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Industry Standard of Lagging is it positive or negative?


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#1 vskneifl

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Posted 25 March 2012 - 03:36 AM

So I have been trying to understand ABC and ACB rotation relative to CW or CCW rotation and which is positive degrees and which is negative in degrees and if lagging is considered positive or negative direction as well. I did find this website http://www.marcspage....uk/pq/3120.htm that helped explain it but it talks about two different standards. Does anyone know or have a good explanation of this and what standard is used for the test?

#2 Wildsoldier PE

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Posted 25 March 2012 - 02:22 PM

Usually in the exam they specify sequence if not i always asume positive sequenence. Regarding lagging and leading you use the voltage as your base...when they say is lagging pf means that the current lags the voltage by x degrees ....if it leads means that the current leads the voltage by x degrees. When its lagging its said that is an inductive circuit...when its leadding its said that is a capacitive circuit....in reality most electical installations have a lagging power factor thats why capacitors are added at the load side to fix the power factor...good aproach to fix the power factor is usually to .95 lagging.

#3 vskneifl

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Posted 25 March 2012 - 03:52 PM

That helps, thanks.

#4 Peele1

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Posted 25 March 2012 - 05:18 PM

I tried to stay out of the leading/lagging positive/negative and just use pure vectors. I think that back in the old days before pocket calculators that could do complex math, these notations were more important. Same thing with a lot of the formulas, I do them all in vector format. Remember that electrical power equations are often 100+ years old, so there were no calculators at all, and just tables. Vectors are the way to go IMHO.

If you know that S = V I* (# of phases)^1/2 and V = I Z you are good for 1/2 of the test. And, always draw it out on paper.

#5 vskneifl

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Posted 25 March 2012 - 06:18 PM

I have tried to use the vectors as well I do fine on the magnitudes I just have trouble knowing when to add or subtract the angles. I have even created a phasing template to help me keep it straight (see attached pdf) on the simple problems I due fine but the more complex ones that I just can't seem to get right like NCEES #111, I understand how they did it but I was trying to calculate VaA and VBb and add up the loop of voltages. They give you VAB and IaA and so I figured since it is balanced IbB would be the same just 120 degrees rotated but I am not sure which direction. With an ABC rotation and phase b in the negative direction IbB will be -140 or is it in the positive direction making IbB 100 degrees.

The problems #111 is a balanced delta load, sequence ABC and they give you VAB of 12.5kV 0 degrees which is the VLL of the delta load then they give you a line impedance of (5+10j) Ohms/phase and then IaA = 70 angle of -20 degrees. Point "a" and "b" are the ends of the lines that connect to the Delta load at points "A" and "B".

Any guidance or help would be appreciated. Thanks

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