Secondary of transformer
Actually it is single phase there is no phase shift. If you think of the secondary windings as reversible pumps. The pumps run the same direction at the same time and switch at the same time. The two pumps start from a stop, pump one direction until the pressure reaches 120 RMS, them slows to a stop reverses direction and pumps in the opposite direction to 120 RMS. Now if we measure the difference in pressure from 1B to 1A and the difference in pressure from 2C to 2B we would see the pressure go from 0 to 120 RMS to 0 to -120 RMS on both loops.. Now you tie the two pumps to gather at B. now when pump 2 pumps the pressure up to 120 RMS that is applied to the inlet of pump 1 at the same time it is pumping in the same direction so it applies 120 RMS so point then goes to 240 RMS. The pumps reverse and you get the flow and pressure in the opposite direction. If we reference the pressure difference from point B to A and point B to C point A appears to be going to + 120 RMS at the same time as point C is going to -120 RMS and vice versa, even though the pumps are pumping in the same direction. If a load is connected from point A to point C it sees a pressure difference of 140 RMS while loads connected from point A to B and points C to B only see a pressure difference of 120 RMS.
Be sure you live your life, because you are a long time dead.-Scottish Proverb
Holy mackeral, all you guys never cease to amaze me with the depth of your knowledge.
Jack ... I get what you're saying .... I did one mistake there is no phase shift between the primary and the secondary.
the way I figure it we are using two 120 volt supplies in series to produce the 240 volts along with the center tap to drain the difference between the 2 unbalanced loads which creates a split phase.
Which would be aiding ( boosting ) each other resulting in the 240 volts.
This means that one split 120 volt leg would have to be 180 degrees out of phase with the other 120 volt leg. Otherwise they would be opposing ( bucking ) each other resulting in 0 volts.
So mathematically the polar equation :
(120v @ 0degree) minus (- 120v @180 degree) = 240 v @ 0degree.
Just a thought.
Last edited by canuk; 09-11-2008 at 07:11 PM.
"" an ounce of perception -- a pound of obscure "
I wish member djohns would sound in here because distribution is his gig but he’s been working a ton of hours in LA.
As far as phase angles go my guess is that it has a lot to do with how a utility distributes power and the types of transformers they use.
My office is fed overhead with three phase and three pole set transformers feed our service from three overhead phases that I know are 120 degrees apart.
The city block is fed from lines that run down the alley. One half is commercial three phase but the other side of the block is residential single phase.
If you look down the alley each single phase house service is fed with one “pot” or transformer fed with two alternating phases as in AB, next BC, next AC and so on.
Given that I know that each houses service has two hots that are 120 degrees apart. This means that the transformers are fed off of a three phase delta supply.
If your having trouble understanding electric speak just think of it this way:
A generator rotates in a full circle and a full circle contains 360 degrees. So for three phase draw a triangle inside of the circle. That is the symbol for “delta” . Then 360 divided by 3 is 120 degrees. Each point were the triangle meets the circle is where the generator is tapped to send power out to the distribution system
Keep in mind that you can tap a winding anywhere you want and ground it. For single phase it is a center tap as has been pointed out here. The “neutral” is not carried on the transmission lines. It is created by center tapping the transformers secondary winding and grounding it.
rabbitgun ... sure sure ... I was just trying to make it simpler with a single high voltage line and you had to add in the 3 phase delta or wye configuration
"" an ounce of perception -- a pound of obscure "
that was interesting.
we have Y or wye transformers that feed the power here to the services. i'm told that delta is not nearly as safe and is not usually used for distribution of the lower voltages for homes.
Well....I started this thread with my question, and I am totally overwhelmed with the depth of knowledge of the members responding. Thanks to all who have responded!
Bsum1 said it well...holy mackerel, you guys are very knowledgable indeed!
So...here is a photo of the power pole and transformer outside my house (see attached). Obviously, the very top wire is the high-voltage line...somebody said it was about 7000 volts or so. This I assume is the single phase distribution from the power grid. The transformer is obvious with the 3 lines for distribution to the houses (2 split single phase hots and a center-tap neutral)....so far so good. I get it this far.
What I can't understand is why there is only one high-voltage line at the top of the pole. The schematic diagrams drawn earlier show the primary winding of the transformer with two wires on the high voltage side...where is this second wire? Rabbitgun alluded to this by saying that the neutral is not carried on the transmission lines, but I don't understand how this is possible.