The Keyphasor

 

The Keyphasor transducer is usually a proximity probe mounted observing a keyway or projection which provides a large change in gap in front of the proximity probe (Figure 2). Since the proximity probe a gapthe to voltage keyway passes the a voltage pulse results.isSince keywaytransducer, passes the when probethe once-per-revolution of probe the shaft, the Keyphasor pulse occurs at a frequency equal to the speed of the machine. Therefore, its frequency is the same as the running speed of the machine.

  WAVEFORMS AND ORBITS

To properly understand the full operation of a Keyphasor, it is first necessary to understand the waveform or orbital pattern on an oscilloscope (Figure 3). When vibration probe inputs are put into a dual channel oscilloscope and observed on the cathode ray tube, the dynamic motion (vibration) may be displayed in time base or orbital form. A waveform or orbit pattern on an oscilloscope is simply a dot moving very rapidly such that to the eye it appears as a continuous line. The rapidly Copyright © 2016 General Electric Company. All rights reserved

 

moving dot represents the centerline motion of the shaft. The orbit is a path of the High Spot at that lateral position of the probes. The Keyphasor pulse, when fed to the Z intensity input of the oscilloscope, simply intensifies the dot at the instant in time when the keyway (event) is in front of  the Keyphasor transducer. Therefore, the Keyphasor dot on the orbit or waveform represents the centerline location of the shaft in its path of travel (or High Spot) at the instant that the keyway is in front of the Keyphasor transducer. By utilizing this Keyphasor technique we get a physical reference to the shaft. The rotor can be stopped with the keyway in front of the Keyphasor  transducer and the exact angular location of a High Spot of the shaft that was observed at that particular instant in time can be determined. In addition to Z intensity axis input the Keyphasor  should also be used to trigger the oscilloscope when in the time base mode.

  PHASE ANGLE MEASUREMENTS FOR BALANCING

The Keyphasor (shaft reference) technique tells where the rotor High Spot is at a particular instant in time when used with a phase measuring instrument (Figure 4). If we define the phase angle as the number of degrees from the Keyphasor pulse to the following positive peak of dynamic motion (vibration) as sensed by a vibration probe, the use of the shaft reference technique allows the direct phase angle readout to define the High Spot of the rotor system. The logic of this method of phase measurement is as follows: the Keyphasor pulse occurs the instant the Keyphasor transducer and the shaft reference (notch or  projection) are aligned. The positive vibration peak occurs "0" number of degrees (phase angle) later in time as the shaft High Spot passes beneath the vibration input probe. The machine rotor may then be stopped and rotated to align the Keyphasor transducer and its shaft reference. If  the rotor is then rotated in the direction of rotor rotation the number of degrees as defined by the phase angle, the High Spot of the rotor will lie directly under the vibration probe.

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  By utilizinginstrument the ability which of the Keyphasor toHigh define the of High a rotor, it is to provide a balancing can read the Spot the Spot rotor on directly with a possible physical shaft reference. This enables the addition or subtraction of weight at the proper angular location on the rotor to improve rotor balance and helps to minimize the number of machine runs necessary to balance or trim balance a machine. By utilizing it with oscilloscope orbit presentations, it is possible to trim balance machinery in the field as well as on the balance stand. In a classical rotor dynamics situation, below the first balance resonance (critical) the phase angle as referenced by the Keyphasor provides an accurate indication of the High Spot on the rotor when the keyway is aligned with the Keyphasor probe. Well below the first balance resonance this High Spot indicates the angular location of the residual unbalance on the rotor (Heavy Spot). Assuming no thermal or permanent bows of the rotor, the trial weight could be added 1 80 ° from this angular  location and a finer degree of balance would be obtained. As the rotor increases in speed through its balance resonance region an amplitude increase occurs with a corresponding 180 ° phase shift (lag) of the High Spot from the Heavy Spot. Well above the first balance resonance the amplitude has decreased and the full 1 80 ° phase shift has occurred. Since the 1 80 ° phase shift affects the relationship of the High Spot on the rotor with respect to the unbalance, the balancing rules of the Keyphasor change accordingly. Above the first balance resonance the phase angle, as referenced by the Keyphasor, defines the location of the High Spot on the rotor where weight must be added to obtain a finer balance (Figure 4). The phase angle as defined utilizing the shaft reference (Keyphasor) technique always defines the High Spot on the rotor. With a fundamental knowledge of rotor dynamics it is possible to more easily and more directly balance rotors without trial and error techniques. In a working machine an exact 1 80 ° phase shift may or may not be seen on Bode' plots. Every Copyright © 2016 General Electric Company. All rights reserved

