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The Concept of Phase


Phase is a measure of relative time difference between two sine waves. Even though phase is truly a time difference, it is almost always measured in terms of angle, either degrees or radians. This represents normalization to the time taken by one cycle of the wave in question, without regard to its true time period.

The phase difference between two waveforms is often called a phase shift. A phase shift of 360 degrees is a time delay of one cycle, or one period of the wave, which actually amounts to no phase shift at all. A phase shift of 90 degrees is a shift of 1/4 of the period of the wave, etc. Phase shift may be considered positive or negative, i.e., one waveform may be delayed relative to another one, or one waveform may be advanced relative to another one. These conditions are called phase lag and phase lead respectively.


In this example, the lower curve is shifted 90 degrees with respect to the upper curve. This is a time lag of one-fourth of the period of the wave. You could also say the upper waveform has a 90 degree phase lead.

Phase can also be measured with reference to a particular time. An example of this is the phase of an imbalance component in a rotor with reference to a fixed point on the rotor, such as a key way. To measure this phase, a triggerpulse must be generated from a certain reference point on the shaft. This trigger can be generated by a tachometer or some type of optical or magnetic probe that senses a discontinuity on the rotor, and is sometimes called a "tach" pulse.


Phase of a Rotor


The phase angle can be measured from the reference position either in the direction of rotation or opposite to the direction of rotation, i.e., phase lag or lead, and different equipment manufacturers use different conventions. In the DLI Balance program software for the DC-7, either direction may be selected at the operator's preference.

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