High Frequency Analysis Rev2

8/2/2019 High Frequency Analysis Rev2

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High Frequency Analysis with Advanced Technologies

Dave Druiff

Emerson Process Management

Asset Optimization

Presentation for:

Vibration Institute - Piedmont Chapter #14

May 2008


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Why they are different.


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High frequency wave action is generallyconsidered to the condition reached when

the dimensions of the wave and theobjects it encounters have similardimensions.


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The condition can be reached in virtuallyall physical wave applications. Electronic,

light, sound, pressure. even traffic andhelicopters can exhibit strange wavebehavior.


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The condition arises when thetransmission medium has discontinuities

and the overall dimensions are such thatwaves upon reaching the discontinuity arereflected back and forth. If thetransmission losses are low, the moving

energy will create standing waves


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These standing waves can be extremelyuseful or very damaging depending on the

situation. A laser or a piano string is asituation which uses the properties ofstanding waves for good. A machinestructural resonance is an example of a

standing wave that is often verydestructive.


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So when is a standing wave likely to becreated. First there must be a discontinuity

in the transmission medium. An emptyroom is a good example of discontinuities.The room is full of air as a soundtransmission medium and the rigid flat

walls create a near ideal discontinuity.


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Second there must be a source of soundenergy at a constant frequency. If a

speaker is positioned at one end of theroom and supplied with a source ofsinusoidal energy it will launch a soundwave (longitudinally) toward the other end

of the room. Assume for the moment theonly discontinuity of interest is the one atthe far end of the room.


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In air sound travels about 1100 feet per secondand sea level. We can calculate the wavelengthof any given frequency by using the equation

wavelength = 1100 / f in Hz. For example, if theroom is 10 feet long and the frequency is 20Hertz the wavelength is about 1100 fps /20 Hz or55 feet. If the waveform is very long compared to

the room length the pressure at any point in theroom will pretty much uniformly rise and lower.


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A person standing anywhere in the roomwill hear the constant tone.

If the frequency is raised to 1100 Hz. Thewavelength will now be about 1 foot. Thesituation will be similar when the individualis near the speaker, however as the

listener moves toward the far end of theroom the conditions will change.


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When one of his ears is about 6 inchesfrom the wall he will not be able to hear

the tone. This is because the incomingtone from the speaker and the reflectionfrom the wall (the discontinuity) are verynearly the same amplitude and are exactly

180 degrees out of phase.


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The reflection cancels the incoming directsound and the ear has nothing to hear.

Notice this condition will occur only is thetone is constant in frequency. It is howevernot necessary for the tone to be anyspecific frequency. The frequency must

only be high enough for multiple reflected(Standing) waves to be created with in theroom.


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The exact position of the null will move about butit will always exist if the discontinuity is highlyreflective and the dimensions are suitable. When

the dimensions The point of the above laboriousdiscussion is this.

If the wave energy being evaluated and thedimensions of the object transmitting the wave

energy become similar the possibility ofdeveloping null points in the transmission path isvery likely.


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When we make high frequency vibration orsound measurements, (As in Peakvue) we

have a very good possibility of missinginformation if the care is not taken to scanthe unit surface for energy peaks.


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Demonstration of acoustic standingwaves. (500 - 1000 Hertz)

Plug one ear with a finger

Move near to a clear flat wall. (2 to 4 footrange). Move unplugged ear about untilyou notice even though the sound level is

not being changed as it is being generatedyou can still find high and low levels of thesound.


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Repeat with 50 Hertz sound.

Standing waves are no longer created. The

room is to small.


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Why It works


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All most all metals have a similar fatiguefailure curve. Cyclic stress is plotted

vertically and number of cycles to failure isplotted on the horizontally. Most metals donot ever reach a stable condition such thateventual failure is not guaranteed. This

true no matter how much the cyclic stressis reduced.


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The cyclic stress eventually causes afailure of the metal. This failure will occuraround some microscopic defect in the

metals crystalline structure. The actualfailure may occur on the surface of themetal (Visible) or it may below the surface(Invisible) The crack will grow and

eventually a piece of the bearing elementwill separate form its parent metal (Aspall).


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When the defect first occurs themicroscopic rubbing of the surfaces willcreate high frequency vibrations known asstress waves. The effect is similar to thenoise produced when a very cold ice cubeis dropped into a warm drink. The ice

makes noises generated by the thermalstress.


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These waves although very weak have theadvantage of being very high in frequency.

