Originally Posted By: bbadger This post was automatically imported from our archived forum.
rbennett wrote:
Joey and Bob
One can not get 2 phase out of a 3 phase box by dropping off one phase
2 phase can be gotten by fancy wiring of transformers
Any help out there from some one that has a short explanation of this issue??
RLB
First lets make clear that three phase has phases A, B and C, if I use only two of these phases I have "single phase" not two phase.
True "Two Phase" is very unusual I have never seen it and I doubt you all will see it. Until recently I had believed you needed a two phase generator but I have learned it can be done with some unusual transformer connections.
That is all I will say about "Two Phase" systems.
Now forgetting two phase systems and going back to Jeff's building, the use of three phase panels in each unit is odd.
For a building of this size ....
...you would definitely want a three phase service for two major reasons.
1) The power company distribution system is three phase if you installed a 1000 to 2000 amp single phase service from a three phase distribution system you would create a current imbalance that is detrimental to the power company's equipment. (They normally would not even consider it.)
2) Capacity. When we talk about capacity we should really use KW (Kilowatts) as using "Amps" does not really show the capacity, let me explain.
A standard home service 240/120 @ 200 amps has a capacity 48 KW (48,000 watts)
A three phase 208Y/120 @ 200 amps has a capacity of 71 KW
A three phase 480Y/277 @ 200 amps has a capacity of 166 KW
All the above are 200 amp services but the capacity in watts ranges from a mere 48,000 watts to 166,000 watts.
Now back to the building in the picture, in this area I would expect a 1000 to 2000 amp 208Y/120 (the Y means 3 phase) service to this building depending on the heating method, electric, gas, oil etc.
We would use the three phase to power the elevators and any large HVAC equipment.
However the feeders to each unit we would be run single phase in a rotating basis to keep the system in balance.
Like this
Unit 1 uses phases A and B
Unit 2 uses phases A and C
Unit 3 uses phases B and C
And on we go like that for however many units there are.
I can not think of any reason to bring 3 phase right to the units, obviously the engineer had a reason as it was done.
I sure would like to know why, 3 phase equipment is much more expensive than single phase equipment.
Jeff do you recall if there where any 3 pole breakers installed in the unit panels?
About the only thing I can think of is each unit has a Air conditioner unit that is 3 phase.
-- Bob Badger
Electrical Construction & Maintenance
Moderator at ECN
Originally Posted By: bbadger This post was automatically imported from our archived forum.
jtedesco wrote:
Bob:
Am I missing something?
I always thought that "balancing" the load would be described by my revision?
Quote:
Unit 1 uses phases A and B
Unit 2 uses phases A and C
Unit 3 uses phases B and C
No your not missing anything and either way is fine.
I actually typed up my reply at about 6 AM this morning, I was unable to post it this morning as the NACHI board was experiencing technical difficulties.
When I finally got back to posting it I guess you already had it covered.
No offense intended, I did not see or read your post.
Bob
-- Bob Badger
Electrical Construction & Maintenance
Moderator at ECN
Originally Posted By: Joey D’Adamo This post was automatically imported from our archived forum.
I’m not an EE nor an EC but I’m not sure I see how two phases of a three phase supply is the same as single phase. For one, it’s not completely balanced. In a regular home 120/240V, you have one phase with the neutral from the center tap of a transformer, and they are 180 deg apart. In the two legs from a three phase case, they are 120 deg apart, and that will leave a 240 deg gap where the third phase would’ve gone. So how can that be single phase? I’d say its two phases of a three phase supply. Which isn’t two phase either.
Originally Posted By: rbennett This post was automatically imported from our archived forum.
Joey
Don't want to re-invent a very good wheel so I don't want to write a lesson plan on how transformers work. The people that are the masters of education can do that.
So here is the short answer. Like most things it is a matter of communication.
1. As to US residential 240-120 service, if the wave forms were 180 degrees out of phase they would equal zero volts when the two hot leads are used to get 240 (220) volts.
2. Depending on where one establishes ones reference point, phase relationships can be almost anything.
3. Terms like "in or out of phase - phase shift - voltage-current phase relationship (power factor) - split phase - etc." can get in the way of basic understanding.
4. Electrical knowledge for the HI should not that of an electrician. When it is we are in a heap of trouble.
5. Electrical standards are a must primarily for the success of our economy (Single phase products for one market - three phase products for another) Two phase products are very hard to find in the US.
