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