Phase rotation convention 3

[spartanv12]

The younger staff I work with are as confused as hell about phase rotation convention, our original phase rotation test instruments had a needle that rotated anti-clockwise in the direction of the arrow for a,b,c rotation. This is as all us oldies expect to see and also how we draw vector diagrams. However all the new instruments on the market have a, b, c as clockwise in the direction of the arrow, in effect it's still the same just a visual differance.
But why the change, when did it happen, was I asleep?

 

[davidbeach]

Excellent questions for which I have no answers. I had a real problem on one electrical infrastructure replacement project when the contractor was telling me that the system rotated clockwise when I was expecting counter-clockwise. It took a while, but we determined that we were actually talking about the same thing. My phasors rotated counter-clockwise as the should and his tester rotated clockwise. Best thing is to have a known reference and use it to test all rotation testers, marking them as to the "correct" direction of rotation for your facility.

 

[alehman]

I frequently hear electricians talk of clockwise and counterclockwise as though these were absolute terms. In reality I think this question hits the nail on the head - it depends entirely on the test instrument. David has the right answer - always do a direct comparison.

Another tricky situation is connecting two circuits from the same source (e.g. tie breaker). In this case you must not only match rotation but also match position. A-B-C is the same rotation as B-C-A but they cannot be connected togather.

See also... thread238-8600

 

[edison123]

alehman,

Your comment

"A-B-C is the same rotation as B-C-A but they cannot be connected togather."

is interesting.

Could you pls tell why A-B-C sequence cannot be connected in B-C-A ?

 

[davidbeach]

What alehman is talking about is two circuits from a single source and paralleling the two circuits together with A of one connected to B of the other, B to C, and C to A. Excessive fault current will flow. If the two sources are never connected together, then no problems if one is connected A-B-C and the other B-C-A.

 

[edison123]

davidbeach,

single source ... missed that one. thx.

OK. I have another question.

Before synchronizing a generator to the grid, is it advisable to use the phase sequence indicator to check the sequenc ?

 

[davidbeach]

My preference when doing the initial start up of a system that will have a generator parallel to the grid is to heat up the common switchgear from the utility and see what the phase rotation is on the relay at the point of common coupling. Then kill the utility and heat up the gear from the generator and check the phase rotation at the same relay. This way there are no changes in connections or instrumentation used to determine phase rotation. If both sources produce the same phase rotation, all is good and it doesn't matter which direction the rotation is or what it is called.

 

[edison123]

Thx davidbeach. That is exactly what I do except that I use a phase sequence indicator. I use only one PT for both the utility backfeed and the generator feed. That way I rule out any PT wiring change.

 

[Skogsgurra]

This is a question that all of us should have asked long time ago. I never thought about it before and now I have started thinking. This is my little contribution to confusion:

Vector diagrams are always rotating counter-clockwise. The reason is obvious, angles in a cartesian diagram grow from zero towards positive values when rotating clockwise. No problems there.

An induction motor is supposed to run in clockwise direction when connected to a grid with positive phase sequence. This is when definitions start to blurr.

So what is positive rotation? Is it when the motor rotates clockwise - or is it when the vectors rotate CCW? Or both?

The answer is that the sequence shall be A-B-C (L1-L2-L3 in Europe) and the only safe way to find out (at least in my opinion) is to connect an oscilloscope and read the timing between the phases. Using a rotating phase sequence checker is OK as long as it has an arrow showing what is "Right" or "Wrong" - it doesn't matter if "Right" is left or right. It just has to be right - or should I say Correct? There are simple rotation checkers with two lamps. One labeled "Right" and the other one labeled "Wrong". No need to discuss the result and no confusion.

More food for confusion: The European motor manufacturers had a convention (it is long time ago, about forty years) that a motor's sense of rotation should be defined looking from motor body along the shaft. This was changed so that we now shall look on the shaft end when telling if it rotates CW or CCW. I think that this never got through to all people involved and I still hear arguments about this.