Originally Posted By: Greg Fretwell This post was automatically imported from our archived forum.
Joey, the 2 phases of a 3 phase look like single phase because there is no reference to tell them any different. The voltage delta will still rise and fall at 60 hz. The only real difference is, you will only see a 208v swing instead of 240.
If you really want to see something unusual take a peek at corner grounded delta. There you have 3 phase using single phase equipment, 2 hots and a ground, but it is 3 phase.
Originally Posted By: Greg Fretwell This post was automatically imported from our archived forum.
I just remembered another fairly common configuration that is somewhat mind blowing.
You can have center tapped delta (AKA wild leg or red leg)
They take a basic transformer like you are used to seeing then add a second transformer fed from another primary leg wired in an open delta on the output.
The first transformer looks like your normal 120/120/240 single phase to the load. If you use those 2 legs with the output of the second transformer you get 240v delta 3phase … from the same service.
You may see this in areas with light industrial bays, mixed with small mom and pop commercial and residential.
Naples Fla has a lot of it. The tip off is 2 transformers feeding a 3 phase 4 wire service. One (the center tapped one) may be larger than the other since it serves all the 120v loads along with the 3p load. They call it wild leg because the 3d leg is 208 above ground.
Originally Posted By: roconnor This post was automatically imported from our archived forum.
Joey … to try to keep it short and simple, a 3-phase supply will have three (3) hot wires where you can get three separate high side voltage measurements between the hot wires (e.g. A to B, B to C, and C to A).
If you take two of those hot wires wires (say A and B) there is only one high side voltage to measure ... therefore it's "single phase" even though you are using two of the three hot wires.
Originally Posted By: jburkeson This post was automatically imported from our archived forum.
bbadger wrote:
First lets make clear that three phase has phases A, B and C, if I use only two of these phases I have "single phase" not two phase.
True "Two Phase" is very unusual I have never seen it and I doubt you all will see it. Until recently I had believed you needed a two phase generator but I have learned it can be done with some unusual transformer connections.
That is all I will say about "Two Phase" systems.
I have seen it installed in some urban sections of central New Jersey (North Plainfield), I do not believe it had anything to do with the way it was generated because both 2-phase and 3-phase were available in the same neighborhood. The configuration was 2-transformers on the pole and 4-wires consisting of 2-hots & 2-neutrals 120/240.
All I really remember about 2-phase was that there were about 6 questions on it when I took the NJ licensing test back in 85'.
-- Joseph Burkeson, RPI (Hooperette)
?Anyone who has proclaimed violence his method inexorably must choose lying as his principle.?
~ Aleksandr Solzhenitsyn
Originally Posted By: pdickerson This post was automatically imported from our archived forum.
Joey D.,
I understand this stuff reasonably well, but delivering a concise and understandable explanation...well, it might be educational for both of us.
A piece of equipment cannot tell the difference between two hots 180 degrees out of phase (as in a center tapped xformer supplying power to a typical residence) or two hots 120 degrees out of phase (as in two phases of a 3 phase power supply. Let's ignore the fact that the RMS voltage between the two examples will likely be different (240 and 208 respectively).
The piece of equipment only knows that the voltage difference between the two hot wires is the correct magnitude and is fluctuating 60 times a second. That's all it cares about.
Hypothetically, you could take a standard 120 volt outlet (hot, neutral, gr), run the hot through a 2:1 transformer to give you a 240 volt hot, and operate a 240 v single phase appliance using the 240 volt hot and the neutral. The appliance doesn't care what the actual voltage on each wire is, it only sees the relative voltage between the two wires.
In Europe and most of the rest of the world, a standard outlet has a ground, a neutral, and a 240 volt hot. You can power a European coffee machine at your home in the U.S. by supplying it the 2 120 volt hots and ground from your panel. The coffee machine is happy either way.
Originally Posted By: Joey D’Adamo This post was automatically imported from our archived forum.
Thanks for all of the explanations, all! I did some side research in addition to your comments and I think I get it now.
I kept thinking in terms of motor loads and figured the phase angles would be "unbalanced" and mess up a "single phase" motor. But I did some research and it turns out a single phase 240v motor runs just fine on 208v with a boost buck transformer to get it up to 240v without changing the phase offset between the two hots.
So I get it now... thanks everyone! 