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 FIMechE; CP Eng FIEAust RPEQ Design Detail & Development
http://waterhammer.hopout.com.au/ Skype address geoffrey.d.stone
Why Are VFDs Specified for Pumps
Process conditions Electrical supply
are not fully developed Variable process conditions Poor pump selection Future process upgrades Energy efficiencyReduced operating cost Prior art-Industry
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
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, Change pump
temperature or flow control valves Bypass valves Larger suction tanks or sumps Holding tank Pump for longer periods Stop/start controls
impeller diameter Alternate pump type Multiple pumps Different sized pumps
Pump Considerations
Pump Selection-The Issues
Duty point(s) Casing pressure
Static head (Hs)
Friction loss (Hf) Dead head Transients Design factors
- head - flow - NPSHa
rating Efficiency Specific speed Moment of inertia Curve shape Stability over range Best efficiency point 1st Critical speed
System Design-Issues
Software allows the Motor/VFD
analysis of systems Excessive design factors used Pump suppliers design factors New vs. Old pipe friction losses Pipe wall /lining tolerances
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
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)
Pump Curve #3-VFD, control valve or
reduced impeller viable
Pumps using VFDsConsiderations
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 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
BJTs (Bipolar Junction
Today >20kHz
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 Overspeed capability
HV) Braking requirements Power Power loss Line & load side Ride through time harmonics Audible noise Load torque Length/type of cable Speed range Power factor Speed regulation correction Failure mode Altitude Acceleration/decelerat Motor, insulation and ion times VFD life Mechanical engineers are required to Efficiency
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 motors 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 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 15C, 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 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 VFD voltage peaks (Vp) < 1,000V and dV/dT <500 V/s but VFD motors are subjected to 5000V/s 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 Damage peaks breakdown dieelectric strength of insulation Die electric strength reduced by humidity & temperature Corona & partial discharge destroy motors Standard motors design life reduced by up to
IEC 34-17 and DIN VDE 530 Repeated voltage
Commercial Considerations
Costs of a Pump/VFD Installation
Capex
VFD components with
Opex
VFD inefficiency
a design life < 10years Larger switchroom Increased air conditioning Screened cable Harmonic protection Special motors Supplementary fans Increase in noise
95% Inefficiency of motor Supplementary fans Special motor spares Air conditioning energy Reduced life of motor Spares for VFD Spares costs oversize pump
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 Geoff Stone
geoffrey.stone@yahoo.c o.uk
sulzerpumps.com mcnallyinstitute.com
eng-tips.com
nidi.org pumpsystemsmatter.org
Tel 0402 35 2313
Or 02 8850 2313
aft.com
toshont.com/vfdapp.htm virtualpipeline.spaces.live.com
canterburyengineeringassociat
es.com
