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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 are 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 size
Electrical supply restraint
Soft starting
Braking- Dynamic or hold
Unlimited number of starts
and stops
Waterhammer mitigationFatigue
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
or flow control valves
Bypass valves
Larger suction tanks or
sumps
Holding tank
Pump for longer
periods
Stop/start controls
Change pump impeller
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
1st Critical speed
System Design-Issues
Software 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 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 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- 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 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
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)
Overspeed capability
Power
Braking requirements
Line & load side harmonics
Power loss
Load torque
Ride through time
Speed range
Audible noise
Speed regulation
Length/type of cable
Failure mode
Power factor correction
Acceleration/deceleration
Altitude
times
Efficiency
Motor, 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 length
Cables 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
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 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 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
Damage
IEC 34-17 and DIN VDE 530 VFD Repeated voltage peaks
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
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
VFD components with a
design life < 10years
Larger switchroom
Increased air conditioning
Screened cable
Harmonic protection
Special motors
Supplementary fans
Increase in noise mitigation
Increased design costs
Opex
VFD inefficiency ≤ 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 (FMECA)
Increase in noise
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
[email protected]
Tel 0402 35 2313
sulzerpumps.com
mcnallyinstitute.com
eng-tips.com
Or
nidi.org
02 8850 2313
pumpsystemsmatter.org
aft.com
toshont.com/vfdapp.htm
virtualpipeline.spaces.live.com
canterburyengineeringassociates.com