7154 VFD7154 VFDPresentation #2Presentation #2
May 2002May 2002
Paul WeingartnerPaul Weingartner
Inverter typesInverter types
VVIVVI
CSICSI
PWM – current technologyPWM – current technology
AccelerationAcceleration
Linear rampLinear ramp
S-curveS-curve
DeaccelerationDeacceleration
If the drive is ramped down too quickly, the If the drive is ramped down too quickly, the motor becomes a generator and will drive motor becomes a generator and will drive the DC bus voltage upthe DC bus voltage up
This can cause the drive to fault due to a This can cause the drive to fault due to a bus overvoltagebus overvoltage
Skip frequenciesSkip frequencies
Every mechanical system has a resonant Every mechanical system has a resonant frequency(s).frequency(s).
Drive operation at that resonant frequency Drive operation at that resonant frequency can cause problems and potentially can cause problems and potentially damage the systemdamage the system
VFDs can be programmed to skip over VFDs can be programmed to skip over these frequencies (i.e. not dwell on them)these frequencies (i.e. not dwell on them)
Flux vector drivesFlux vector drives
Use encoder feedback to obtain exact Use encoder feedback to obtain exact information on what the motor is doinginformation on what the motor is doing
Adding an encoder and feedback board to Adding an encoder and feedback board to a drive will typically add $1000 in costa drive will typically add $1000 in cost
Vector drive benefitsVector drive benefits
Speed regulation to 0.01%Speed regulation to 0.01%
High torque at zero speedHigh torque at zero speed
Torque linearityTorque linearity
Fast response to load changesFast response to load changes
Sensorless vector drivesSensorless vector drives
Do not have the encoder for external Do not have the encoder for external feedbackfeedback
Feedback is derived from the motor Feedback is derived from the motor terminalsterminals
Better control than a standard VFDBetter control than a standard VFD
Needs to go thru an auto-tune procedureNeeds to go thru an auto-tune procedure
Needs to be given motor characteristics in Needs to be given motor characteristics in order to achieve the superior controlorder to achieve the superior control
Installation issues of VFDsInstallation issues of VFDs
HarmonicsHarmonics
SWR / Cable length issuesSWR / Cable length issues
Bearing currentsBearing currents
Carrier frequencyCarrier frequency
Radio Frequency Interference (RFI)Radio Frequency Interference (RFI)
HarmonicsHarmonics
In the United States, 60 Hertz power is the In the United States, 60 Hertz power is the standardstandard
In Europe, 50 Hz power is usedIn Europe, 50 Hz power is used
TerminologyTerminology
60 Hz is the fundamental frequency60 Hz is the fundamental frequency Also known as the 1Also known as the 1stst harmonic harmonic
The frequency of any harmonic can be The frequency of any harmonic can be calculated by the number of the harmonic calculated by the number of the harmonic multiplied by the fundamental frequencymultiplied by the fundamental frequency Example: The 5Example: The 5thth harmonic would be 5 x 60 harmonic would be 5 x 60
Hz = 300 HzHz = 300 Hz
Which harmonicsWhich harmonics
Only even number harmonics are present Only even number harmonics are present in VFD systemsin VFD systems
Most important harmonicsMost important harmonics 33rdrd
55thth
77thth
Harmonic propertiesHarmonic properties
33rdrd - also known as a “triplen” – Zero - also known as a “triplen” – Zero sequence harmonicsequence harmonic
55thth - Negative sequence harmonic - Negative sequence harmonic
33rdrd Harmonic Harmonic
Causes neutral currents to flowCauses neutral currents to flow
NEC specifies that the neutral should be NEC specifies that the neutral should be either one wire gauge larger than the either one wire gauge larger than the gauge of the phase line or two lines should gauge of the phase line or two lines should be pulled for the neutralbe pulled for the neutral
Can cause overheating in transformers Can cause overheating in transformers and premature failureand premature failure
55thth Harmonic Harmonic
Counter-rotating (i.e. Counter-rotating (i.e. BAD!)BAD!)
Harmonic distortionHarmonic distortion
Measure of the sum of all of the harmonic Measure of the sum of all of the harmonic components compared to the fundamental components compared to the fundamental 60 Hz current60 Hz current
IEEE-519IEEE-519
Total Harmonic Distortion (THD)Total Harmonic Distortion (THD)
Total Demand Distortion (TDD)Total Demand Distortion (TDD)
Typically, a THD of less than 5% is Typically, a THD of less than 5% is considered goodconsidered good
Mitigating harmonic problemsMitigating harmonic problems
Installing line reactors and/or filtersInstalling line reactors and/or filters
Reactor – is another term for inductorReactor – is another term for inductor
Filter – implies RC, RL or LC componentsFilter – implies RC, RL or LC components
Standing Wave Ratio (SWR)Standing Wave Ratio (SWR)
Transmission line effects due to high Transmission line effects due to high frequency pulsesfrequency pulses
Maximum power transfer theoremMaximum power transfer theorem
Creates high voltages at the motor Creates high voltages at the motor terminalsterminals
What can be doneWhat can be done
Minimize the cabling lengthMinimize the cabling length
Use inverter rated motorsUse inverter rated motors
Don’t use a high carrier frequencyDon’t use a high carrier frequency
Use reactorsUse reactors
Bearing currentsBearing currents
Using VFDs to drive motors create Using VFDs to drive motors create common mode voltages that often will common mode voltages that often will reach ground through the motor shaftreach ground through the motor shaft This causes the grease to break downThis causes the grease to break down Worse it can cause arc pitting of the bearings Worse it can cause arc pitting of the bearings
and introduce metal fragmentsand introduce metal fragments
Mitigating bearing current problemsMitigating bearing current problems
Lower the carrier frequencyLower the carrier frequency
Use insultated bearingsUse insultated bearings
Ground the shaft with a slip ringGround the shaft with a slip ring
RFIRFI
Anytime there is distortion in a waveform, Anytime there is distortion in a waveform, harmonics are present.harmonics are present.Since harmonics are multiples of the Since harmonics are multiples of the fundamental frequency and these fundamental frequency and these harmonics exist in meaningful amplitudes harmonics exist in meaningful amplitudes at over 100x the fundamental frequency, at over 100x the fundamental frequency, RF energy is releasedRF energy is releasedThis RF energy can cause problems for This RF energy can cause problems for sensitive equipment and sensorssensitive equipment and sensors
Mitigating RFI problemsMitigating RFI problems
Use shielded cableUse shielded cable
Don’t run signal wires in the same conduit Don’t run signal wires in the same conduit as power wiresas power wires
Use proper grounding methodsUse proper grounding methods Avoid ground loopsAvoid ground loops
VentilationVentilation
Ventilation for the enclosure for a VFD Ventilation for the enclosure for a VFD (and any other equipment) should be (and any other equipment) should be considered due to the heat generatedconsidered due to the heat generated
Cooling fans – also need to be checked for Cooling fans – also need to be checked for proper air flow and if filters are present, proper air flow and if filters are present, need to be cleaned periodicallyneed to be cleaned periodically