A couple 3phase questions...

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?

Reply to
Toller
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Rather than try to go into detail I took a look on the web. There are many sites that give good explanations:

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Reply to
Rich256

Hi, Sounds like you are confused between real 3 phase(star ot Y) and Edison circuit. 2 phase? Maybe bi-phase? Draw them out on a piece of paper then you'll wee what is what. Tony

Reply to
Tony Hwang

Hi, Oops, Star = Y or delta. Tony

Reply to
Tony Hwang
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 phase).

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!

Reply to
Bill

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.

Reply to
Jeff

When all three lines are used,it is three phase: A-B, B-C, and A-C

Reply to
RBM

So, if I wanted to refer to three phase, where L-N was 120v, I would call it

208v 3 Phase since a-b, b-c, and a-c are all 208v?
Reply to
Toller

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 etc.

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 the sinusoid).

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 breaks.

In practice, 3-phase motors can be cheaper, smaller, quieter, easier to start, and run cooler and more efficiently for a given HP.

Reply to
Beachcomber

And just as he begins to understand the three phase wye, we'll throw in a delta with a wild leg!!

Reply to
RBM

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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Reply to
Bill

I doubt that you would see a building that has 120 V outlets and a tiny return. Too much of a chance of loading one phase.

But when the loads are hard wired to 3 phase the return can be small or not in existance. I remember one military application where the power was a grounded delta.

Reply to
Rich256

This is Turtle.

Hummmmmm, i see Pie are round and Corn Pond Are square.

TURTLE

Reply to
TURTLE

Reminds me of a boy downhome in the Missouri Ozarks, his folks sent him off to college to get him educated. Since they had spent so much money on him, his dad wanted him to impress the locals, so when he came home, his dad said "Say something smart, Junior!" Junior thought a while and said "Pie are square" Dad was totally humiliated, "Damn fool is stupider now than when left, everyone knows pie are round, cornbread are square".

Dan

Reply to
Dan

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.

Comments?

Jeff

Reply to
Jeff Wisnia

Synergy is the word for 3 phase. The three wires working in concert is what gives you a gain in efficiency.

Reply to
Bill

Reply to
Don Young

Reply to
Don Young

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.

bud--

Reply to
Bud--

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.

Jeff

Reply to
Jeff Wisnia

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