Wild leg and compressors

[e57]

Mark I think, and Larry feel more then free to correct me here, the question becomes which phase is really 'A'?

In my experience the power company does not identify any phase as A B or C.

I can certainly get clockwise rotation without knowing which phase is actually A B or C.

Bob, I think I have been saying the same thing? Or trying to. But if you have a high-leg you can choose to call it B or C, and A will be different depending on that choice. Choose poorly, and you have counter clock-wise. Which is why I often go to a building to put in say an air conditioner, and have to double and triple check rotation before I leave so we don't end up owning that AC unit. If rotation were standard I could walk in ABC-123, and leave.



Likewise the NEC does not dictate color codes to us spare a few, two of which are in a high-leg system. (neutral and a high-leg) And on those it matter which is which. But changing the other two changes rotation and it too can matter which is which. (Moving the high-leg C-B after a meter cabinet changes rotation.) But the code doesn't actually come out and say it needs to be and maintain one direction over the other.

Heres my local code pet peev -

120/240 volt 3-phase delta circuits - “A” phase black, “B” (high leg) phase purple, “C” phase red

hardworkingstiff, I am not sure I have ever seen a set up like that, and don't think it could be called 3 phase without having all 3 phases present.

 

[e57]

Someone tell me what is meant by "rotation".

Is it a matter of motor direction?

Or does it pertain to the phase sequence?

Yes, and yes.

An article or two:
http://ecmweb.com/mag/electric_determining_motor_rotation/
http://www.tegam.com/html/phase_motor.htm
http://www.lawrencepumps.com/newsletter/news_v03_i3_mar06.html
http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=get_topic;f=3;t=002789;p=
http://www.shermanlab.com/xmwang/javappl/acMotor1.html (Note this one is counter clock-wise - maybe from New Zealand? )

 

[LarryFine]

I cant sleep either Larry - back pain....

I can relate! My back is bad, related to a hip joint that needs replacement.

First the primaries, be it wye (not to common, but I have one in my back yard) open or closed delta on the distribution will be roughly the same KV. (no high-legs there.)

I know the primaries are equal voltage to earth. However, I think you're mistaken about the wye primaries being rare. Almost every transformer (or group) I see has a single primary bushing, meaning that the other primary conductor is internally grounded to the can and thus the system neutral.

And you NEED 3 primaries for an open or closed delta. 3 for wye, and grounding the center but not totaly nessesary. Everyone DUCK! But if you want a nuetral thats where it goes...

Again, I must disagree. In your upper drawing, if you connect what is labeled as L2 to the system neutral, the upper drawing shows an open-Delta secondary perfectly; add a grounded center tap to the bottom secondary, and you have a 240/120v 4-wire system.

The open-Delta secondary only uses two phases on the primary. Otherwise, there would be no reason to use it. The primary is made up of two legs of a wye system (like our 208v 1ph circuit tapped from a 3-ph Y; add the neutral, and you restore the 120-deg. timing).

I really intended for the primary to not be part of my questions, but I believe the two-phases-and-the-neutral to be an integral part of the open-Delta system.

Does it matter which leads or follows? Oh yes! Fire up 3 phase motors and equipment counter clock-wise and they dont work right. (unless they are self correcting - but you wont see that often.) Do it on an elevator an you're in for a wild ride. Do it with the high leg and you changed voltage on the system and better just call the fire dept.

Of course, all of this is true, and is what I referred to as 'relative phase'. Just as with a stereo system, where it's imperative that relative phase be maintained (so all speaker cones move in unison with an in-phase signal), absolute phase may or may not be relevant.

After all, absolute phase must be compared to some extarnal standard for comparison. I agree we need to know the phase rotation comparison for motor direction, but does it really matter (hi-leg voltage aside) which phase is which?

What's in a name? Why can't A phase be the high leg? It seems to me that the meter terminals' labeling is where a name matters. It's not that B or C phase must be the hi-leg, it's that we must land the hi-leg conductor on a specific terminal; then we name it what we want.

Sure, if every motor in an installation happened to spin the wron way, we can swap the two non-hi-leg phases to restore everything, but hi-leg or not, what standard pre-identifies any particular conductor as a specific phase? Relative, yes; absolute, no.

 

[LarryFine]

Mark I think, and Larry feel more then free to correct me here, the question becomes which phase is really 'A'?