 

balance resonance exhibits 1 80 ° phase shift, assuming some degree of unbalance is present. The actual observed phase shift is influenced by the presence of all residual bows from lower  rpm responses of the rotor. A total amount of uncorrected phase angle change passing through a balance resonance can vary from 0 ° to 360 o depending upon the magnitude and angular location of bow with respect to the residual unbalances in the rotor. (NOTE: Proximity probes allow direct observation of rotor bow because they can observe slow-roll.) The use of an instrument such as the DVF 2 allows for compensation of phase angle readings to eliminate any residual bows from balancing data. If the fundamental rotor dynamics of the particular machine are known then it is possible to do a one-run trim balance of the rotor utilizing the Keyphasor technique. In fact, standard procedure on some high speed air cycle machines (for ground support maintenance facilities of many airline companies) is to trim balance with one machine run. This discussion has perhaps been an over simplification of phase angle measurements tor balancing. It is not meant to be construed as a new method tor balancing but simply an easier and more meaningful method of referencing the phase angle of the vibration on rotating machinery. POLAR PLOT

The Keyphasor provides a vital signal for techniques used to plot rotor unbalance response. Two widely used techniques today are the Polar plot and the Bode' plot, which require primary measurements of amplitude, rpm, and phase angle. The Keyphasor provides the rpm signal and a reference pulse for phase angle measurements when a Vector Filter Phase Meter with 1 X rpm filter is used. The Polar plot is a plot of the phasor (rotating vector) of the amplitude of rotative speed (or forcing function speed) and phase angle as a function of speed. On rotating machinery, the Polar plot is the amplitude and phase reading from a transducer showing the response of the machine to its residual or deliberate unbalance as a function of speed. Mass unbalance manifests itself as a phasor. The magnitude (p-p mils) and the direction (phase angle) of the resultant motion with respect to a known reference point (Keyphasor) can be described on a Polar plot, which displays the in-phase and quadrature components of the phasor. These component values will change as the magnitude and direction of the phasor quantity changes with speed. A classic Polar plot of a true balanced rotor will look like this:

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Polar plots may be plotted directly,from the DVF 2 with the use of an X-Y plotter. BODE' PLOT

The Keyphasor pulse also allows a plot of rotative speed amplitude ( 1 X) of a given measurement against rpm, along with the phase lag angle of that amplitude phasor against rpm. The Bode' plot provides this relationship and is most useful in showing the rpm of various resonances of  a machine. The rotative speed absolute amplitude must be nulled with rotor speed below each resonant rpm. Failure to do so always yields a "Brody" (false Bode').

  The DVF 2 offers the capability of zero nulling for display of proper Bode' plots. Bode' plots may be plotted directly from the DVF 2 with the use of an X-Y-Y plotter. GLITCH FILTERED ORBITS

In attempting to utilize the Keyphasor and an oscilloscope for balancing on machines where a high mechanical or electrical runout (glitch) level is present, it is sometimes necessary to filter the high frequency glitch from the waveform before making the phase angle measurement (Figure 5). In these instances it is extremely important that any phase angle shift of the electronic filters being utilized be taken into account. If two matched tunable filters are utilized with both of them being exactly tuned to 1 X rpm, zero phase angle shift from the instrument can result. In this case, the 1 X rpm running speed frequency of a machine vibration is obtained from the filters and input into an oscilloscope in conjunction with a Keyphasor pulse input to the Z intensity axis of the oscilloscope. This provides 1 X rpm filtered orbits with a phase angle reference. Dual low pass filtering is a successful means of observing orbital patterns with relatively high glitch levels. However, caution must be used since phase angle can be shifted by low pass filtering depending upon the filter cutoff frequencies. Utilizing "autotuning" filters like the Digital Vector Filter Phase Meter eliminates the problem of instrument phase distortion. The Digital Runout Compensator can also be used to minimize problems associated with glitch. The DVF 2 also has the ability to null out glitch vectors in addition to any bow vectors which may be present. Copyright © 2016 General Electric Company. All rights reserved

