3 Phase Power versus 1 Phase Power

[Road Guy]

Whats the difference in english?

 

[benbo]

Whats the difference in english?

Wikipedia has a pretty good explanation -

http://en.wikipedia.org/wiki/Three-phase_electric_power

If you remember your waves, it is just three electrical sinusoids of basically the same voltage and frequency, that are out of phase, or shifted so they peak at different times - usually 120 degrees from each other. Single phase power is just one single phase.

There are reasons why this is beneficial for rotating machinery and distribution systems.

If you don't understand this, don't worry, I don't fully either. So I probably won't be able to answer any complicated questions. But I'm sure some power person like mudpuppy or Flyer will step up to give a better explanation.

 

[mudpuppy]

Benbo's explanation is correct. Is there anything more specific you wanted to know RG?

There are reasons why this is beneficial for rotating machinery and distribution systems.

Two reasons come to mind immediately. First, to deliver single phase power you need two wires. To deliver three phase you need three. So by adding the third wire you get triple the power delivered--that's a really good ROI.

Second is the efficiency gain in motors (and generators). In a single phase motor, you get zero power delivered when the sinusoid crosses zero. With a three phase motor, when one phase crosses zero, the other two are not at zero so you are always getting power delivered. This makes for a smoother-running, more efficient motor.

There are probably more reasons; those are just the ones that popped into my head first.

 

[Road Guy]

that answered it, had a sub trying to sell me the 1 phase, when we have basically already priced the 3 phase..

thanks

 

[WaterholeWally]

[SIZE=18pt]As previously noted, the choice between electrical service depends on equipment and available service from POCO. I've been on service calls where the equipment was improperly wired by other trades (HVACR). Most of the problems were a result of improperly labeled panels. I've worked on two-phase (2 phase) systems and still hear about them when electricians go out on calls to old facilities that still use the equipment.[/SIZE]

 

[Art]

Benbo's explanation is correct. Is there anything more specific you wanted to know RG?
Two reasons come to mind immediately. First, to deliver single phase power you need two wires. To deliver three phase you need three. So by adding the third wire you get triple the power delivered--that's a really good ROI.

Second is the efficiency gain in motors (and generators). In a single phase motor, you get zero power delivered when the sinusoid crosses zero. With a three phase motor, when one phase crosses zero, the other two are not at zero so you are always getting power delivered. This makes for a smoother-running, more efficient motor.

There are probably more reasons; those are just the ones that popped into my head first.

actually you get ~73% more power (or 173% as much, not 300% or triple) for 50% more conductor (assuming the same V and I and ignoring pf)

P1 = VI

P3 =sqrt(3) VI...or 1.73...

or for a given load you have smaller I, smaller conductors and CB's...

eg, for a 4800 VA load at 480V, I1= 10, I3 = 5.78, a good deal smaller, less heating losses also...

it helps with continuity of power flow, as you stated, no zero's summing the phases...

it makes starting AC motors easier, since the fields are rotating, you don't need a capacitor to induce a phase shift and start the motor...

in addition in a balanced system the currents cancel, so the N carries no I...

there's a real beauty to the scheme

there are also advantages in a fault condition...the magnitudes are less...

imho the primary advantage is with rotating machinery...

 

[mudpuppy]

actually you get ~73% more power (or 173% as much, not 300% or triple) for 50% more conductor (assuming the same V and I and ignoring pf)P1 = VI

P3 =sqrt(3) VI...or 1.73...

This is only true if you are equating the voltage over the single-phase load to the line-line voltage of the three-phase load. I'm equating the voltage over the single-phase load to the line-neutral voltage of the three-phase load.

 

[Art]

This is only true if you are equating the voltage over the single-phase load to the line-line voltage of the three-phase load. I'm equating the voltage over the single-phase load to the line-neutral voltage of the three-phase load.

then you need 4 wires...

or you're comparing an 831V circuit to a 480V one...

the base assumption is 1 phase V = the same as the Vll 3 phase...

if the I are the same, I3l = I1 = 10

it's still the same...why?

because the Iln does not equal Ill, it differs by the sqrt(3)

and it's a single phase circuit, as in 120-208, not a 3 phase circuit...

P1 = 480 x 10 ~ 4800 VA

P3 = 3 x 480 x 10/(sqrt(3)) ~ 8324 VA

8324/4800 ~ 1.73...not 3 times as much...

but if you're saying:

V1 = 480

V3ll = 831, V3ln = 480

I1 or line = 10

I3ln or line = 10, I3l = 17.3

then yes, 3 times as much, for 73% more voltage and current, ie insulation and conductor...

 

[mudpuppy]

then yes, 3 times as much, for 73% more voltage and current, ie insulation and conductor...

No, there is not more current, just higher voltage:

P1 = V1*I1

P3 = Sqrt(3)*V3*I3

Then if I make I3 = I1 and V3 = Sqrt(3)*V1 then

P3 = Sqrt(3)*Sqrt(3)*V1*I1

P3 = 3*V1*I1

P3 = 3*P1

So by adding another conductor that can carry the same amount of current, you can triple the power delivered. Yes, you may have to increase the insulation level and clearances, but my guess is this is relatively less expensive to the conductor cost (which is why distribution voltages keep creeping up--our standard for new construction is now 24.9 kV).