To top off: Some countries use wordings like "With Sun" or "Against Sun". This may be good practice in polar countries where we all know how the sun moves. But it has proven disastrous when exporting across the Equator. Stick with CW and CCW. Clocks still have hands...

Gunnar Englund

http://www.gke.org

 

[dpc]

The IEEE convention for phasors give CCW "rotation" for a positive A-B-C phase sequence.

I have had the same experience with the commerical phase sequence testers. They like to use CW for positive phase sequence, to correspond with motor rotation.

This is just a case of looking at the same situation from two different points of view.

Edison123's method of using a phase sequence indicator to check generator phasing will work provide you use the same set of PTs as mentioned.

In the old days, when men were men, we racked out the breaker, cranked open the shutters and went in the cubicle with phasing sticks and checked across the generator breaker line and load stabs. My testosterone poisoning has subsided to a level where I would not recommend this procedure today, especially with increased awareness of the arc-flash hazard this obviously creates.

 

[ScottI2R]

DPC,
I guess that would explain the great tan you always have! Just kiddin!

Skoggs,
So given your last comment, is it CAR-I-BE-AN or is it CA-RIB-I-AN?

Spartanv12,
FYI, I still stick with the tried and true, vector calculations I was "learned" : CCW for phase rotation.

Juss some humor for everyone. Hope its appreciated, it's been a long day! Merry Christmas all! And may next year put this one to shame!!!!

Scott

In a hundred years, it isn't going to matter anyway.

 

[Skogsgurra]

ScottI2R,

Completely clueless there. Need to know. Could it have anything with Leeward and Windward to do? A Merry Christmas to you and all!

Gunnar Englund

http://www.gke.org

 

[jschwei314]

My hat is off to Mr. SpartanV12.

I have wanted to ask this question also but way lead on to way and I never did ask this question.

I too was bit by the Clockwise and Counter Clockwise statement. I wanted ABC rotation, to me, counter clockwise.

I have always wonder why, when I arrived on site and found CBA rotation.

 

[peetey]

I am a Journeyman Meterman by trade. We use phase rotation to plot vectors all the time. As a matter of convention we consider CCW To be ok rotation or ABC rotation. So depending on where they plot it can be ABC, BCA or CAB for ok. If it is reverse it is clockwise. So it can be CBA, BAC or ACB depending. When doing Q metering or metering with phase shifters it is imperative to know rotation. This is what we call phase displaced metering. We have a few phase rotation meters that are wired to go CCW for ABC rotation. I have heard of some electricians that bump a motor for proper direction. If
A-AN is 0 degrees then A-BN is 240 degrees and A-CN is 120 degrees, so the phasing is "looking backwards on a sine wave" BCA. B=240, C=120, A=0 this is ABC rotation.

--peetey

 

[alehman]

skogs - great post. I always wondered if systems rotated in the opposite direction in the southern hemisphere!

 

[jghrist]

Does anyone know how some utilities in the Southeast USA ended up with clockwise phase rotation? Duke Power and Georgia Power both use X-Y-Z CW rotation.

I was told once that Georgia Power did it to make things difficult for Georgia Tech students. ;-)

 

[ScottI2R]

Skoggs,
It is 2 different ways to pronounce Caribbean. Based on how you accent the syllables! Am I in A-mer-i-ca or am I in Am-er-ic-ca. Sorry it does not relate to our lovely forum, but I was just trying to add some Holiday Cheer as it would be! God Bless and have a wonderful one yourself!

Scott

In a hundred years, it isn't going to matter anyway.

 

[peetey]

Try this link for the best phase sequence explanation I have found.

http://www.themeterguy.com/theory/sequence.htm

--peetey

 

[alehman]

http://www.themeterguy.com/Theory/sequence.htm

 

[peetey]

Thank you Alehman, I did not set an apropriate Hypertext link.

--peetey