In my experience the power company does not identify any phase as A B or C.

I can certainly get clockwise rotation without knowing which phase is actually A B or C.

Nothing to correct. In order to identify any particular wire as a particular phase, it must be compared to a reference of some sort, and in a way that the timing comparison is accurate to better than 1/3 of a cycle.

 

[LarryFine]

Could it be that the NEC is just trying to get us all to communicate better? Since the NEC has dictated where the phases are, when I say use A-phase on a circuit, we all know that we need to pick the phase that is on the left (or B in the middle and C on the right) of the panel bus. Does it really matter which is which? It may only be a means to provide better communciation.

Exactly. Again, relative phase matters, for proper rotation on rotation-sensitive equipment, but absolute phase is not relevant.

The open deltas I've seen (not many) use 2-phases and one neutral on the primary side of the 2 transformers.

Correctamundo.

 

[LarryFine]

. . . if you have a high-leg you can choose to call it B or C, and A will be different depending on that choice. Choose poorly, and you have counter clock-wise. Which is why I often go to a building to put in say an air conditioner, and have to double and triple check rotation before I leave so we don't end up owning that AC unit. If rotation were standard I could walk in ABC-123, and leave.

If I'm not mistaken, 3-ph AC compressors that use pistons do not care about rotation, while scroll compressors do. If the condenser fan motor is not 3-ph, it doesn't care; if it is 3-ph, it's all the rotation-direction indicator you need for that unit.

Moving the high-leg C-B after a meter cabinet changes rotation.

Unless you move all three wires in the same direction (eg. A to B, B to C, C to A), which maintains the relative phase rotation direction.

hardworkingstiff, I am not sure I have ever seen a set up like that, and don't think it could be called 3 phase without having all 3 phases present.

A wye-wye system, or a Delta-primary would not be able to reproduce a 3-ph system without all three phases, but two lines of a wye-primary system, along with the neutral, can indeed "synthesize" the missing third phase secondary.

 

[LarryFine]

Notice that, on the page on e57's first link, the rotation direction and phase identifcation are compared to other connections on the premisies:

http://ecmweb.com/mag/electric_determining_motor_rotation/

 

[e57]

Hmmm.... Where to begin with 5 posts in a row - quite a roll!

Where to begin? OK 2 phase power 120 degrees apart, while in therory I can see it, yet have no confirmation of it. Not having known of ever connecting to one.... Not saying it is not posible, but for me its one of those leap of faith things.... As it is already an act of smoke and mirrors to have an open delta in the first place. 3 phases - 2 transformers although it is a significant lose of real and apparent power. Then to have 3 phases produced from only 2, with even less real power? To make an analogy, if your already on 5 cylinders - hey lets see how slow we can go with only 2?

~Again, relative phase matters, for proper rotation on rotation-sensitive equipment, but absolute phase is not relevant. ~~~~ Unless you move all three wires in the same direction (eg. A to B, B to C, C to A), which maintains the relative phase rotation direction

I have a tendancey - really it is a curse! - of being misunderstood, that is what I'm trying to say - the code says nothing about what (CW or CCW), or maintaining rotation at all! However it really does matter the day you have to buy equipment that you destroy in the process of finding out.

FYI piston compressors in AC units are less sensitive and will handle CCW rotations but won't work right - but that is like saying your car works right when it exhausts through the carborator. Yes - turns over - but it wont get you anywhere efficiently. And some equipment a single CCW turn and you own it! I know of a motor controler that has a rotation meter on it - it also has memory that can be read - if it is attempted to be connected to load with reverse rotation - NO WARRANTEE!

 

[gpedens]

Wild leg and compressors

Mr. Kurt , since your motor is single phase your breaker will need to be rated at nominal 240 service as your phase to phase voltage is 240. You probably have a 3 phase open delta service using two single phase transformers. As long as the name plate current for each transfromer is not exceeded you will be okay. Maintaining load balance in the transformer with the "neutral" tap is probably what your buddies were refering to. As you say voltage is voltage. Your motor will pull more current on 208 than 230. This will shorten the life of the motor a little and it will run a little slower. The main problem with "wildleg" services is the neutral when expanding or adding another service, especially in older services or buildings. Folks tend to think the "neutrals" of the old and new are the same. They aren't and there may be a substantial voltage differance between them.