three phase connection, RMS, uses

[PG1995]

Hi

We have a single phase connection. It's 220V. I think it's RMS value so the peak voltage would be around 311V. Two wires are used for this single phase connection (forget the earth wire we don't use it here, we use circuit breaker for every room). I don't know anything about three phase connection. I want to when such a three phase connection is required and what its advantages are as compared to single phase. Please remember that I'm a beginner so please try to explain things using math formulas. I'm after conceptual understanding. Thanks.

RMS value is a kind of 'average' voltage. If RMS is 220V then that means it would deliver the same amount of power as a 220V DC supply. Correct?

The following picture shows that Phase 2's peak is 120° out of phase from the Phase 1's and Phase 2's.

**broken link removed**

All the three phases (three voltage waves) have the same peak voltage which means same RMS value. If the peak voltage for all the phases is 311V then the RMS would be 220V. Correct? Then, doesn't it look as if we have three 220V DC supplies in parallel? Please tell me.

How many wires are used for three phase connection ignoring the ground wire?

I have been told that three phase connection is used for power hungry devices. If we have three batteries in parallel then they have the capacity to deliver more power than a single battery.

Please guide me with the above stuff. Thanks a lot for your help and your time.

Regards
PG

PS: Are you thinking why don't we have the ground wire? A circuit breaker is used in every room. So, if someone accidently touches the wires/some electric device create a short circuit the circuit breaker will shut off the current by tripping off. Why don't they use the ground wire? I believe they do it to save money. The electric company would need only two wires instead of three by not using the ground!

 

[carbonzit]

You've got part of it, but some of your posting is so confused I can't really tell what you know.

In a nutshell: 3-phase AC power, as the term suggests, consists of 3 separate phases of the same frequency (50 or 60 Hz, typically). The phases are spaced 120° apart, which makes sense: 360/3 = 120.

A 3-phase feed requires 3 wires.

The advantage of 3-phase over single-phase power is generally for rotating magnetic devices, like motors. Instead of two "pushes" per rotation, as with single-phase, there are 3 "pushes" with 3-phase.

There are lots of pages out there on the web that explain how this stuff works. You also need to know about the two ways of connecting 3-phase devices ("delta" and "wye").

And no, if you don't have 3-phase power (which you won't if you have regular residential service), you can't use it. There are 3-phase converters available, but it would be hard to think of a reason you'd actually need 3-phase power, unless you're operating some kind of really big power machinery in a workshop.

Your description of grounding is completely off-base. The power company, in fact, only supplies 3 wires to your house for residential service (2 "hots" of 120 volts and a "neutral", in a 120-0-120 arrangement). They don't supply a ground at all. That's up to the electrician who wires the house, who's supposed to install an approved grounding device, generally a rod driven into the ground, as well as bonding the ground to any metal water (not gas!) pipes entering the house.

Again, you should read up on this. Tons of stuff out there.

 

[MikeMl]

...
The advantage of 3-phase over single-phase power is generally for rotating magnetic devices, like motors. Instead of two "pushes" per rotation, as with single-phase, there are 3 "pushes" with 3-phase...

I would have said there are 6 pushes per revolution for 3Φ vs 2 pushes for singleΦ.

 

[carbonzit]

I would have said there are 6 pushes per revolution for 3Φ vs 2 pushes for singleΦ.

Yepper. That is correct.

 

[crutschow]

As carbonzit noted, 3-phase power is used for larger electrical motors as it provides a rotating field and constant power, which 1-phase does not provide (thus you need a starting method for single phase motors that a three phase motor doesn't require).

The continuous power characteristic of 3-phase power is interesting. It turns out the sum of the instantaneous power levels of the three-phases is a constant (DC) value. This means there are no power fluctuation in the output of a three phase motor (the peak strength of the continuously rotating magnetic field in the motor has a constant value). There are no individual pushes per revolution, it is a continuous push. Thus a three phase motor is smoother and quieter than a single phase motor.

