I know 3 phase is 120v line-neutral, and 208v line-line, but what is it when
all three lines are used? Or is it just not referred to at all, since
voltage between three lines doesn't mean anything?
Can 208v be called 2 phase? I know 2 phase is properly an obsolete system
that involved 2 120v line, 180 degrees out of phase; but is the term now
ever used for 208v?
3 phase is used for things like large electric motors.
These motors use a 3 wire connection. Like A, B, and C.
And these motors will run more "efficiently" on three phase.
It all starts at the electrical generation plant. An electrical generator is
turning around in circles. So you take 3 wires off of the generator like
"pieces of a pie" cut into 3 pieces, then send these 3 wires out on the
electric lines (notice high up electric lines have 3 wires - they are 3
Then usually businesses will have a 3 phase service, almost never a home.
The 3 phase services will have 4 wires coming in. The 4th is a neutral.
Then the business may have a large electric motor, and 3 wires are
connected. Then the 3 wires power or push the electric motor from the 3
different "pie pieces", or 3 different points on a clock, just as it was
received from the generator.
Sort of like a 3 cylinder engine instead of a one cylinder engine!
Why three phase. Remember power is proportional to voltage squared. If you
look at the sum of the power for each of the three phases you will find that
it is constant. So motors that use three phase power are much smoother than
2 phase motors and smaller for the same horsepower I believe. Also large DC
power sources use three phase power as it requires much less filtering.
Notice high-voltage transmission lines are always in groups of three for the
same reason. The voltage of three phase power supplied in moderate settings
is 208 so that 120 can be obtained using a "Y" type configuration.
Correct. The voltage naming convention refers to the phase-to-phase
voltage, not the phase-to-ground voltage. If you know the phase to
phase voltage, you may obtain the phase-to-ground voltage by dividing
the phase-to-phase voltage by the square root of 3 (which is 1.73).
208/1.73 = 120
480/1.73 = 277
In general terms, a three phase circuit is more powerful than a single
phase circuit because it delivers more power from point A to B per
unit of copper conductor and hence, is much more efficient.
AC Single Phase Circuits actually deliver varying sinusoidoil pulses
of power that are a zero 120 times per second (the zero crossings of
AC Three phase circuits supplying a balance load such as a motor are
delivering the same level of power continuously, (but rotating in
intensity among the 3 conductors). It is interesting that DC
circuits also deliver power constantly without the zero crossing
In practice, 3-phase motors can be cheaper, smaller, quieter, easier
to start, and run cooler and more efficiently for a given HP.
I'm having difficulty following that one point, Beachcomber.
If by "unit of copper conductor" you mean the pounds of copper needed to
make the conductors going between points A and B, then;
It seems to me that for each indivdual conductor running between points
A and B, the power loss in that conductor is just going to be equal to
the rms current squared times the total resistance of the conductor, and
the power delivered to the load by that conductor is going to be equal
to the rms current times the rms voltage at the load, assuming that the
load has a unity power factor of course.
Those two powers (power loss and power delivered) remain the same
whether that conductor happens to be part of a single phase or a
multiphase transmission system, so the efficiency of the transmission
system (power delivered to the load less power lost in heating the
conductors, divided by power entering the line) should be constant if
the voltage and pounds of copper used stay the same.
I seem to recall a discussion that if you assume equal voltages, currents,
and resistances, thus equal loss in each conductor, then adding the third
conductor increases losses by 50 percent (1.5 times the two conductor loss)
for a power delivery increase of 73 percent (1.73 times the two conductor
power). I think in a balanced three phase system that Power equals RMS
Current times RMS Voltage times the Square Root of Three (1.73). Right now I
don't have the math handy to show it but I think it is correct.
> conductors, divided by power entering the line) should be constant if
> the voltage and pounds of copper used stay the same.
