Variable Frequency Drives . Are they worth it?

Presented by

Geoffrey D Stone C.Eng FIMechE; CP Eng FIEAust RPEQ

Design Detail & Development http://waterhammer.hopout.com.au/                 Skype address

[email protected]

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