The ripple voltage of rectified 3-phase is also much lower that rectified single phase so it's better when you need a large amount of DC power from the mains. This is also one of the reasons that automobile alternators are all 3-phase.

A general advantage of 3-phase power from the power company's point of view, is that it requires the least amount of copper wire to transmit a given amount of AC power. Only DC transmission requires less copper.

 

[PG1995]

Thanks a lot, carbonzit, MikeMl.

carbonzit: In a single phase connection at any instance when in one wire electrons are pushed, in the other wire the electrons are being pulled. At any time one wire is sucking in the electrons. It's all about push and pull of electrons.

In the three phase, the Phase 1 wire is at its peak of 'pushing' which wire would be at its peak 'pulling'?

Regards
PG

 

[carbonzit]

The ripple voltage of rectified 3-phase is also much lower that rectified single phase so it's better when you need a large amount of DC power from the mains. This is also one of the reasons that automobile alternators are all 3-phase.

As Johnny Carson used to say, "I did not know that" (that car alternators are 3-phase). Thanks for the info.

 

[carbonzit]

In the three phase, the Phase 1 wire is at its peak of 'pushing' which wire would be at its peak 'pulling'?

Good question. For the answer, just look at your waveform diagram and look at what Phase 2 and Phase 3 are doing when Phase 1 is at its positive peak.

3-phase is pretty strange and wonderful stuff, for such a simple concept ...

 

[RCinFLA]

Main advantage for three phase is lower wiring current for same net VA load. Used mostly for industrial loads. From an alternator or motor perspective, it yields a near constant shaft mechanical torque load.

At residential end, the three phases are generally split up to serve different areas (block, road, etc.). Power company try to balance the load from the split off areas as best they can. Typical residential distribution is 7 kV or 12 kV lines that feed a local transformer serving a few homes.

In USA, the HV to 240vac transformer is an auto-transformer winding configuration with a center tapped secondary of 240 vac. In Europe there is no center tap just straight 220vac-240 vac delivered to residence. The USA arrangement has the center tap called the neutral connection. There are 240 vac loads like pumps, electric range, electric dryer, air conditioners, etc. The 120vac outlets are one side of the 240 vac line to neutral center tap providing 120 vac. The breaker box should try to distribute the 120 vac branches to balance the loads on either 240 vac lines. The more the 120 vac loads are balanced on either side to center tap neutral, the lower the neutral return wire current back to the transformer.

If you have overhead service lines, the neutral line is bare uninsulated support cable with the two 240 vac insulated wires wrapped around it. Neutral line is grounded at a single point near your service entrance.

The lower outlet socket voltage of USA is considered a bit safer then Europe's 240 vac outlets and arguably provides a bit better lightning damage avoidance. Most of Europe is 50 Hz versus 60 Hz for USA. 60 Hz is an advantage to transformer core size for a given power.

 

[crutschow]

In the three phase, the Phase 1 wire is at its peak of 'pushing' which wire would be at its peak 'pulling'?

In a 3-phase connection there is no one wire "pulling". It's a combination of the other two wires based upon the relative voltage between the three sets of wires.

If you use just 2 of the 3 wires to generate single-phase then the push and pull is obviously just between those two wires.

 

[KeepItSimpleStupid]

Single phase 240/120 VAC residential in the US does have two components 180 degrees out of phase relative to a common terminal. For 2 phase to be useful, you would have to remove the common terminal which you cannot do. The power company feed is single phase. The center tapped transformer does the wierdness.

FWIW Two phase power did exist at one time in the US and my friend actually has a 2 phase motor from where he used to work. Three phase residential does exist, but it's very very uncommon. A DC distribution system existed for a short time. The utilities may use DC power transmission especially for underwater applications. AC creates too much heat in the outer protective metal shield.

 

[crutschow]

Single phase 240/120 VAC residential in the US does have two components 180 degrees out of phase relative to a common terminal. For 2 phase to be useful, you would have to remove the common terminal which you cannot do. The power company feed is single phase. The center tapped transformer does the wierdness.

