# Phase angle for 3 phase power



## GinaB (Feb 4, 2014)

I am looking for a good explanation of the phase angles in a 3 phase system. Any recommendations.


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## knight1fox3 (Feb 4, 2014)

When we are dealing with a single-phase mains supply and purely resistive (or nearly so) loads, normal maths (V = I.R, P = V² / R, etc) is adequate. When reactive components (inductors, capacitors) are concerned and we need a graphical representation that will help us to visualise the relationships between voltages and currents in different parts of the circuit, we turn to a device called a ‘phasor’. A phasor is quite simply a line that has a length and a direction, and it rotates. The length represents the magnitude of the voltage or current, and the angle represents its relationship to some reference (that we can choose to suit our circumstances. We can illustrate the relationship between the three voltages of a 3-phase with three phasors 120° apart. If we connected a 3-channel oscilloscope to the supply we might see something like this:






http://openenergymonitor.org/emon/sites/default/files/phasors.svg

The phasors rotate at the supply frequency. The three phasors are 120° apart, and the three voltage waveforms too are 120° apart — 1 complete cycle being 360°.


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