127360 Phase Measurements

How to Make Phase Measurements

Phase Measurements I.

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Phase Angle Measurements and Phase Reference Review Phase angle is the timing relationship, measured in degrees, between two events. A. Relative Phase is the timing relationship measured in degrees, from a point on one signal to the nearest corresponding point on another signal. 0°

ONE CYCLE

360°

Signal A (Y)

RELATIVE PHASE

Signal B (X) ONE CYCLE 0°

360°

TIME

There are five rules used to measure relative phase: 1. two vibration signals 2. same frequency 3. same units (mils, in/s, or g’s) 4. either signal may be the reference signal 5. relative phase is measured between 0 and 180 degrees leading or lagging.

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Machinery Diagnostics

By knowing the relative phase relationship between two orthogonal (xy) transducers and their location on the machine, one can determine the direction of precession.

360°

0°

ONE CYCLE Signal A (Y)

RELATIVE PHASE

Signal B (X) ONE CYCLE 0°

360°

TIME

Q. On the figure above, what is the relative phase between the two vibration signals? A. Q. Is the direction of precession “X to Y” or “Y to X” ? (Y is 90° CCW of X) A.

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B. Longitudinally: We can look at the response along the length of the shaft. This can give us an idea of the deflection shape of the shaft and possibly the location of a malfunction source (i.e. instability).

1

2

3

4

1

2

3

4

“A”

“B”

Q: What is the phase relationship between probes 2 and 3 for example “A”? A: Q: What is the phase relationship between probes 2 and 3 for example “B”? A:

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Machinery Diagnostics

C. Absolute Phase is the number of degrees of vibration cycle from when the Keyphasor® fires (once-per-turn reference pulse) to the first positive peak in the vibration signal. 1. The rules for measuring absolute phase are: a. two signals (vibration signal filtered to single frequency and reference signal) b. filtered vibration frequency must be an integer multiple of reference signal c. absolute phase is measured from the reference signal and is therefore always a lag angle, measured from 0 to 360 degrees. d. the 0° location is defined as the point on the shaft under the reference vibration transducer when the reference signal occurs. Explanation of Reference Signal 0° -V

One Probe Revolution

0°

-V

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One Revolution

The reference signal can come from a variety of instruments: 1. Keyphasor probe (projection or notch) 2. optical pickup (reflective tape) 3. strobe light (match mark on shaft and casing) (Beware of aliasing at multiples or submultiples of running speed) 4. magnetic pickup (changes in emf) Important: The reference pulse must be a onceper-turn signal.

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Phase Measurements

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2. 1X absolute phase measurement

Vibration Signal

Phase Lag

Time

Keyphasor® Signal Degrees of Rotation

0°

360°

In this example the vibration frequency is 1X. This is evident because one vibration cycle matches one rotation of the shaft (Keyphasor pulse to the next Keyphasor pulse). The phase angle, measured in degrees of vibration cycle, is measured from the leading edge of the blank spot to the first positive peak of vibration. The measurement is from the reference signal to the positive peak of vibration. Absolute phase, therefore, is always a lag angle. Q. What is the absolute phase of the vibration signal shown? A.

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Machinery Diagnostics

3. nX absolute phase measurement One Shaft Revolution One Vibration

0°

360°

Cycle

Time

Absolute phase measurement for other vibration frequencies is done in the same manner. In this example the vibration frequency is 2X. This is known because two complete vibration cycles occur in one rotation of the shaft, Keyphasor pulse to Keyphasor pulse. The absolute phase angle is still measured in degrees of vibration cycle from the leading edge of the blank spot to the first positive peak of vibration. The divisions on the oscilloscope can be used to accurately measure the vibration cycle degrees. Q. What is the absolute phase of the vibration signal shown? A.

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D. Additional Uses of Phase Angle Measurements 1. Phase angle is a vibration measurement that can be used extensively in machinery diagnostics. Listed below are a few of the many phase angle applications. a. shaft balancing b. shaft crack detection c. shaft/structural resonance detection d. shaft mode shape e. direction of precession f. location of fluid induced instability source 2. Laterally (or radially): When we look at transducers in a single measurement plane on the shaft, using phase we can measure: a. direction of precession (using XY vibration transducers and relative phase) b. direction of the response (using a vibration transducer and a reference signal)

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