May I ask a question about the single vs two phase stuff

[Russs57]

I always try to understand other people's point of views.

With that in mind let me ask the following. Say you have a complex, non-periodic waveform. Yet say over a half cycle it might be close to periodic. Now if we break it down with Fourier transformation to individual sine waves, and phase shift them by 180 degrees, and then sum them back together......well are we back to 180 = -1?

This might be worth playing around with. Haven't had a chance yet myself.

https://www.desmos.com/calculator/cktkoo3hex

 

[Adamjamma]

But, which one for all those phase questions as part of the tests here? I think the bottom one is what I use but.. not sure..lol..

Answering myself..sort of.. just looked in textbook, which is like fifteen years old..lol... and description calls for 120 degree math not 180 degree math so it would be the 208 wave form.. IThink..lol

 

[jumper]

Where did you get this? This is gold to me.

Just pics I snagged from the net.

Have all kinds of similar stuff.

 

[jumper]

Answering myself..sort of.. just looked in textbook, which is like fifteen years old..lol... and description calls for 120 degree math not 180 degree math so it would be the 208 wave form.. IThink..lol

Use 0 and 180 degrees for 120/240V.

Use 0 and 120 degrees for 120/208V.

 

[Besoeker]

Where did you get this? This is gold to me.

But they are not correct.

 

[jumper]

But they are not correct.

They are just graphs to illustrate. They have to be used in context.

 

[Besoeker]

They are just graphs to illustrate. They have to be used in context.

Why not make them correct? It is no more difficult.
And showing the wrong amplitude is misleading.

 

[jumper]

Why not make them correct? It is no more difficult.
And showing the wrong amplitude is misleading.

It all depends on the level of the person I am speaking to.

Many average electricians have no clue about Vp Vp-p, just Vrms.

At that level, the graphs are helpful. They help me explain the 120-0-120 set up.

 

[Besoeker]

It all depends on the level of the person I am speaking to.

Many average electricians have no clue about Vp Vp-p, just Vrms.

But that's my point. The graphs are not showing Vrms.

 

[jumper]

But that's my point. The graphs are not showing Vrms.

My point is that they are simple graphics to illustrate the basic math for our two single phase supplies. 120/240V and 120/208V

and

 

[Besoeker]

My point is that they are simple graphics to illustrate the basic math for our two single phase supplies. 120/240V and 120/208V

What they illustrate just doesn't happen. If the basic electrician doesn't understand Vp-p, why show him sine waves at all, especially those that are incorrect representations?

Why not just draw a circle with two 120V dimensioned diametrically opposed spokes rather than incorrectly dimensioned sine waves?
Yes, I strongly believe that you need pitch the explanation at a level that can be understood by those receiving it. I equally strongly believe that you should not present duff information.

 

[wwhitney]

Why not make them correct?

Since it bugs you so much, here's some amateur image manipulation to fix the scale:

Cheers, Wayne

 

[Besoeker]

Since it bugs you so much, here's some amateur image manipulation to fix the scale:
Cheers, Wayne

Well done that man :thumbsup:
The manipulation to correct it wouldn't have been required had it been done right in the first place.

 

[jumper]

Well done that man :thumbsup:
The manipulation to correct it wouldn't have been required had it been done right in the first place.

Yes, but more than half the average sparkies would be clueless about those amplitudes. Their little meter only has Vrms. I only use those types of for simple illustration.

And I never use the word phase for the 120V. I say signals.

 

[jumper]

Since it bugs you so much, here's some amateur image manipulation to fix the scale:

Cheers, Wayne

View attachment 21008 View attachment 21009

Cool. I intend to use both now. Thank you.

 

[Russs57]

Surprised you didn’t catch grief for the 240 or 208 pointing to the peak of the sine wave

You really feel 50% have no idea what root, mean, square is? That is sad if it is true.

 

[jumper]

Surprised you didn’t catch grief for the 240 or 208 pointing to the peak of the sine wave

You really feel 50% have no idea what root, mean, square is? That is sad if it is true.

The level of education needed to get licensed in many places is pitiful.

One or two electrical courses and a code class is not uncommon.

I do what I gotta do. If I have to fudge a little at times, so be it.

 

[Besoeker]

Yes, but more than half the average sparkies would be clueless about those amplitudes.

OK. So why confuse them with (incorrect) sine wave representations?
What am I missing here?

 

[wwhitney]

With that in mind let me ask the following. Say you have a complex, non-periodic waveform. Yet say over a half cycle it might be close to periodic. Now if we break it down with Fourier transformation to individual sine waves, and phase shift them by 180 degrees, and then sum them back together......well are we back to 180 = -1?

So, it's been a couple decades since I took real or complex analysis, but a little research plus my recollection tells me:

Let F(t) be a differentiable, periodic function with period T, meaning f(t + T) = f(t) for all t. Then its Fourier series will be convergent everywhere, that is F(t) can be expressed as an infinite sum of sine and cosine waves of the form f_n(t) = A_n sin(2 pi n t/T) and g_n(t) = B_n cos(2 pi n t/T).

Then F(t) will be anti-periodic with period T/2, meaning F(t + T/2) = - F(t), if and only if A_n = B_n = 0 for all even n, that is, F has only odd harmonics.

Cheers, Wayne

 

[jumper]

What am I missing here?

That most guys will never touch a scope or such. Sine waves are something they just kinda understand in a vague and abstract way at best.

The pics are just simple illustrations to show simple points to simple people.

Trust me they serve a purpose at a certain level for field purposes.