What is a Keyphasor? How Does KeyPhasor Works?

The keyphasor is a transducer installed on the rotating shaft of the equipment. It produces a voltage pulse per revolution of the shaft that is used to detect the problem in the machine.

Importance of Keyphasor

The operator or machinery specialists can identify developing machine problems using keyphasor-generated information. The information distinguishes serious problems, from less serious ones.

More than one-third of the information about the machine’s performance status is generated by the keyphasor signal. This information relies heavily on phase (relative and absolute). Without phase information, the overall machine condition and machine cannot be understood and analyzed.

Why is PHASE information important?

It is possible le detect even slight changes to the machinery using phase information. Otherwise, that may go unnoticed. Information during startup, shutdown, and steady-state data provides critical information management. In the absence of the phase information, a large gap in the usefulness of the above information. Operators can be alerted to phase changes before costly machine damage occurs.

What is Phase?

It can be seen in the below figure, phase is the timing relationship in degrees between two or more signals. Phase information provides us with the direction of vibration.

What is Phase?

What is Phase Angle?

The phase angle is the number of degrees from the keyphasor pulse to the first positive peak of the vibration.

What is Phase Angle?



Keyphasor is a registered trademark of Bentley Nevada, but it has become a popular usage term. Key phasor is a proximity transducer used to observe a once-per-revolution event, usually a notch (keyway) or projection on the shaft.

The system includes a proximity probe, extension cable, and proximitor (proximity sensor).

Principle of Keyphasor

The Keyphasor signal is an electrical pulse. It is generated once per turn provided by a transducer, usually a probe of proximity that is used for the measurement of the speed of rotation of the machine. It provides the phase retardation angle of the vibration. It means that, as a phase zero-point reference to determine where the phase occurs vibration in the turbine rotor, it is known as a phase reference.

The keyphasor signal is essential in the generation of much of the information regarding the state of the machine, including the speed of rotation from the axis.

Because of loss of Keyphasor information severely affects the monitoring and diagnosis of the machine, installing the redundancy keyphasor probes is a recommended practice.

This is especially critical for machines with transducers mounted internally. It is necessary to install a spare transducer, with the extension cable routed external to the machine to the proximity junction box (JB).

The keyphasor signal is used to monitor, diagnose and manage systems. This device generated filtered vibration amplitude, delay angle of phase, velocity, and a wide variety of information, including information vector to balance the rotor. It is also an essential element in the measurement of the slow rotation rotor of the roller or the information of disconnection.

It is recommended that the keyphasor is located as close as possible to the thrust support area to minimize thermal growth effects that may move the transducer’s reference marker. For the notch to be effective, it must be built into the rotor, not into the coupling or component.

In general, mounting the keyphasor axially is not recommended since it may interfere with the thrust-bearing position accelerometer.

Using the instruments of diagnostics and software, reference sign locations must be properly documented for use and configuration.

Examples of PHASE ANGLE applications

  • Shaft crack detection
  • Rub detection
  • Shaft balancing
  • Direction of vibration
  • Shaft/structural resonance detection
  • Shaft mode shape

Shaft crack detection

In rotating machinery, shaft cracking is a common and dangerous problem. Cracks can lead to shaft fractures, which are disastrous for rotating machinery and cause significant economic losses. It is very important and necessary to early detection and diagnoses the shaft crack of rotating machinery timely and accurate, avoiding severe damage and expensive repairs.

Rub detection

Absolute and relative phases are useful for detecting rub conditions. Changes in steady state phase, reverse components in orbits, and abnormal rates of change of phase during transient conditions can be indicators of rub condition.

Shaft balancing

For proper rotor balancing, absolute phase angle information is essential. Phase information is used to compensate for non-dynamic influences (slow rotor vector) and to establish influence vectors for individual machines. Once slow roll, unbalance response, and influence vectors are established, it is possible for the machinery specialists to balance a rotor in a minimal number of runs.

Shaft mode shape

By comparing the absolute or relative phase between different measurement planes, the rotor’s inboard end of the machine to that of a probe signal with the same angular orientation at the outboard end of the machine.

Notch or a Projection

What is Phase Angle?

There are two methods for obtaining the electrical pulse, possibly using a notch or a projection. It will always be more advisable to make a notch rather than a projection. A notch is easier to set up and less likely to damage the probe if there is any disturbance to the turbine.

keyphasor diagram

When the marker is a notch, the probe gap is adjusted while the smooth surface of the shaft is visible, not the notch. However, when the marker is a projection, the probe gas is adjusted while looking at the top of the projection.

A notch comprises the following typical measurements.

  • Length: 15 mm (0.6 inches)
  • Width: 100mm
  • Depth: Between 1.5 and 2.5 mm (0.6 to 0.1 inches).
  • Radius: 5mm
  • Transducer: 8 mm proximity probe.
  • Configuration: Gap at 1.3 mm from the rotor.

Some characteristics to consider when making the notch are that all edges must be perfectly rounded to avoid the formation of cracks in the turbine shaft.

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