Electrical question

Stephen Newell

Well-known Member
If the power lines from the utility just have two wires as shown, how does the transformer divide two lines going into your house out of phase with each other. It looks to me like if they could divide it out like that they could divide it again and give you three phase if you needed it.
a172803.jpg
 
Primary coil in transformer has two terminals, one connected to each one of the highline.

Secondary coil has a center tap as well, this is the neutral. End taps produce two 120v terminals reference center tap, and 240v across end to end.
 
Three-phase power consists of three phases 120 degrees out-of-phase with each other. Single-phase legs are 180 degrees out-of-phase. A regular transformer will not produce three-phase.
 
Two hot equal 220 volts coming in and then earth ground. If you look at every pole the pole has a ground wire on them and it goes up to the ground/neutral wire of the pole
 
A transformer can not create phases. There is some confusion here as the secondary of the pole transformer is sometimes called "split-phase" but it is only a center-tapped secondary giving you 120 VAC and 240 VAC.

"a split single-phase is not a two-phase system".
In order to get three-phase power, you need three phase hot wires (here in front of the house it is about 12470 Volts from any hot wire to any hot wire and 7200 Volts from any hot wire to neutral). And then three transformers would be needed. So if you do not have three hot wires on your power poles, you can not get three phase power (unless you get a rotary converter or a VFD).
Please be very careful if you ever see any downed hot wires.

DeereMark recently retired EE though never touching any Voltage higher than 480 (at least not on purpose).
 
That two wire HY Primary service is ONLY Single Phase, it could be 9600 or so volts across Hot to the Grounded Neutral......... The Transformer serves two houses simply by their being in parallel at the Transformers LV Secondary output..........The Transformer takes the two wire 9600 HV Primary input and steps it down to 240 VAC Single Phase Line to Line...........However that 240 volt LV Secondary winding is center tapped such that its only half or 120 VAC either Line to the center tap..............The Center tap is bonded to a Grounding Electrode Conductor (NO 4 Bare Copper) and also the can and maybe guy wires etc which runs down the pole to a Grounding Electrode (driven into earth copper ground rod) and is now referred to as the Neutral and/or the Grounded Conductor, while the two outer Hots are referred to the UNgrounded Conductors............Its 240 VAC Like to Line and 120 VAC either Line to the center tap Neutral..........The home service is referred to as 120/240 Volt Single Phase Three Wire

Hope this helps

John T Long retired electrical power distribution design engineer
 
(quoted from post at 09:24:43 09/24/17) If the power lines from the utility just have two wires as shown, how does the transformer divide two lines going into your house out of phase with each other. It looks to me like if they could divide it out like that they could divide it again and give you three phase if you needed it.
a172803.jpg

That photo had me confused for a minute, in my area the bottom line is always the neutral and anything above that is a hot line.
 
I'm sorry but I'm not sure I understanding the concept of center tap. I had always thought you had two separate hot wires coming from the utility which the power was reduced to 120v at the transformer. This is very confusing getting two wires out of phase from each other off of one high voltage line.
 
Nope, you have one Hot UN Grounded Conductor and one Neutral Grounded Conductor single phase, typically 9600 or so volts with respect to each other on that aerial feed. At every third pole we used to run up a No 4 bare copper wire from a driven ground rod and bond to the Neutral to keep the utility grid at a common low voltage reference IE mother earth potential

Nowwwwwwwwww imagine a LV Secondary winding that steps that down to 240 volts single phase line to line SO FAR SO GOOD

Now right in the dead center of that LV Secondary Transformer winding T in a tap wire.

As its in the dead center then its only 1/2 or 120 from that center tap out to either end.

To the house you run the 2 hots and the Neutral and you have 2 legs of 120 (each 180 out of phase with the other) with respect to the center tapped Neutral and 240 line to line

Got it??

John T Retired electrical power distribution design engineer
 
We ALWAYS had the hot on top so if it broke it fell on the Neutral so the fuse or breakers tripped so there wasnt a hot laying on the ground

John T
 
I think I get it, its the amount of time it takes for the electricity to pass from one side of the winding to the other which makes it go from positive to negative.
 
No, the alternating current is caused by the rotor on the alternators at the power plant switching polarity as they rotate. Automotive alternators are three phase, they just have full wave rectifiers to convert to DC with a ripple. The ripple is smoothed out by the battery.
 
Our REA has neutral on the bottom...I think public utility does as well. One reason I like that is when moving grain bins that might touch, I don"t get electrocuted! But I do run a 2x4 up and over the bin, down the backside. Just in case...plus the wire won"t snag and break. Same with phone lines. Here, REA neutral has 24 foot clearance on most roads.
 
(quoted from post at 22:50:02 09/24/17) I think I get it, its the amount of time it takes for the electricity to pass from one side of the winding to the other which makes it go from positive to negative.
orry, but no cupie doll for you!
 