This means the inherent machine vibration ---- 1X,

2X, looseness, vane pass, etc. is well below thestress wave frequencies. It is therefore possibleto utilize a high pass filter to remove all of theinherent vibration allowing the entire dynamicrange of the analyzer to used to process thestress waves.


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The stress waves are created by thebearing elements and therefore they havethe same bearing element repetition rateas the directly created bearing faultfrequencies. They are however notcoherent with each other which means

special processing is required to extractuseful results.


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Two separate operations are performed.

Since the duration is very short the

analyzer is programmed to sample as fasta possible, ensuring there as manyindividual wave peaks are captured as ispossible. All of these values are then

sorted to find the biggest peak present ineach desired time block


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Since they are not coherent there is noadvantage to maintaining the positive andnegative peaks as such. The second step is to

full wave rectify the chosen peaks to make themunipolar. The final step is to store the values anthough they were waveform samples andperform the FFT as normal. Since the stress

waves are created by the bearing element faultenergy the they will possess the timing of theirorigion and fault frequencies will be present.


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Peak Vue processing can identify defectsthat while real are below the bearingsurface and cannot be seen by any form ofmicroscopic inspection. It is probably bestto use Peak Vue as an early warning tooland wait until normal bearing frequencies

appear before scheduling a maintenanceaction.


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Where do the troubles start?


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A VFD is an AC to DC to AC converter

Step 1-----3 phase AC is rectified to make it unidirectional, filtered, and turned into the

equivalent of battery (DC) power. This step iscommon in many, many house hold andindustrial products. There are a few problemsthat are created in the form of distorted currentwaveforms. Such distortion is of littleconsequence and can be minimized with aslightly more expensive power supply design.


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The DC power (plus volts and minus volts)must now be changed back to 3 phase ACvoltages at the frequency desired. Ideally,

the new AC would be as good a sine waveas that created by turbo generators. Thiscan be done. The concept is no different tothat used in stereo power amplifiers. The

difficulty is the conversion process will notbe efficient. (About 50 percent of theenergy will be lost as heat.)


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If instead of linearly reshaping the DC intothe AC sin wave, It is possible to useswitching devices to create an wavepattern having the energy proportions andtiming of a sin wave but actually havingonly two voltage levels (on and off). This

process is much more energy efficient. Intheory, 100 percent.


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The problem is the switching must be veryfast compared to the frequency of thepower desired (30 90 Hertz). Typicalswitching frequencies range from 2 Khz to40 Khz. The bulk of the energy is shapedto a sine wave that the motor can use to

create the necessary rotating magneticfield needed to produce useful power.


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The problems begin with the remainder ofthe energy which the motor subjected to.This energy has very high frequencyspectral lines. High frequency componentssee capacitance as a short cut back home.

A 60 Hz AC motor is designed to see only

60 Hz energy plus maybe a small amountof harmonic distortion.


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Volt for Volt, a 6 Khz signal will produce100 times of the capacitive current a 60 Hzsignal will produce (Same capacitor).

The motor was not designed toaccommodate such currents. In general,the currents are intent upon getting theground. The best path to ground may wellbe through the bearings, hence flutingproblems.


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Not all insulation systems can effectivelyaccommodate high frequency energy. Theinsulation itself may absorb the energycreating local heating.

The iron laminations will likely have higherlosses at the higher frequencies resulting

in more local heating.


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Speeding or slowing a motor can createcooling problems.

A motor running at a higher speed usuallyhas to produce more horsepower to carrythe load.

Physical noise may be a problem for

nearby humans or other sensitiveprocesses.


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VFD switching energy can be filtered out.

It only takes a large low pass filter on each

phase. Large inductors and capacitors areexpensive.

These filters must be designed for thetask or they will themselves fail for many

of the same reasons the motors do.


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Ceramic Bearings will eliminate electricaldamage

Use of external forced cooling air will helpremove excess heat and prevents areduction of airflow when the motor is runslower than normal.

Remember a 10 HP 3600 RPM motor canonly yield 5 HP at 1800 RPM. (Usually noteven that much)


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Precision balancing and alignment goeshand in hand with any speed increases.

A VFD is the finest way ever created tolocate machine resonances.


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Do not use a VFD with an old motor.

Buy a new motor designed for VFD use.

Preferably buy the VFD and the motorfrom the same supplier.

Are VFD s going to go away. No!!!

They will be around for a long time and thepain will subside as experience spreads.

High Frequency Analysis Rev2

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