# three phase connection, RMS, advantages

Discussion in 'Homework Help' started by PG1995, Apr 17, 2011.

1. ### PG1995 Thread Starter Well-Known Member

Apr 15, 2011
813
6
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.

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.

Regards
PG

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
I wouldn't fancy living / working there if that is in fact the case - perhaps you mean there are residual current device (RCD) or Earth Leakage circuit breakers used in every room.

3. ### PG1995 Thread Starter Well-Known Member

Apr 15, 2011
813
6
Yes, this device:

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.

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
As you say in your earlier post your knowledge in this area & electrical matters in general is limited. At this stage you probably don't have sufficient understanding of your electrical installation to make statements concerning the earthing which are probably quite wrong.

Put the issue of earthing aside for the moment.

As to answering questions related to AC and the like you are asking about things which are written about in detail in books and on a multitude of web sites. Rather than asking forum members to simply confirm what you have been told or read elsewhere, you might pose your questions in a way that might attract some comment. That is, comment other than a simple "yes" or "no".

Asking someone to confirm that the RMS value of an AC quantity produces the same heating effect as the equivalent DC value, is a case in point. The simple answer is - yes. Unfortunately (from your perspective) it requires some knowledge of mathematics to prove this on paper. You can of course confirm this in a practical way by direct measurement, but may then have to explain why some discrepancies in the experimental results might have occurred. For example the load used to make the measurement of power drawn from supposedly "equivalent" AC and DC sources may not be a pure resistor - it could have some inductance, thereby introducing additional complexity in the physical behavior. The experimenter would need to make a carefully considered approach in conducting the experiment to prove equivalence of the two quantities. The experimenter also need to understand why the instruments used to take such measurements in the two cases can be trusted.

Your question about a 3-phase system being equivalent to 3 DC systems in parallel is reasonable. It invites further comment. You might gain some insights by reading about the origins of AC and DC power and the historical and technical reasons for the adoption AC as the prime (but not exclusive) means of electrical power distribution. There are economic & technical reasons why 3-phase AC distribution and powering of electrical devices (machines) is used rather than single phase. I'm not sure how much detail you are after ....

Good luck with your pursuit of knowledge - you obviously have an inquiring mind and a genuine desire to learn. Both admirable qualities. Keep asking your questions.

5. ### PG1995 Thread Starter Well-Known Member

Apr 15, 2011
813
6
Well, Sir, I'm genuinely impressed and encouraged by your words. Thanks a lot. People like you make this world a good place to live in, otherwise there is a lot of junk out there!

Yes, I do understand there are ample of books, and websites full of information on any topic one can come up with. But we all learn in different ways. If Google and the internet had answers to everything then there wouldn't be any need for forums like this. There wouldn't be need to write new books. And for a boy like me whose first language is not English (not even second!) the internet could be confusing place. Yesterday I was reading a Wikipedia article it took me more than two hours to finish it because I had to consult the dictionary several times. Yes, I did learn many new words!

I hope you understand that I'm a beginner so the things which seem very simple and plain to you are very difficult and obscure for me to understand.

Best regards
PG

6. ### Georacer Moderator

Nov 25, 2009
5,181
1,289
t_n_k wrote an extensive and almost complete post about the things you should turn your attention to and sources of knowledge.

He talked about how AC and DC systems could be compared in terms of voltage and power.

What I would like you to also notice is that there are applications where a 3-phase power supply is a must and can't be substituted with a single AC line or 3 DC supplies.
One of these are the AC electrical motors. AC motors have low consumption and high robustness compared to DC motors but generally require a multi-phase system to operate.

DC and AC are two different continents of the same planet and you should learn about both of them if you want to explore it thoroughly.

7. ### PG1995 Thread Starter Well-Known Member

Apr 15, 2011
813
6
Thank you, Georacer.

I will ask some follow-on questions soon.

Best wishes
PG