The Pos to Neg swing is because a coil of wire is passing by a magnetic field (at the generating station) which induces a voltage, and since it keeps passing by a North magnetic Field and then a South magnetic field (changing MAGNETIC polarity), in turn the voltage keeps changing ELECTRICAL polarity.

John T
 
AHHHHHHHHHHHH a pictures is worth a thousand words lol THANKS FOR POSTING IT. Now add to that picture a connection at that center tap to a big No 4 bare copper wire (Grounding Electrode Conductor) which runs down the pole to a driven in earth ground rod (Grounding Electrode) and you have a Grounded Conductor IE a Neutral ..................while the two outer Hots are referred to in the trade as UNgrounded conductors.

John T
 
A NOT so recently retired EE here. Our 3 phase HV Primary is also 12,470 Y 7200 Volt Three Phase Four Wire. Our typical Single Phase service is 9600 volt. Home service, of course, is 120/240 Volt Single Phase Three Wire. WE had a lot of 240 or 480 three phase three wire floating Delta at our facility, but I preferred a grounded system so installed a 240 volt three phase CORNER GROUNDED delta one time and the electricians complained saying one phase was dead WELL DUH lol

Nice chattin with you Sparky

John T BSEE, JD
 
(quoted from post at 11:53:51 09/25/17) I was always told that the phase is "derived" from the transformer. One in two out -- makes sense .
orry, but you were told wrong. Transformer with single phase in will be single phase out, yes, even the center tapped winding.
 
(quoted from post at 17:54:54 09/24/17)
(quoted from post at 09:24:43 09/24/17) If the power lines from the utility just have two wires as shown, how does the transformer divide two lines going into your house out of phase with each other. It looks to me like if they could divide it out like that they could divide it again and give you three phase if you needed it.
a172803.jpg

That photo had me confused for a minute, in my area the bottom line is always the neutral and anything above that is a hot line.

here top is 15000 volts bottom line is neutral
 
(quoted from post at 13:15:29 09/25/17)
(quoted from post at 17:54:54 09/24/17)
(quoted from post at 09:24:43 09/24/17) If the power lines from the utility just have two wires as shown, how does the transformer divide two lines going into your house out of phase with each other. It looks to me like if they could divide it out like that they could divide it again and give you three phase if you needed it.
a172803.jpg

That photo had me confused for a minute, in my area the bottom line is always the neutral and anything above that is a hot line.

here top is 15000 volts bottom line is neutral
believe it is most common to run the hot above the neutral in areas & heights where human contact is more likely, but run ground on top in other areas (think cross country transmission) for lightening protection.
 
That is pretty high for phase to ground, remember phase to ground is phase to phase divided by square root of 3, typically in the 6-7K range.
 
(quoted from post at 17:40:55 09/25/17) That is pretty high for phase to ground, remember phase to ground is phase to phase divided by square root of 3, typically in the 6-7K range.
hen our rural area started to grow population, the co-op handled it by doubling voltage. 7200 to 14,400.
 
Stephen, electricity in free air travels at the speed of light. In a conductor a tad slower.....think about how fast you can connect to the other side of the world when punching your "send" button on your computer. The power generator supplying power to your power lines is oscillating from zero to max positive to zero to max negative back to zero 60 times per second and that sets how fast your current changes (alternating current) in your power lines.

Just like a magnet has a N and S pole, the "electromagnetic" effect of the current passing through your transformer on the pole generates a voltage across the high voltage input (14,400 from my coop) and output winding, (center tapped to "earth" ground) at 240V for a reference number....(mine usually runs around 248V across the secondary).

Just like with the magnet, if you were to sit 2 "button magnets" on top of each other, your total magnetic field across the stack would be N-S as is the 240V secondary of the transformer and the total strength would be the sum of both but you still have NS and NS per magnet.

So if you take your secondary of 240V and center tap it to ground you have a circuit similar to 2 button magnets sitting on each other, the top of which is sitting at N and the bottom sitting at S.......or in the case of electricity, the top output terminal is sitting at + while the bottom of it is at -. Just like the button magnets little N and S poles on each magnet, any and all taps in between are essentially little + and - circuits. Being equal in number of turns, their output is equal and opposite in polarity since you are sending the top of one and the bottom of the other to the house with the mid point grounded.

In your circuit breaker box you have 2 power strips feeding alternate breakers so that if you put in a dual breaker it spans the output of the whole transformer and gives you 240V. However if you only use one breaker it only connects to one input strip and you only use 120V. The fact that you have 2 appliances connected to different sides of your electrical service and each is fed 180 degrees out of phase is of no consequence. One doesn't care what the other is doing.....if staying within it's ratings and not shorting out or something.
 
Yepper. My COOP did just that about 10 years ago. All they had to do was to increase the insulator by 1 disc (from 1 to 2) for the hot line and add a new transformer per location.
On the transformers, not that bad of a deal as the they were getting old and starting to be a maintenance problem. Usually the line voltage is rock solid at 124. Good thing they did.
Lots of new people moving into the area and the power requirements are skyrocketing.
 

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