Pump Applications Using VFDs Are VFDs worth it

Pump Applications Using VFDs Are VFDs worth it for pump applications? Have they been oversold to the market? Presented by Geoffrey D Stone C. Eng FIMech. E; CP Eng FIEAust RPEQ Design Detail & Development (geoffrey. stone@yahoo. co. uk)

Why Are VFDs Specified for Pumps Process conditions are Electrical supply not 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 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 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 Change pump impeller or flow control valves Bypass valves Larger suction tanks or sumps Holding tank Pump for longer periods Stop/start controls 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 1 st Critical speed

System Design-Issues Software allows the Motor/VFD Efficiency analysis of systems Excessive design factors used Pump suppliers design factors New vs. Old pipe friction losses Pipe wall /lining tolerances Wire to Water k. W The original Affinity Laws are based on systems with no static head Affinity Laws overstate energy savings Revise the 2 nd 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

Pump Curve #3 -VFD, control valve or reduced impeller viable

Pumps using VFDs. Considerations Energy 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 k. W-hr per m 3 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- < 600 Hz Today >20 k. Hz 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) Power Line & load side harmonics Load torque Speed range Speed regulation Failure mode Acceleration/deceleratio n times Efficiency engineers Mechanical Overspeed capability Braking requirements Power loss Ride through time Audible noise Length/type of cable Power factor correction Altitude Motor, insulation and VFD life 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 25 m is the recommended cable length Cables longer than 25 m 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.

Motor Cooling Below 25 hz 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 increased by anything between 2 and 15 d. BA 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 Repeated voltage peaks VFD voltage peaks (Vp) < 1, 000 V and d. V/d. T <500 V/µs but VFD motors are subjected to 5000 V/µs and 1, 500 V Voltage peaks depend on carrier frequency d. V/d. T affects the insulation between turns, the high voltage spikes affect the insulation between phases and phase to ground 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 VFD inefficiency ≤ design life < 10 years Larger switchroom Increased air conditioning Screened cable Harmonic protection Special motors Supplementary fans Increase in noise mitigation 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

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 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 links This presentation was by sulzerpumps. com Geoff Stone mcnallyinstitute. com geoffrey. stone@yahoo. c o. uk eng-tips. com nidi. org Tel 0402 35 2313 Or pumpsystemsmatter. org 02 8850 2313 aft. com toshont. com/vfdapp. htm virtualpipeline. spaces. live. com canterburyengineeringassociates. com