vfp example
TRANSCRIPT
Principles and Applications of Variable Frequency Drives
Presented byRandall L. Foulke, P.E., BCEE
NC AWWA-WEA Spring ConferenceApril 6, 2009
New Bern, North Carolina
What is a variable-frequency drive?
A variable-frequency drive (VFD) is a system for controlling the rotational
speed or torque of an alternating current (AC) electric motor by controlling the
frequency of the electric power supplied to the motor. By extension, a VFD also controls horsepower. A VFD is a type of
an adjustable-speed drive (ASD).
NC AWWA-WEASpring Conference
April 6, 2009
NC AWWA-WEASpring Conference
April 6, 2009
Types of Adjustable Speed Drives
Mechanical Adjustable Speed DrivesVariable pitch drivesTraction drives
Hydraulic Adjustable Speed DrivesHydrostatic drivesHydrodynamic drivesHydroviscous drives
Continuously Variable Transmission (CVT)
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Types of Adjustable Speed Drives
Electric adjustable speed drivesDC motor drivesEddy current drivesAC motor drives
Slip controlled drivesMultiple speed motorsAdjustable-frequency drives (AFD)
Also known as variable-frequency drives
NC AWWA-WEASpring Conference
April 6, 2009
Why Use Adjustable Speed Drives
Saving energyProcess control
Smoother operationAcceleration/deceleration controlDifferent operating speed for each process modeCompensate for changing process variablesAllow slow operation for setup purposesAdjust rate of process parametersAllow accurate positioningControl torque or tension
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Why use VFDs
Assumes the need for adjustable speedAdjustable outputSteady state condition
Reduced capital expenditureSolid state electronic power conversion devices
Reduced maintenance
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Alternate Terms for VFD
Adjustable-frequency drive (AFD)Variable-speed drive (VSD)MicrodriveInverter driveVariable voltage variable frequency (VVVF)
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Saving Energy with VFDsAffinity Laws
Law 1. With impeller diameter (D) held constant:Law 1a. Flow is proportional to shaft speed: Q1/Q2 = (N1/N2)Law 1b. Pressure or Head is proportional to the square of shaft speed: H1/H2 = (N1/N2)2
Law 1c. Power is proportional to the cube of shaft speed: P1/P2 = (N1/N2)3
whereQ is the volumetric flow rate (e.g. CFM or GPM), D is the impeller diameter (e.g. in), N is the shaft rotational speed (e.g. rpm), H is the pressure or head developed by the fan/pump, and P is the shaft power. These laws assume that the pump/fan efficiency remains constant. In other words, η1 = η2 .
Reducing pump speed by 50% results in a power consumption drop to 12.5%
NC AWWA-WEASpring Conference
April 6, 2009
Saving Energy with VFDsMotor Starting
Start motors – Resistance of motor, equipment, and process
Simple switching at full loadRequires additional energy – inrush current
Typical inrush current is at least 300% of rated current of motor – produces less than 50% of rated torque of motor
VFD can start motor at low frequency and avoid high inrush currentConstant-speed motor starts significantly increase energy usage
Each motor start increases energy usage at least 300%
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Operating Principles of AC Motors
Synchronous speed of an AC motor determined by frequency of AC supply and number of poles in stator winding.