Granted you will probably want to add a fourth wire for safety's sake, but it can be smaller than the current-carrying conductors (at least on the utility side).

 

[Art]

No, there is not more current, just higher voltage:
P1 = V1*I1

P3 = Sqrt(3)*V3*I3

Then if I make I3 = I1 and V3 = Sqrt(3)*V1 then

P3 = Sqrt(3)*Sqrt(3)*V1*I1

P3 = 3*V1*I1

P3 = 3*P1

So by adding another conductor that can carry the same amount of current, you can triple the power delivered. Yes, you may have to increase the insulation level and clearances, but my guess is this is relatively less expensive to the conductor cost (which is why distribution voltages keep creeping up--our standard for new construction is now 24.9 kV).

Granted you will probably want to add a fourth wire for safety's sake, but it can be smaller than the current-carrying conductors (at least on the utility side).

so you're comparing V1 line current to V3 line to neutral, not V3 line

because V3 ln = V3ll/sqtr(3)

if V1 = 10A and V3ll = 10A, then V3ln = ~5.8A??

so P1 = 480 x 10 = 480

P3 = sqrt(3) x 831 x 10/1.73 = 8310 or 73% more...

or using the line-neut values...

P3 = 3 x 831/1.73 x 10/1.73 = 8310 or 73% more...

same

what you are saying is that V3ln = 10A or V3ll = 17.3A

you need a larger conductor to carry 17.3 amps...

so although you add 50% more wires, you add more area (Cu) also...

so to get 3 times the power you need an extra wire, and all 3 must be larger, plus a neutral (full sized)...so 2 wires, and all 4 are larger than the 1PH conductor...

apples & oranges

 

[Art]

from Federal-Pacifics website

quote: By the use of three conductors a three-phase system can provide 173% more power than the two conductors of a single-phase system.

a very good primer

http://www.federalpacific.com/university/t...s/chapter3.html

 

[mudpuppy]

if V1 = 10A and V3ll = 10A, then V3ln = ~5.8A??

I'm not entirely sure what you're trying to say here, but I think I disagree. I'm saying in the single phase system the line current is 10 A. In the three phase system the line current is 10 A.

Forgive my bad drawing, but maybe this makes it clearer what I'm trying to say:

 

[Art]

I'm not entirely sure what you're trying to say here, but I think I disagree. I'm saying in the single phase system the line current is 10 A. In the three phase system the line current is 10 A.
Forgive my bad drawing, but maybe this makes it clearer what I'm trying to say:

for any comparison to be valid, the voltages MUST be equal...

so if the single phase V is 480, the 3 phase must be 480 (or ph to neut 277)

the single phase power is 480 x 10 ~4800

3 phase power:

sqrt(3) 10 480 ~ 8314 VA

or

3 x 277 x 10 ~ 8310 VA, the same and both 173% more, not 300%....

now, I'm wrong as much as I'm right...lol

but Federal-Pacific knows a thing or two (or sqrt(3)) about power, and THEY say it's 173% more for equivalent circuit parameters...

so do the textbooks...

 

[mudpuppy]

for any comparison to be valid, the voltages MUST be equal...

The voltages ARE equal in my case. The voltage across each phase of the wye-connected load is the same as the voltage across the single phase load!

But we are just arguing semantics at this point. I do agree that if the line-line voltage in the three-phase system is the same as the applied voltage on the single phase system, then yes, the 3-phase power is 173% of the single-phase power. I just don't see the reason for making that assumption. Different ways of thinking about things I guess.

 

[Art]

The voltages ARE equal in my case. The voltage across each phase of the wye-connected load is the same as the voltage across the single phase load!
But we are just arguing semantics at this point. I do agree that if the line-line voltage in the three-phase system is the same as the applied voltage on the single phase system, then yes, the 3-phase power is 173% of the single-phase power. I just don't see the reason for making that assumption. Different ways of thinking about things I guess.

because if you don't equate the 1ph voltage to the 3ph (line-line) voltage, but instead choose the line-neut, you are comparing 1 equivalent single phase circuit to 3 equivalent single phase circuits...of course the power will be 3 times as much...

but it is not a valid comparison because the phase relationship is not factored in...

each phase to line voltage in a 3 phase system is NOT equivalent to a single phase voltage...they are not in phase...so they do not sum linearly, ie, 3 times, but are summed geometrically, by a factor of, you guessed it...sqrt(3)

 

[geofs_PE]

The choice would depend on the application, the size and type of loads to be powered. Also, in many areas, the power company doesn't provide three phase service, many areas have only single phase or possibly "high-leg three phase." The cost of installation for three phase vs. single phase would also be a consideration as three phase equipment (transformer, breakers, etc.) is typically more expensive.