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I have a confusion interpreting the lagging leading concept when it comes to time plots.

I think in phasors I can see which one is leading or lagging. For example in the below phasor diagram:

enter image description here

How I interpret the above diagram is that A is leading B; and B is leading C. And I think it is because we imagine the phasors are rotating counter clockwise and the reference point is the green line (positive x axis) the first phasor we encounter is A then B and then C.

But what should be the practical way of thinking if we are only given time plots as shown?:

enter image description here

Above in the first plot there are three voltages with 120 degree phase shifts. And in the second plane voltage and current plots are given for a single line.

For these plot what is leading what? What should be the way of thinking here? Do we have to see the plots from the time zero or?

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  • \$\begingroup\$ If you compare cos(x) with sin(x), would you say cos is leading, or lagging? \$\endgroup\$ – a concerned citizen May 8 '18 at 18:01
  • \$\begingroup\$ It can be both?? 30 lagging means 330 leading; isn't it confusing? Depends where we take the reference? \$\endgroup\$ – panic attack May 8 '18 at 18:03
  • \$\begingroup\$ I see, then say you're walking in fron of me. Would you say I am lagging, or leading? For reference, considering me being ahead of you with the circumference of the Earth, minus the distance, is not the right answer. :-) \$\endgroup\$ – a concerned citizen May 8 '18 at 18:08
  • \$\begingroup\$ Phase angle is a relative measure, not absolute. You nominate the reference (\$0^o\$) phasor, and choose the one that makes the subsequent analysis easiest. \$\endgroup\$ – Chu May 8 '18 at 22:38
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And I think it is because we imagine the phasors are rotating counter clockwise and the reference point is the green line (positive x axis) ...

That is fine.

... the first phasor we encounter is A then B and then C.

No. If we are rotating counter-clockwise then A will be followed by C and then B. (Imagine A has just crossed the green line. Which one is next?)

For these plot what is leading what? What should be the way of thinking here? Do we have to see the plots from the time zero or? (sic)

A simple way to do this is to decide which is going to be the reference signal. Usually we choose the voltage.

enter image description here

Figure 1. Leading or lagging? Voltage is blue. Current is yellow.

Next find the zero crossing point for the voltage. This is point (1) on the graph.

Then find the zero-cross for the current. This is point (2) on the graph.

Since these are within 180° of each other reading from left to right it makes sense to say that the voltage is leading the current. The lead is about 3 ms on a 20 ms cycle so that gives \$ \frac {3}{20}360 = 54° \$.

CIVIL

As an extra help, if you remember "C-I-V-I-L": in a Capacitor I leads V but V leads I in an inductor. From this we can conclude that these voltage and current measurements were taken on an inductive load.


From the comments:

Ok so we have to find zero crossing points for current and voltages where these points are less than 180 degrees apart from each other; then the point on the left side is the leading quantity.

In power circuits the leading or lagging is 90° max so that's a good way to approach it. If you got a lag of 300° it would be much more sensible to convert this into a lead of 60°.

That's cool we dont need the initial point for this; any steady state part of the plots work to interpret.

Zero is an easy point to use. You could use the peak or trough also but the curve is flat (zero slope) at those points whereas it crosses the 0 line at a steep angle making it much more convenient to measure.

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  • \$\begingroup\$ CIVIL, woaw. I think I'll finally learn this thing. It is similar to the Swedish abbreviation "SIV" which stands for "syra i vatten" which translates to "Acid in Water", which means you should pour strong acids into water, not water into strong acids. The more you know \$\endgroup\$ – Harry Svensson May 8 '18 at 18:18
  • \$\begingroup\$ I think taking current as the reference more common in phasor representation: electronics-tutorials.ws/accircuits/acp155.gif \$\endgroup\$ – panic attack May 8 '18 at 18:24
  • \$\begingroup\$ And how about the first plot? Is it meaningless to talk about leading lagging for those voltages? \$\endgroup\$ – panic attack May 8 '18 at 18:26
  • \$\begingroup\$ They're all voltages. We talk about leading and lagging of currents with respect to voltages (or the other way around). \$\endgroup\$ – Transistor May 8 '18 at 18:27
  • \$\begingroup\$ Ok so we have to find zero crossing points for current and voltages where these points are less than 180 degrees apart from each other; then the point on the left side is the leading quantity(because time increases from left to right direction in cartesian representation). That's cool we don't need the initial point for this; any steady state part of the plots work to interpret. \$\endgroup\$ – panic attack May 8 '18 at 18:32

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