RPM = (120 x f) / pRPM = revolutions per minute, f = AC power frequency (hertz), p =
number of poles(120 x 60) / 4 = 1800 RPM
Induction motor at full load = 1750 RPM
Speed controller with input of 50 hertz (Hz)(120 x 50) / 4 = 1500 RPM
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VFD TypesAll VFDs use output devices only as switches, turning them only on or off
Using linear output devices impractical, since dissipated power = power delivered to load
Drive typesConstant currentCycloconverterConstant voltage – Most Common
Pulse width modulation to control frequency and voltage
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VFD System
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VFD System DescriptionMotors
3 Ø induction motorMost economicalSynchronous motors may offer some advantages
VFD controllerSolid state electronic power conversionUse of rectifiers and inverters to convert AC to DC to quasi-sinusoidal ACWill accept 1Ø power, but must derate systemEmbedded microprocessor – governs operation
Limited ability for configuration programming and parameter adjustment
Allowable spacing between motors and controllers can vary by factor of 2.5:1
Higher carrier switching frequencies (CSF) dictate short distances
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VFD Diagram
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Types of Switches
TransistorsThyristorsInsulated-gate bipolar transistor (IGBT)
Introduced in 1980sMost used device in VFDs for inverter circuits in first decade of 21st century
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Drive Output Waveform
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Common Torque
If torque is maintained, then applied voltage must be decreased in same ratio to frequencyUsual method for adjusting motor voltage is pulse-width modulation (PWM)
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Selection of VFDs
PWM type of IGBT is newest and best performing
6 pulse and 18 pulse available, with 6 pulse most common – no difference in control performance18 controls harmonics for motor and facility electrical system18 pulse only used for 100 hp and above
Small VFDs (<20hp) most manufacturers same, >20 hp best to select from well known manfacturers
Allen-Bradley, Danfoss, Robicon (Siemens)
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Selection of VFDs
Fully rated NEMA bypass – expensive optionIncludes across the line starter in parallel with VFDFailure of VFD removes circuit from system and NEMA bypass operates motor at full speed
Use one or more constant speed process units in parallel with VFD process units in multiple unit applications
Also reduces capital costMay cause unbalanced wear unless provided with selector switches for VFD controlled units
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VFD Operator Interface
Commonly known as human machine interface (HMI)Operator controls
Start/stopAdjust SpeedSwitch between manual and automaticReceive external process controlDisplays of indication and metersKeypad can be remote from controller
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VFD Examples
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How VFDs Are Controlled
ManuallyAdjust frequency by set point locally or remotely
AutomaticallyAdjust frequency based on maintaining process parameter –level, flow, DO, densityAdjust frequency based on relationship to another process parameter – flow, nitrogen
Control input adjusts the frequency output from the VFD to motor changing the RPM of the motor and process unit.
Normally from 60 Hz down, but can be operated above 60 Hz, but will reduce available torque from motor (sometimes call “field weakening”). Example – operation at 75 Hz reduces torque to 80%.
NC AWWA-WEASpring Conference
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How VFDs Operate
StartingInitial application of low frequency and voltage (2 Hz or less) – avoids high inrush currentApplied frequency and voltage increased at controlled rate without drawing excessive current
Develop 150% of torque with only 50% of rated current
As load accelerates, available torque drops somewhat, then rises to peak at motor full load
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How VFDs Operate
StoppingFrequency and voltage are ramped down at a controlled rate, approaching zero, motor shuts offSmall amount of braking torque available to decelerate load faster than if allowed to coastBraking circuit can be added to obtain additional braking torque turning the motor into a generator
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Maintaining VFDsVFD – Basically a computer and a power supplyMaintenance requirements
Keep it cleanKeep it dryKeep the connections tightKeep it cool
Fan injecting dust
Corrosion caused by moisture
Arcing caused by loose contacts
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Available VFD Power Ratings
Low voltage110 v to 690 v¼ hp to 1000 hp
Medium voltageUp to 2400/4160 v @ 60 Hz and 3000 v @ 50 HzUp to 5000 hpSome applications use a transformer between low voltage drive and medium voltage load
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Limitations of VFDs
ReprogrammingMay require factory technician
May have limited operating rangeGenerally with pumps 50% turndown
Location of drive unitsDifferent transmission-line impedance of the cable and motor result in reflection of pulses, which can put high stress on cable and eventual insulation failure –long runs of 480 volts and frequent concern for motors of 600 volts and above.
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Disadvantages of VFDs
Additional point of failure – Increased riskEquipment failureMisinterpret communication signals – Process Control
Additional cost of equipmentMicroelectronics have mitigated this disadvantage
Complexity of control circuitryAdditional operator and maintenance training, skill and knowledge
NC AWWA-WEASpring Conference
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Disadvantages of VFDs
HarmonicsPotential for development in process unit –destructive stressElectrical system – if VFD >25% of load, must take steps to mitigate
Requires clean conditioned space or AC cooler on panel
Sensitive to heatFrequent/constant running at full speed decreases efficiency and life of VFD
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Alternative/Innovative Uses of VFDs
Use to control surge during start-up/shut-down of process units (pumps)Use to adapt to varying head conditions, manually or automatically, while maintaining flow rateUse braking circuit to return power to sourceUse of single VFD to control more than one process unit (additional pumps)
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Useful Application Information
Single phase to three phase – common trick when site does not have 3Ø power to supply 3Ø pumps
Used for larger lift stations in rural and remote locationsIssue – VFD size usually doubled to accept higher current on input side of electric supply
Generator sizing – VFD motor requires smaller generator than across the line starting
VFDs limit inrush current by up to 50%VFD cost can be offset by reduced generator size
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Is a VFD Right for You
Varying process conditionsBetter controlFine tuning of process
Control surgesFrequent start-stops of equipmentNeed to control energy costsAutomatic control
