Biggest misunderstandings with 3 phase power......

Status
Not open for further replies.
D

dionysius

Senior Member
Location
WA
I find many people struggle with 3 phase power.

Consider PF as 1 for all of this to keep it simple.

My method is to always revert to single phase in calculations.

On rating labels the specified Amps are always per phase. This confuses many I find.

So lets say the current is listed as 10A on a 3 phase motor label.

So if VL-L = 480VAC (System Voltage is always VL-L) then VL-N= 480/1.732 = 277

Thus power per phase is 277x10 = 2770W =2.7kW.

And for the motor with all 3 phases it is 3x2770 = 8310W = 8.3kW

____________________________________________________________

Most of our folks here are experts on this. Can you list anything about 3 phase or other that you find confuses people.

Do any of you have a really good mechanical or fluid or other analogy to help people understand why 3 phase is so superb and why it was the choice across the planet. There could be four, five, even ten phases. Why not???

Also is it accurate to tell a customer that one 3 phase 200A panel is the same (even better) than 3 panels like his one at home with single split phase 200A??? Assume he does not know what an induction motor really means.
 
dionysius said:
Also is it accurate to tell a customer that one 3 phase 200A panel is the same (even better) than 3 panels like his one at home with single split phase 200A??? Assume he does not know what an induction motor really means.
Click to expand...

I think not. Lets compare one 240 volt, 200 amp, single phase panel against one 208 volt, 200 amp, three phase panel.

240x200 = 48,000 KW

208x200x1.73= 71,968 KW

So one three phase is not even equal to two of the single phase panels.
 
dionysius said:
Can you list anything about 3 phase or other that you find confuses people.
Click to expand...

Yes, when people try to simplify it by coming up with analogies. They end up trying to explain every detail instead of speaking in generalities.
 
Assume 10 A per line
V = 240
same conductor size for 1 and 3 phase
no N

1ph
240 x 10 = 2400 va
2 conductors

3 ph
sqrt3 x 10 x 240 = 4160 va
3 conductors

with 3 ph you get 73% more power with only 50% more Cu
 
I think the best way to explain 3 phase is to note that it is just three singles phase sources (or loads) that "happen" to be 120 degrees apart. That would be 6 wires, but because of the phasing difference, we can connect those 6 wires two ways (delta or wye) that reduces the number of wires to three (or four).
 
electrofelon said:
I think the best way to explain 3 phase is to note that it is just three singles phase sources (or loads) that "happen" to be 120 degrees apart. That would be 6 wires, but because of the phasing difference, we can connect those 6 wires two ways (delta or wye) that reduces the number of wires to three (or four).
Click to expand...


Yep, that's about as clear as mud to a novice!
 
electrofelon said:
Lets hear your vastly superior pedagogical method. Presumably if one is explaining three phase, the student already has the basics of single phase down. Seems like a straightforward and logical extension to me.
Click to expand...

I didn't say I had one nor offered one.

I was talking about customers not students as the OP mentioned "customers".

Didn't mean to hurt your feelings!:p
 
iwire said:
I think not. Lets compare one 240 volt, 200 amp, single phase panel against one 208 volt, 200 amp, three phase panel.

240x200 = 48,000 KW

208x200x1.73= 71,968 KW

So one three phase is not even equal to two of the single phase panels.
Click to expand...

For an apples to apples comparison should it not be:
240x200x1.73 = 83,040 kW ????

I am using same VL-L for each. But I do get your point. It is less than double. Disappointing.

Why was I thinking times 3???? I wanted to think 3 times the power intuitively.
 
Ingenieur said:
Assume 10 A per line
V = 240
same conductor size for 1 and 3 phase
no N

1ph
240 x 10 = 2400 va
2 conductors

3 ph
sqrt3 x 10 x 240 = 4160 va
3 conductors

with 3 ph you get 73% more power with only 50% more Cu
Click to expand...


I know by the formula you are correct.

Please help me here.

Intuitively to me it is 3 single phases. The fact that they are 120 deg staggered does not matter to the power. Consider water flowing through 3 pipes as an analogy. Over a period of minutes each of the three pipes will deliver the same amount of water.

Why am I not seeing this:dunce::dunce::dunce:
 
dionysius said:
Do any of you have a really good mechanical or fluid or other analogy to help people understand why 3 phase is so superb and why it was the choice across the planet. There could be four, five, even ten phases. Why not???
Click to expand...

