Phase versus Polarity

[Smart $]

That is being nit-picky and has nothing to do with the point dbuckley was making.

Yes, but we do have apprentices reading these posts, most likely... don't want to give them a false impression early in their learning curve!

 

[mivey]

TTBOMK all contacts are intended to make at the same time.

no argument there (from me anyway)

 

[mivey]

Yes, but we do have apprentices reading these posts, most likely... don't want to give them a false impression early in their learning curve!

I did not think about that

 

[crossman]

I'd just like to note that this representation is inaccurate. I don't know of any commonly used split- or multi-phase generation system that does not have any output prior to the delay time. The delay or lag or shift is only the reference point in the cycle, but the waveform exists at t = 0.

I agree with Smart$ on this. He is entirely correct, for example, when you start a generator and get it up to speed and you say "start measuring voltages now", or when you close a breaker from the utiltiy company, all the various terminals have their appropriate voltages being produced. There is no way that one of the voltages could sit at zero for the first cycle after the breaker is closed. And like I mentioned, this doesn't even happen in the generator.

So the diagram that was posted with the nine transformers and claiming that the final output would be lagging in time before the light came on is just not correct.

edit: correct grammar

 

[mivey]

I agree with him too. I was just trying to think of a delay example and did a poor job.

 

[mivey]

So the diagram that was posted with the nine transformers and claiming that the final output would be lagging in time before the light came on is just not correct.

I agreed with what he was illustrating in his post and thought the post illustrated that point nicely.

Unless I read it wrong he said IF they produced a time shift (which they do not)

Now if each transformer introduces 180 degrees of phase shift, or a delay of 8.333 milliseconds

I just did not agree that the terms "phase shift" and "phase difference" meant the same thing.

I took "phase shift" and "time shift" to be related.

I took "phase difference" to be a change in reference point.

[edit typo]

 

[mivey]

By the way crossman, I'm enjoying "AC Current Flow Question" and think you are dead on. I'm in the 60 mA camp.

 

[crossman]

No doubt, the past couple of weeks have been extremelyinteresting in the theoretical department here on the forum. I am getting the feeling it has all just about run its course, which will be okay to me. My head hurts.

 

[dbuckley]

I too think Carl summed up math versus real world in post #15, in particular the pertinent last sentence. Pretty much all the time, if the math model gives the right answer it is "good enough".

Now, as an exercise for the reader at home, try replacing the input sinewave by something that isn't the same for the first and second half-cycles. So the first half (the positive half) is, say, sine, and the second half (the negative half) is say square, and then redraw the diagrams for phase shift and polarity invert, and note something very odd happens...

(I'd do it myself but I'm not where I need to be to do diagrams and post them at the moment, if anyone else wants to post them, please go ahead)

 

[mivey]

I am getting the feeling it has all just about run its course

Don't stop playing yet. I think you bring a lot to the table. How about your perspective on:
Electrical Calculations/Engineering
Delta system neutral point

 

[Smart $]

...In the three phase world, a delta-wye transformer produces a 30 degree phase difference on its output, without any sort of time delay elements...

Here's an example I'll toss in for entertainment purposes. A bank of two 3? 208Y/120 transformers with a common N and reversed primaries. This system could power three 120/240 systems with performance basically indistinguishable from that of typical systems.

 

[crossman]

Don't stop playing yet. I think you bring a lot to the table. How about your perspective on: Electrical Calculations/Engineering
Delta system neutral point

I appreciate your comment and I reciprocate and appreciate your thoughts too. As for the 4-wire delta neutral point, I am on the side that the neutral is only considered a neutral for the single-phase system.

 

[ELA]

I think it is mostly semantics or a differnet way of looking at the same thing.
For me if the waverform is "continuous and repetitive" then I don't care if it is 180 degree phase shifted or inverted.
For a non continuous or non repetitive wave then I like phase shift best.