Two components 180 degrees out of phase is not two-phase power. The power to the home is from a standard center-tapped transformer output. Any center-tapped transformer output gives two components 180 degrees apart when referenced to the center-tap. No weirdness there.

Two-phase power systems have two separate lines with the voltages 90 degrees apart. That gives a rotating component to the motor magnetic field so a two-phase motor would be self-starting.

 

[KeepItSimpleStupid]

I totally agree with what you said. Clarification welcomed. I've seen people mistakingly try to refer to the 240/120 system as two phase because of the out of phase "components" with the center tap as the reference. The main point I'm trying to make is that the power company provides a high voltage single phase feed, but you get two 180 degree out of phase 120 lines with respect to neutral/ground.

 

[carbonzit]

I totally agree with what you said. Clarification welcomed. I've seen people mistakingly try to refer to the 240/120 system as two phase because of the out of phase "components" with the center tap as the reference. The main point I'm trying to make is that the power company provides a high voltage single phase feed, but you get two 180 degree out of phase 120 lines with respect to neutral/ground.

I'll say this with great trepidation, because I've seen this subject raised elsewhere and it's resulted in huge arguments.

Technically speaking, ordinary household power is 2-phase power. It has to be: there is one set of conductors (left high side & neutral) that is 180° out of phase with the other set (right high side & neutral). If that isn't the very definition of two phases, I don't know what is, and furthermore, I'll eat my hat (as they used to say). Doesn't matter that the two phases are trivially derived from a center-tapped transformer.

Now, traditionally, this is never referred to as 2-phase power. No electrician in their right mind would ever call this 2-phase power. And they'd give you funny looks down at the electric company if you used that term.

What is called two-phase power is that extremely weird version, now practically obsolete, where there's a second phase that's 90° out of phase with the other one. Very strange, not very useful, and most people have never heard of it. (It was mentioned here earlier, I believe.)

But all our household power does have two separate and distinct phases, even though nobody refers to it that way. So I won't argue the point further.

 

[KeepItSimpleStupid]

Good job carbonzit!

 

[crutschow]

I'll say this with great trepidation, because I've seen this subject raised elsewhere and it's resulted in huge arguments.

Technically speaking, ordinary household power is 2-phase power. It has to be: there is one set of conductors (left high side & neutral) that is 180° out of phase with the other set (right high side & neutral). If that isn't the very definition of two phases, I don't know what is, and furthermore, I'll eat my hat (as they used to say). Doesn't matter that the two phases are trivially derived from a center-tapped transformer.

Now, traditionally, this is never referred to as 2-phase power. No electrician in their right mind would ever call this 2-phase power. And they'd give you funny looks down at the electric company if you used that term.

What is called two-phase power is that extremely weird version, now practically obsolete, where there's a second phase that's 90° out of phase with the other one. Very strange, not very useful, and most people have never heard of it. (It was mentioned here earlier, I believe.)

But all our household power does have two separate and distinct phases, even though nobody refers to it that way. So I won't argue the point further.

It always starts an argument when someone wants to redefine a word differently from the normal accepted definition. So here goes:

Technically speaking, that is not correct. If you can find me any technical paper that describes the single-phase power coming into the house as 2-phase, I will eat my hat. You can call the two waveforms 180 degrees out of phase 2-phase, if you like (everyone is welcome to their own opinion), but that is not an accepted definition of 2-phase power. Technically, inverting the phase of a signal does not make it a 2-phase signal.

2-phase power, traditionally or otherwise, always means two waveforms 90 degrees out-of-phase. The reason is, that it provides a rotating field for motors. It wasn't just used in obsolete power system, it was also used in mechanical servo systems used before solid-state control was invented.

 

[Reloadron]

I have always refered to the power entry to my house (Cleveland, Ohio, USA) as being a single split phase and as to the thinking on it .....

