Spider DSA User’s Manual
273
single-channel phase, we speak of its phase is relative to the start point of its data
block.)
In rotating machine analysis, the phase of the first order of a rotor can be directly
mapped to an angular difference between a signal and a reference. The reference
signal can be another channel of measurement, or the tachometer signal. The
phase difference between two waveforms is often called a phase shift or phase
delay. Phase shift may be considered positive or negative, i.e., one waveform may
be delayed relative to another one, or may precede it in time. These conditions are
called phase lag and phase lead respectively.
An example of this is the phase of an imbalance component in a rotor with
reference to a fixed point on the rotating rotor, such as a shaft keyway. To
measure this phase, a trigger-pulse must be generated whenever the keyway
passes a fixed point. An optical, magnetic or eddy-current probe may be used to
generate such a “tach” pulse signal.
A zero-degree phase delay at a frequency can be depicted as a series of pulses
overlaid with a sine wave where the pulse edge is exactly located in peak position
of the sine wave.
In the figure above, a section of the tachometer signal is shown on its own and
then overlaid on the vibration signal. The tachometer signal in this example
crosses the vibration signal at the same point on each cycle. If the phase of the
vibration signal were to change (as it would with a slight speed change), then its
position relative to the tachometer pulse would also change. Extracting the first
order magnitude and phase, gives the curves shown below. The phase is now
constant near -60
o
as it should be for such a signal. Because the rotating period of
the signal is about 20 ms, -60
o
corresponds to a 20*60/360=3.3 ms delay.
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