Presented by
Geoffrey D Stone C.Eng FIMechE; CP Eng FIEAust RPEQ
Design Detail & Development http://waterhammer.hopout.com.au/ Skype address geoffrey.d.stone
Pump Applications Using VFDs
Are VFDs worth it for pump applications?
Have they been oversold to the market?
Why Are VFDs Specified for PumpsProcess conditions are
not fully developedVariable process
conditionsPoor pump selectionFuture process upgradesEnergy efficiency-
Reduced operating costPrior art-Industry
practiceOver-speeding a pump to
reduce pump frame size
Electrical supply restraint-Soft starting
Braking- Dynamic or hold
Unlimited number of starts and stops
Waterhammer mitigation-Fatigue
Ignorance -Engineer having no understanding of other process control solutions
Pump Speed Control SolutionsMechanicalCone & disc variatorCyclic variatorVee belt & pulleysGearboxInternal combustion
engineScoop control fluid
couplingsHydraulic drive
ElectricalVariable Frequency
DriveEddy current driveTwo speed motorDirect Current drivesSlip ring motorsMultiple pole motorsRelay pulsed motors
Process Solutions-AlternativesPressure,
temperature or flow control valves
Bypass valvesLarger suction
tanks or sumpsHolding tankPump for longer
periodsStop/start controls
Change pump impeller diameter
Alternate pump type
Multiple pumpsDifferent sized
pumps
Pump Considerations
Pump Selection-The IssuesDuty point(s)Static head (Hs)Friction loss (Hf)Dead headTransientsDesign factors
- head- flow- NPSHa
Casing pressure rating
EfficiencySpecific speedMoment of inertiaCurve shapeStability over
rangeBest efficiency
point1st Critical speed
System Design-IssuesSoftware allows
the analysis of systems
Excessive design factors used
Pump suppliers design factors
New vs. Old pipe friction losses
Pipe wall /lining tolerances
Motor/VFD EfficiencyWire to Water kWThe 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-VFD Viable
Pump Curve #2-VFD Not Viable
Existing Pump Oversize? 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 impeller 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-VFD, control valve or reduced impeller viable
Pumps using VFDs- ConsiderationsEnergy savings with a VFD occurs for duties
reduced to between 60% to 85% of the BEP.Where duty is reduced to only 85% of BEP, a
control valve or reduced impeller 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 Transistor)
SCRs (Silicon Controlled Rectifier)
GTO (Gate Turn Off Thyristor)
IGBT (Insulated Gate Bipolar Transistor)- these offer the benefits of higher frequencies and increased efficiencies
Electrical Factors to be ConsideredVoltage (LV, MV or
HV)PowerLine & load side
harmonicsLoad torqueSpeed rangeSpeed regulationFailure modeAcceleration/
deceleration timesEfficiency
Overspeed capabilityBraking requirementsPower lossRide through timeAudible noiseLength/type of cablePower factor
correctionAltitudeMotor, 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 lengthCables 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
(Courtesy WEG motors)
Motor CoolingBelow 25hz motor fan speed will not
cool motorSupplementary fan requiredAdded cost of drive, cable, SCA,
controls, access and maintenanceReduced 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.
CurrentA 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 LevelDue 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 increased 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 LifeStandards Damage
IEC 34-17 and DIN VDE 530 VFD 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
Repeated voltage peaks 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 Opex
VFD components with a design life < 10years
Larger switchroom Increased air
conditioningScreened cableHarmonic protectionSpecial motorsSupplementary fansIncrease in noise
mitigationIncreased design costs
VFD inefficiency ≤ 95%Inefficiency of motor Supplementary fansSpecial motor sparesAir conditioning energyReduced life of motorSpares for VFDSpares costs oversize
pumpRisk & reliability (FMECA)Increase in noise
Commercial-OtherEngineers 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 for larger motors 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 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 linksThis presentation was by
Geoff [email protected]
Tel 0402 35 2313
Or
02 8850 2313
sulzerpumps.commcnallyinstitute.comeng-tips.comnidi.orgpumpsystemsmatter.orgaft.comtoshont.com/vfdapp.htmvirtualpipeline.spaces.live.co
mcanterburyengineeringassoci
ates.com