Three phase is the least complex system that does what more phases would do only slightly better.


dionysius said:
Also is it accurate to tell a customer that one 3 phase 200A panel is the same (even better) than 3 panels like his one at home with single split phase 200A??? Assume he does not know what an induction motor really means.
Click to expand...

Never had a customer ask me that. Wild leg and delta 3 ph services wouldnt be as good as center tapped single phase for many smaller applications.

Easiest answer for the layman who inquires "Why do we have 3ph service?" would be "because you have equipment that requires it."

My knowledge of 3ph power and motors is pretty weak; not going beyond the basics to explain it.
 
The thing about three phase that is confusing (if you ask me, and on this rare occasion you did as a matter of fact :D) is trying to think of it as three single phase sources when powering a three phase load. I think if you have to better understand three phase, then break the load down into three single phase loads, keeping the source and lines as three phase. The hardest part to understanding how three phase works is how currents combine onto lines then separate at source(s) and load(s)...
 
dionysius said:
I know by the formula you are correct.

Please help me here.

Intuitively to me it is 3 single phases. The fact that they are 120 deg staggered does not matter to the power. Consider water flowing through 3 pipes as an analogy. Over a period of minutes each of the three pipes will deliver the same amount of water.

Why am I not seeing this:dunce::dunce::dunce:
Click to expand...

If split phase had 208V l-l, it would have 104V l-n. 208/120V 3ph has 120v l-n. 120/104 = 1.1538. Now multiply that by 3ph having 3 hots vs split phase's 2 (3/2 = 1.5) 1.5 x 1.1538= 1.7307..., which is basically √3.

Coincidence?
 
dionysius said:
... Can you list anything about 3 phase or other that you find confuses people. ...
Click to expand...
Trying to use a mechanical, fluid or other analogy usually confuses people. Go back to the basics: Voltage is the amount of charge on an electron; current is the number of electrons circulating. AC is sinusoidal. The three phases lag each other by 120°. Do the Math and it will become clear.

dionysius said:
... why 3 phase is so superb and why it was the choice across the planet. There could be four, five, even ten phases. Why not??? ...
Click to expand...
For a given voltage, no other number of phases transmits more power with less copper.

6-phase & 12-phase, et c., transmit an equal amount of power over the same mass of copper, but the additional insulators, bigger/more crossbars and such makes them less economical for transmission & distribution.

6-phase & 12-phase are commonly used in rectifiers, inverters, variable-frequency motor drives and such to reduce ripple, improve waveshape and/or reduce harmonics. Ford uses 8-diode 6-phase rectifiers in their vehicle alternators.

dionysius said:
... Also is it accurate to tell a customer that one 3 phase 200A panel is the same (even better) ...
Click to expand...
What's your definition of "better"? Mine is effective, well-matched to the need & economical.
 
drcampbell said:
Trying to use a mechanical, fluid or other analogy usually confuses people. Go back to the basics: Voltage is the amount of charge on an electron; current is the number of electrons circulating. AC is sinusoidal. The three phases lag each other by 120°. Do the Math and it will become clear.
Click to expand...

With DC analogies hold up very well. Voltage -> Pressure (PSI); Current -> Gal/min flow rate; Power -> Volume of water per minute.

The problem is AC. Perhaps a circulating pulsing pump like the heart pushing into the veins/arteries might serve. Now if you consider three hearts phased 120 deg apart all pumping into the same one vein/artery load. Now I am running into trouble. Anyone want to take this further???
 
dionysius said:
With DC analogies hold up very well. Voltage -> Pressure (PSI); Current -> Gal/min flow rate; Power -> Volume of water per minute.
Click to expand...

Perhaps they do but why teach that when it has to be forgotten with AC.

I agree with the others, forget teaching analogies and teach the real facts.
 
iwire said:
Perhaps they do but why teach that when it has to be forgotten with AC.

I agree with the others, forget teaching analogies and teach the real facts.
Click to expand...

Real facts are important, but analogies are also important in my opinion. My reason is that you can't see or taste electricity. It became much easier for me to understand the concept of voltage when it was related to water, because I can see water. Of course, every person is different, and one must move beyond the analogies. I am not trying to be argumentative, just suggesting that anything that helps someone to actually understand instead of memorize is good.
 
Status
Not open for further replies.
Top