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Presented by Geoffrey D Stone C.Eng FIMechE; CP Eng FIEAust RPEQ Design Detail & Development  http://waterhammer.hopout.co...
Why Are VFDs Specified for Pumps <ul><li>Process conditions are not fully developed </li></ul><ul><li>Variable process con...
Pump Speed Control Solutions <ul><li>Mechanical </li></ul><ul><li>Cone & disc variator </li></ul><ul><li>Cyclic variator <...
Process Solutions-Alternatives <ul><li>Pressure, temperature or flow control valves </li></ul><ul><li>Bypass valves </li><...
Pump Considerations
Pump Selection-The Issues <ul><li>Duty point(s) </li></ul><ul><li>Static head (Hs) </li></ul><ul><li>Friction loss (Hf) </...
System Design-Issues <ul><li>Software allows the analysis of systems </li></ul><ul><li>Excessive design factors used </li>...
Pump Curve #1- VFD Viable
Pump Curve #2- VFD Not Viable
Existing Pump Oversize? <ul><li>This is a common pump dilemma that VFDs are used to solve but the VFD does NOT save the en...
Pump Curve #3 - VFD, control valve or reduced  impeller viable
Pumps using VFDs- Considerations <ul><li>Energy savings with a VFD occurs for duties reduced to between 60% to 85% of the ...
Electrical Design Considerations
What is a Variable Frequency Drive? <ul><li>Legacy- < 600Hz </li></ul><ul><li>Today  >20kHz </li></ul><ul><li>BJTs (Bipola...
Electrical Factors to be Considered <ul><li>Voltage (LV, MV or HV) </li></ul><ul><li>Power </li></ul><ul><li>Line & load s...
Cable <ul><li>Voltage peaks at motor terminals can be increased to 2 times the peaks of the VFD output for a long cable </...
Motor Considerations
Bearing Damage –Induced Shaft Voltage <ul><li>Induced Shaft Current Types </li></ul><ul><li>Conductive mode bearing curren...
Motor Cooling <ul><li>Below 25hz motor fan speed will not cool motor </li></ul><ul><li>Supplementary fan required </li></u...
Efficiency <ul><li>Published motor efficiency data is based on a pure sinusoidal voltage  </li></ul><ul><li>The high frequ...
Current <ul><li>A higher r.m.s. current to supply the same output (about 10% higher)  </li></ul><ul><li>Increase in motor ...
Noise Level <ul><li>Due to the harmonics, the motor noise level will increase when it is operated using a VFD </li></ul><u...
Motor Design Life <ul><li>Standards </li></ul><ul><li>Damage </li></ul><ul><li>IEC 34-17 and DIN VDE 530  VFD  voltage pea...
Commercial Considerations
Costs of a Pump/VFD Installation <ul><li>Capex </li></ul><ul><li>Opex </li></ul><ul><li>VFD components with a design life ...
Commercial-Other <ul><li>Engineers who use  suppliers  to select pumps or process solutions lose engineering control of th...
Conclusions <ul><li>Engineers need to specify all operating & electrical conditions to pump, motor & VFD supplier </li></u...
Questions <ul><li>Please ask questions remembering I am a mechanical engineer! </li></ul>
Useful links <ul><li>This presentation was by  </li></ul><ul><li>Geoff Stone </li></ul><ul><li>[email_address] </li></ul><...
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Variable Frequency Drives . Are they worth it?

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Variable Frequency Drives . Are they worth it?

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Variable Frequency Drives . Are they worth it?

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

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