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Understanding Phasor Diagrams

phasors-graphic

Introduction

Phasor diagrams can be very helpful in assessing and diagnosing a power or power quality situation if you know what you are looking at and what to look for. This blog covers the basics of understanding phasor diagrams and what it could possibly mean for the circuit(s) you are monitoring.

 

Viewing Phasor Diagrams with PowerSight

For most PowerSight users, the only way to view a phasor diagram is in PSM-A. When viewing a waveform, click the Phasors button in the top left corner of the window. You can alternate to a text view to summarize the values you see on the graph.

 

For PS5000 owners, you can view live phasors right on the analyzer’s screen by pressing the Phasors key on the keypad.

 

Explaining a Phasor Diagram

Phasor diagrams present each voltage and current as a vector on a graph. A vector combines two measurement properties into one object. In this case, the properties are magnitude (of voltage or current) and phase lag.

Normally, phase 1 voltage is considered to be the phase reference signal, so its angle is 0 degrees. If the phase 1 current lags, it will be slightly above it. In a three-phase circuit, normally the other two phases will be 120 degrees before and after phase 1 and the phase lag of each current relative to its associated voltage will be similar. The data graphs on the left of the phasor display show the actual degrees of all voltages and currents in relation to the phase 1 voltage and show the phase angle between the voltage and current of each phase.

Phasors-Black-Background

 

What Information You Can Pull from Phasor Diagrams

PSM-A allows you to identify a few power variables in the phasors window.

  • Unbalance: The unbalance measurements appear to the left of the phasor diagram. PSM-A performs unbalance calculations on both the voltage and current of a waveform set.
  • Phase Lag: The phase lag is listed to the right of the phasor diagram as the difference between the voltage and current angles. The diagram shows a visual representation of phase lag for a quick and clear idea of which phase(s) have significant phase lag.
  • Power Factor: The power factor measurements appear to the right of the diagram. This can be affected by phase lag or harmonics, so if the power factor is low without significant phase lag, it is probably due to harmonics.

Conclusion

In an ideal 3-phase system, V1/I1 will have an angle of 0°, V2/I2 will have an angle of 120°, and V3/I3 will have an angle of 240°. This is rarely the case, however. Phasors quickly show you how unbalanced the voltage and current is, as well as how much the current lags or leads the voltage.

You can view live and logged phasor diagrams on PSM-A (remotely via Bluetooth for live phasors) or on the color graphic screen of the PS5000. From here, you can begin to diagnose the power you are monitoring and plan for appropriate solutions to excessive phase lag/lead or unbalance.