (Gate Turn Off
Thyristor)
Bipolar Transistor)- these
offer the benefits of higher
frequencies and increased
efficiencies
Electrical Factors to be Considered
 Voltage (LV, MV or HV)
 Overspeed capability
 Power
 Braking requirements
 Line & load side harmonics
 Power loss
 Load torque
 Ride through time
 Speed range
 Audible noise
 Speed regulation
 Length/type of cable
 Failure mode
 Power factor correction
 Acceleration/deceleration
 Altitude
times
 Efficiency
 Motor, insulation and VFD
life
Mechanical engineers are required to
understand the electrical issues
Cable
 Voltage peaks at motor terminals can be increased
to 2 times the peaks of the VFD output for a long
cable
 25m is the recommended cable length
 Cables longer than 25m have an inductive load that
affects a motor’s life
 Cables need to be screened to avoid EMI
Motor Considerations
Bearing Damage –Induced Shaft Voltage
Induced Shaft Current Types
1. Conductive mode bearing
current-low speed , good
conductivity.
2. Discharge mode bearing
current-higher inverter
output frequencies-The
capacitive voltage builds
up until it is able to break
down the dielectric
resistance of the grease.
Induced shaft voltage with no
shaft brush or insulated bearing
Motor Cooling
 Below 25hz motor fan speed will not cool motor
 Supplementary fan required
 Added cost of drive, cable, SCA, controls, access and
maintenance
 Reduced reliability
Efficiency
 Published motor efficiency
data is based on a pure
sinusoidal voltage
 The high frequency
harmonics created by VFDs
increase copper and core
losses decreasing the
efficiency of the motor
 Materials behave differently
under these operating
conditions resulting in a
higher efficiency drop when
fed by VFDs.
Current
 A higher r.m.s. current to supply the same
output (about 10% higher)
 Increase in motor operating temperature
 On average, VFD fed motors will have a
temperature increase of about 15°C, at rated
speed and load
Noise Level
 Due to the harmonics, the motor noise level will
increase when it is operated using a VFD
 Experience shows that the sound pressure level at A
scale at motor rated speed is increase by anything
between 2 and 15dBA with a VFD
 This “ extra ” noise level depends mainly on the
inverter switching frequency and harmonic
content.
 Noise mitigation costs increase
Motor Design Life
Standards
Damage
 IEC 34-17 and DIN VDE 530 VFD  Repeated voltage peaks
voltage peaks (Vp) < 1,000V and
dV/dT <500 V/µs but VFD
motors are subjected to 5000V/µs
and 1,500V
 Voltage peaks depend on carrier
frequency
 dV/dT affects the insulation
between turns, the high voltage
spikes affect the insulation between
phases and phase to ground
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breakdown die-electric
strength of insulation
Die electric strength reduced
by humidity & temperature
Corona & partial discharge
destroy motors
Standard motors design life
reduced by up to 75%
Standard insulation varnish is
NOT acceptable
Commercial Considerations
Costs of a Pump/VFD Installation
Capex
 VFD componenets with a
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design life < 10years
Larger switchroom
Increased air conditioning
Screened cable
Harmonic protection
Special motors
Supplementary fans
Increase in noise mitigation
Increased design costs
Opex
 VFD inefficiency ≤ 95%
 Inefficiency of motor
 Supplementary fans
 Special motor spares
 Air conditioning energy
 Reduced life of motor
 Spares for VFD
 Spares costs oversize pump
 Risk & reliability (FMECA)
 Increase in noise
Commercial-Other
 Engineers who use suppliers to select pumps or process
solutions lose engineering control of the procurement
process
 Pump suppliers do not necessarily know, or care, about
the process vs. electrical requirements of the VFD/motor
interface-divided responsibility
 String testing motor/pump/VFD is difficult during the
contract period because of :-time
-manufacture location of components
-responsibility of the other parties equipment
-packing/unpacking/re-packing
Conclusions
 Engineers need to specify all operating & electrical conditions to
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pump, motor & VFD supplier
Invest in the mechanical engineering and specify correctly
Future operating conditions may not occur. If they do they can be
met with alternate solutions
VFDs do not always save energy, Capex or Opex
VFDs do not avoid transients from power loss
VFDs provide a suitable solution to some pump operating
conditions but should not be considered a panacea
“You just can't ever beat the energy efficiency of running a
properly sized pump at 100% BEP rated flow”.
Mechanical engineers have a poor understanding of electric motors
& VFDs and fail to communicate with process or electrical
engineers
Questions
Please ask questions remembering
I am a mechanical engineer!
Useful links
This presentation was by
Geoff Stone
[email protected]
Tel 0402 35 2313
 sulzerpumps.com
 mcnallyinstitute.com
 eng-tips.com
Or
 nidi.org
02 8850 2313
 pumpsystemsmatter.org
 aft.com
 toshont.com/vfdapp.htm
 virtualpipeline.spaces.live.com
 canterburyengineeringassociates.com