A split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications. It is the AC equivalent of the original Edison 3-wire direct current system. Its primary advantage is that it saves conductor material over a single ended single phase system while only requiring single phase on the supply side of the distribution transformer.[1] Since there are two live conductors in the system, it is sometimes incorrectly referred to as "two-phase". The two live or "hot" conductors waveforms are offset by a half-cycle, or 180 degrees offset, when measured against the neutral wire. To avoid confusion with split-phase motor start applications, it is appropriate to call this power distribution system a 3-wire, single-phase, mid-point neutral system.

Not to say the Wikipedia definition definition of stuff like this is always correct but that is how I see it.

Anyway the OP did mention:

In the three phase, the Phase 1 wire is at its peak of 'pushing' which wire would be at its peak 'pulling'?

No. However, if for example Phase A or Phase 1 is at Epk the negative slopes of Phases B&C or 2&3 will be equal and their sum will add up to the Phase A or 1 peak but negative. Look at the image of three phases. If phase 1 is peaked at for example 1.0 at that point in time phase 2 and 3 are both at -0.5.

Ron

 

[carbonzit]

It always starts an argument when someone wants to redefine a word differently from the normal accepted definition. So here goes:

Technically speaking, that is not correct. If you can find me any technical paper that describes the single-phase power coming into the house as 2-phase, I will eat my hat. You can call the two waveforms 180 degrees out of phase 2-phase, if you like (everyone is welcome to their own opinion), but that is not an accepted definition of 2-phase power. Technically, inverting the phase of a signal does not make it a 2-phase signal.

Again, and without really wanting to start a war here, please keep in mind that I'm not trying to redefine any cherished, traditional terms here. I took pains to point out that nobody calls residential electric service 2-phase, OK? (The term most often used is "split phase".)

2-phase power, traditionally or otherwise, always means two waveforms 90 degrees out-of-phase. The reason is, that it provides a rotating field for motors. It wasn't just used in obsolete power system, it was also used in mechanical servo systems used before solid-state control was invented.

OK, let me try this a slightly different way. Again, yes, what you describe is what is traditionally called 2-phase power.

So instead of the 3 wires coming into your house, let's look at another example. Take any ordinary push-pull amplifier with an inverter stage. Look at the signals emanating from that stage, which are 180° apart from each other. Can we not agree that in this case, we have two separate and distinct phases?

Now replace that inverter stage with a center-tapped transformer. Again, we have two signals 180° apart from each other coming from the secondary of that transformer. Again, do we not have two separate and distinct phases?

Now look again at those three wires coming from the transformer outside your house. Two separate and distinct phases, no?

Yes, yes, yes; everyone calls this system "split phase". I promise I will never try to call this "2-phase power", OK? But can I get you to admit that there are, in fact, two separate and distinct phases of power entering your home, 180° apart from each other, even though this is not called 2-phase power?

The most ridiculous argument against this I've heard basically goes something like this: "Well, they would be two separate phases, except that because they're only created from a center-tapped transformer, they're not really two phases." In other words, there's a mathematical "hole" in the concept of phase precisely at 180°. (Now, I might accept that there's such a hole at 0°, except not really: this is the "identity" phase.)

Do you see what I'm getting at here? It's that there are in fact two phases, but for historical reasons and long-standing customary usage, we cannot refer to it that way.

 

[crutschow]

Carbonzit -- OK. Peace. I believe that the term 2-phase should be reserved for phases other than 180 degrees apart to avoid confusion (and I believe you are with me on this). But certainly two signals 180 degrees "out-of-phase" can be said to have two different phases.

As far as a "hole" at 180°, there may not be a mathematical one but there is a unique electrical one. 180° phase shift can be generated basically independent of frequency by a simple inverting amplifier or a transformer. But any other value of phase shift requires a frequency dependent circuit or a rotating generator. So that's why I tend to think of a 180° phase shift as fundamentally different from other values of phase shift.

 

[MikeMl]

Even though there may be a 180deg phase difference between L1-N vs L2-N, who cares? Nobody uses it that way!

Loads are connected either L1-N or L2-N or L1-L2. All three of those are "single phase"!