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

Status
Not open for further replies.

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.
 

iwire

Moderator
Staff member
Location
Massachusetts
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.

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.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Can you list anything about 3 phase or other that you find confuses people.

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.
 

Ingenieur

Senior Member
Location
Earth
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).
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
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).


Yep, that's about as clear as mud to a novice!
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
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.

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
 

dionysius

Senior Member
Location
WA
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.

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.
 

dionysius

Senior Member
Location
WA
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 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:
 

JFletcher

Senior Member
Location
Williamsburg, VA
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???

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


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.

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.
 

Smart $

Esteemed Member
Location
Ohio
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)...
 

JFletcher

Senior Member
Location
Williamsburg, VA
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:

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?
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Registered Professional Engineer
... Can you list anything about 3 phase or other that you find confuses people. ...
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.

... 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??? ...
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.

... Also is it accurate to tell a customer that one 3 phase 200A panel is the same (even better) ...
What's your definition of "better"? Mine is effective, well-matched to the need & economical.
 

dionysius

Senior Member
Location
WA
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.

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???
 

iwire

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

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.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
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.

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