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How to use a phasor diagram?

03-16-2020 | Power quality

When you perform an energy or power quality survey you need to measure days or weeks of data, imagine the disappointment if when you return to the logger after that time you discover that you have failed to capture the correct measurements. The support teams at Fluke all too often get calls from users who don’t understand the measurements they have logged, usually they think the instrument is at fault. Usually though the problem is due to the user making a connection error, this can be due to incorrect voltage connection or revered current connections. The problem can be easily be prevented by checking and double checking those connections before starting the logging session.

Phasor diagram display

The most powerful tool on Fluke’s three phase analyzers to make this check is the phasor diagram display. In one screen, you can quickly see whether you have connected up the voltage and current in the right sequence and that the current sensors are correctly measuring the direction of the current entering the load or in the case of a generator measuring the current provided by the generator.

Once you understand the basics of the phasor diagram you can use it to ensure every time you log you capture the data you need.

The Basics


 
  • Voltage phasors represented with the thick lines with arrowheads with typically 120˚ separation, this varies slightly depending on the amount of unbalance in the system.
  • Current uses the thinner line terminated in a triangle in the same way with around 120 ˚ of separation. The angle between the voltage and current phasor is indicative of the power factor of the circuit, if the angle is small power factor will be good and close to 1. If the power factor is poor, say less than 0.9 the angle will be greater.
  • The length of phasor is an indication of the magnitude of the voltage and current (for convenience the readings of voltage and current are displayed in Volts and Amps). Checking those readings is important to ensure the correct ranges are chosen before logging.
  • The angle between the voltage and current does vary but if it’s greater than 90˚ then that’s due to an incorrect connection.
  • It’s important to note is the rotation of the voltage vectors too, phase L1 should be pointing horizontally to the right, then phase L2 should be next in counter clockwise direction and finally phase L3 should be last. In this example the correct direction is indicated by L1L2L3 at the top of the display.

In this example we see an error:


 
  • Here the current phasor is pointing almost 180˚ away from the voltage. This is because the current sensor (flexible probe or clamp) is pointing in the wrong direction. On the current sensor you will find an arrow, these should all point towards the load. If one or more is pointing in the wrong direction you will see an angle greater than 90˚

Checking that all your phases are correctly sequenced and current sensors are correctly oriented every time you connect to perform a survey will save the frustration of returning to your instrument only to discover you have wasted days or weeks of logging time.

Some of Fluke newer products indicate when there is a connection error and allow automatic correction without having to move any of the connection.