For single phase 120V line to neutral loads - common neutral, assume
1000W each load:
--Single phase supply 3wire (ABN) 2000W supplied
-> 2000W/3 = 667 Watts per wire
--3 phase supply (208/120V) 4 wire (ABCN) 3000W supplied
-> 3000W/4 = 750 Watts per wire - 12% higher
For 3 phase 240V line to line loads - assume 10A per wire:
--Single phase supply 2 wire (AB)- watts supplied = 2400W
-> 2400W/2 = 1200 Watts per wire
--3 phase supply (240V delta) 3 wire (ABC) watts supplied = 10 X 240 X
SQR(3) = 4157W mo
-> 4157/3 = 1386 watts per wire - 15.5% higher
3 phase is also a significant advantage in all but small motors. Also
can be in power supplies.
Here's where I was coming from guys:
Say the 2000W load is composed of two 1000W loads in series, connected
across that single phase 240V supply.
So, there's zero current in the neutral.
And, each of the two wires (A&B) is carrying 2000/240 = 8.33 Amps.
Thus the line losses (in Watts) are equal to that current times the
resistance of each wire. Let's assume 1 ohm resistance in each wire, so
the total line losses for the two wires (AB) will be 16.66W while
powering a 2000W load.
Now take the 3 phase supply (208/120V) 4 wire (ABCN), and let the load
be three 1000 watt loads connected in a star pattern, dissipating a
total of 3000 watts. (It's easier for me to do visualize current flows
with a star rather than a delta.)
Because everything is balanced there's zero current in the neutral in
this case too.
The current in each of the three wires (ABC) is 1000/120 = 8.33 Amps,
just like the single phase example. If they are the same 1 ohm wires,
the total line losses for the three wires (ABC) are 24.99 Watts
Now, 2000/3000 = 16.66/24.99, so the "power tranmission efficiency" per
wire is the same, IF the loads are balanced and you can get away WITHOUT
a neutral wire.
I agree that's not usually a code permitted case, so 'ya got me on the
"wire count efficiency" if the neutral has to be there, even though it's
not carrying any current and dissipating any losses.
True [will be the same loss if one 1000W load is turned off]
Also true [will probably also be the same loss with any total load of 2000W]
If you can ignore the neutral also true. (But not counting the neutral
Beachcomber's post was the power delivered per per pound of copper, in
which case 3 phase provides more power per wire (and per pound of copper).
For efficiency, if we use your example ignoring the neutral, 3 phase is
still more efficient since the losses per wire are the same but the
losses are divided by a larger power delivered for 3 phase. Thus the
percent losses are lower for 3 phase making the efficiency per wire
higher. (Actually losses should have been divided by the power supplied
to the wire.)
If neutrals are included in calculating loss per wire, the 3 phase
efficiency is improved.
At the risk of making a total PIA of myself over this, bud, I was
showing that the losses per wire (In my example 8.33 Watts per wire.)
were the same for each wire, in both the single phase (2 wires) and
three phase (3 wires) examples.
The power in the three phase example (3000 Watts) was 1.5 times the
power of the single phase one (2000 watts) as were the losses in both
So, I still can't agree that the losses expressed as a percentage of the
delivered power (or the supplied power, which as you point out below is
"correcter".) would be different for the two examples I gave.
Agreed, three phase will take less pounds of copper per unit of
delivered power, at the same voltage of course.
Yea that works, but only if you ignore the neutrals. It also requires
the loads to have the same I-V characteristic. In the real world that is
unlikely which would result in too high a voltage on the lower wattage
loads (which is why it is a code violation unless designed as part of a
Ignoring the code, the real world connection would likely be all 240V
loads supplied by single phase 240V 2 wire, or 3 phase 240V 3 wire
delta. Load matching would not be an issue then. In that case the 3
phase delivers more power per wire.
Also when you have a building which has a 3 phase service, a lot of the
stuff is not 3 phase - lighting, 120 V outlets, etc. So you want to balance
the load in the panel. Some circuits will be on one phase, others on the
next, and the rest on the third.
So this is why you might see three big huge wires and a little teeny tiny
4th wire for a 3 phase business electrical service. Most of the load is
balanced between the 3 phases.
Same as with a house where you want half on one leg and the other half on
the other leg.
FYI - Connecting the wires on an electric motor for 3 phase can be complex
to say the least, as there are different types of 3 phase service. Scroll
down to "Three Phase Motors-Single Speed" on the following link...
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