 

  NONSYNCHRONOUS NONSYNCHRON OUS VIBRATION

Besides the Keyphasor' s ability to define phase angle and help define natural rotor response, the Keyphasor lends itself easily to determining whether a vibration motion is ( 1 ) at rotative speed or  an exact ratio thereof, or (2) is at a rate not related to rotative speed. If a motion that is an exact ratio of rotative speed, such as %, 1, 2, etc., is present the Keyphasor dot will appear to be locked onto the waveform of the vibration as displayed on an oscilloscope. If a nonsynchronous waveform is present, the Keyphasor will appear to move over the waveform of the vibration. In many rotor  rub situations the Keyphasor will be unstable or jittery. This ability of the Keyphasor can be very important in observation of whirl phenomena occurring below 50% X rpm. OPTICAL PICKUP PROVIDES TEMPORARY KEYPHASOR

On machine trains where there is no permanently installed Keyphasor proximity probe, a Keyphasor pulse may be obtained by the use of an optical pickup (Figure 6). Highly reflective tape is available which, when placed on the shaft, can provide an "event" for an optical pickup to observe. If highly reflective tape is not available, a paint mark on the shaft can serve this same purpose. With the optical pickup, any place where visible shaft is available a Keyphasor pulse can be observed. Stated another way, any place where a strobe light can observe a shaft, an optical pickup can be used. Copyright © 2016 General Electric Company. All rights reserved

 

 

PROBE INSTALLATION

The Keyphasor proximity probe should be installed observing a notch or projection of sufficient size to create a large voltage pulse Figure 2). It is not desirable to simply put a scratch mark on the shaft and expect the proximity probe to provide a large pulse. This pulse should be of  sufficient magnitude (nominally greater than 5 volts peak-to-peak) to trigger the Z intensity axis of  an oscilloscope and to trigger the time base sweep of the oscilloscope, as well as to trigger a digital tachometer easily without fear of triggering the tachometer on noise. When the Keyphasor pulse is input to the Z intensity axis on an oscilloscope, it provides a reference spot on the dynamic waveform and/or orbital pattern. This appears as a bright spot and a blank. By determining which of these occurs first in time, in the time base mode of the oscilloscope, it is possible to determine the direction of the dynamic motion of the shaft on the orbital pattern. Also since the Keyphasor occurs at a frequency of once-per-revolution, we can determine basic frequencies present in a vibration waveform on an oscilloscope by simply counting the number of cycles of vibration per Keyphasor pulse. In complex waveforms, the basic frequencies can be determined by counting the number of peaks that occur between Keyphasor  pulses.  Assuming  Assumin g there is no gear in a particular particular machine train, it isnecessar isnecessary y to have only one Keyphasor  Keyphasor  Copyright © 2016 General Electric Company. All rights reserved

 

probe mounted on the driver of the machine train. Machine trains that have different speeds (gear  increaser or reducer) must have more than one Keyphasor in order to meet the qualification that the Keyphasor pulse occurs at a frequency equal to once per revolution or 1 X rpm of the running speed of the machine. It is desirable to have the Keyphasor probe installed on the driver of the machine train since it is not uncommon to run the driver uncoupled from the load (for example, steam turbine drive). If the Keyphasor probe was located on the driven equipment, then the uncoupled run would be without the Keyphasor reference pulse. SYSTEM INSTALLATION

On machines with permanent monitoring systems it is desirable to install a permanent proximity probe with its corresponding Proximitor and wiring back to the main panel for the Keyphasor (Figure 7). Most monitoring systems provide for Keyphasor input. With this installation, all vibration data can be recorded and diagnosed at the vibration monitoring system, eliminating the necessity to take measurements at or  on the machine train itself. A permanently installed Keyphasor is recommended for every major  machine train in a plant.

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