# Transformer Explanation (not really solar)

Solar Expert Posts: 259✭✭
I have a question about how transformers work. I understand transformers serve the purpose of stepping up from generator to consumer I order to transmit without much losses and again to step down to a house for consumption.

This means that there should be wires goingin and coming out of a transformer, right?

So here is a picture of one in the street:

On the right, the bottom line seems to go into the right transformer "entry" and then the next two transformer "entries" go down to the middle and top lines on the left. But why if the top line appears to be the neutral?

• Banned Posts: 17,615✭✭✭
Re: Transformer Explanation (not really solar)

Yeah those line transformers can get confusing. Some of them have a single input wire at very high Voltage and reference ground for the other side. Some have multiple secondary taps. The one you show looks like ... a bunch of wires all over the place.

I think this may be a clearer pic of a service transformer. Neutral is CT on the secondary and run to ground at the pole as well as providing N for the house.
• Solar Expert Posts: 3,009✭✭✭✭
Re: Transformer Explanation (not really solar)

The angle from which the photo was taken makes the mess of wires very difficult to follow, but basically the three terminals shown side by side on the side of the transformer are the output terminals. In North America the center one would be Neutral, and is also connected to ground. The two outside terminals would be the two "hots". From either hot output to neutral would deliver 120 volts, while connecting across the two hots and ignoring the neutral would supply 240 volts. The High voltage input would be to a large insulated terminal not visible in the photo and could be either on the top of the unit, or on a side away from the output terminals. The other input terminal could also be to a large insulated terminal, or as one of the input terminals is often connected to ground, that terminal may not have a large insulator like that for the "hot" input.
• Solar Expert Posts: 3,009✭✭✭✭
Re: Transformer Explanation (not really solar)

Yes! Much better photo Cariboocoot! Thanks for sharing.
• Solar Expert Posts: 259✭✭
Re: Transformer Explanation (not really solar)

CT on the secondary?
• Solar Expert Posts: 3,009✭✭✭✭
Re: Transformer Explanation (not really solar)
quique wrote: »
CT on the secondary?

CT = Center Tap, and is required to give you the 120 / 240 volt outputs.
There would be no CT on the inputs of these transformers, only the output (secondary)
• Solar Expert Posts: 259✭✭
Re: Transformer Explanation (not really solar)

Thanks Wayne, here is another picture:

• Solar Expert Posts: 259✭✭
Re: Transformer Explanation (not really solar)

And this is the post before the original one. It sends one line to the original pole and another line out to another pole and the third line just ends right there.

Re: Transformer Explanation (not really solar)

Probably a "Wye" connected three phase circuit (or "Y" or "Star").

Basically, the three lines on the pole are "hot". The center of the Wye can be a ground neutral--So the return path does not have to be electrically isolated (is is at or near "ground" potential").

In some remote areas of the US, they use the earth's ground (dirt) as the return path for current (high voltage out to the remote transformer, ground path back).

http://en.wikipedia.org/wiki/Three-phase_electric_power

Just a guess--I am not a power engineer.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 259✭✭
Re: Transformer Explanation (not really solar)

Ok in the last picture the 3 lines coming in from what would be the right of the picture arrive at the top. From the line closest to us, there is a tap goes to the transformer in my first picture. The middle line sort of skips over via another tap and goes down the crossroad to another transformer. The farthest line simply does at the post.

Why does that line just die at the post?
• Banned Posts: 17,615✭✭✭
Re: Transformer Explanation (not really solar)
quique wrote: »
Ok in the last picture the 3 lines coming in from what would be the right of the picture arrive at the top. From the line closest to us, there is a tap goes to the transformer in my first picture. The middle line sort of skips over via another tap and goes down the crossroad to another transformer. The farthest line simply does at the post.

Why does that line just die at the post?

Because they're done with it there. Sometimes they install a certain type of service as far as 'X' in planning for future development.

Really there are so many variations on line power and transformers that unless you have the utility's map you can't really know what they're doing. You just have to hope they do.
Re: Transformer Explanation (not really solar)

Is this transformer in the US? It would appear to be a 120/240 VAC split phase output (at least it would be in the US).

3 phase Wye can use a grounded neutral... Just like a center tapped 120/240 VAC split phase power system.

My guess is there is a "zero voltage" neutral (just below the street lamp in the post 2 above). That could be the return path for the high voltage power.

It could also be the neutral in the 120/240 wiring too (non-isolated High to Low voltage lines).

If this outside of US/North America, then it appears that the top lead of the "triplet" is the HV Return, and the lower two wires are the local ~230 VAC distribution.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 268✭✭✭
Re: Transformer Explanation (not really solar)

The three top lines are high voltage three phase probably 7,200 volts each. Only one of the three phases continues on to your transformer. You said the second phase continues on to a neighbor transformer. The third phases stop dead at that pole for whatever reason. Usually utilities will continue a loop but not always.

Now that 7,200 volt line continues on to your transformer. Inside the transformer are windings 30 to 1 (ie 240 volts x 30 = 7,200). 30 wounds one side to 1 wound on the other side, thus stepping down the 7,200 volts to 240 volts if current is flowing to your house. If you have a grid connected solar system you send current back thus stepping up the 240 volts to 7,200 volts.
23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring
• Solar Expert Posts: 10,300✭✭✭✭
Re: Transformer Explanation (not really solar)

basically the top wire is the hv input and is a single wire. i believe it appears to be 2 only because it is branching off to go to another pole, but your pic is hard to discern from the angle it was taken.

the 3 wires going out the bottom of it are the 2 hots and neutral with the neutral on the top of those 3 wires running pole to pole. everything below that is low voltage stuff like cable tv and phone lines. it is set up so that the higher the wires are the higher the voltage is.

btw the neutral will act as the second wire for the input too and is in common to both input and output.
• Solar Expert Posts: 9,583✭✭✭✭✭
Re: Transformer Explanation (not really solar)
quique wrote: »

What country sets the poles on their side, how do they keep the transformer oil from running out ?
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Banned Posts: 17,615✭✭✭
Re: Transformer Explanation (not really solar)
mike95490 wrote: »
What country sets the poles on their side, how do they keep the transformer oil from running out ?

Obviously on the Equator. Were it in Australia they'd be upside-down.
• Solar Expert Posts: 259✭✭
Re: Transformer Explanation (not really solar)

The picture was flipped on its side by the forum, I took it upright :-)

This is in Honduras, we use 120/240 split phase.

I'm trying to sketch all this out to post it.

Yes I believe those are indeed 3 HV lines. The middle one goes on to another transformer and the closest line to is also goes to another transformer.

Ive noticed in all transformers that the middle terminal in all transformers is indeed the neutral wire. I'm trying to see

1). Why the transformer not only gets the HV line into the top but also gets an incoming sideline to one of its terminals. I totally understand why there is a middle and left outgoing terminal but does it receive a top central line AND a right incoming line?

2). What is the pattern between lines incoming to the transformer and the ones going out. In other words, Sometimes the transformer incoming side line is L1 or L2 and sometimes the out coming terminal taps into L1 or L2 on the outgoing side.
• Banned Posts: 17,615✭✭✭
Re: Transformer Explanation (not really solar)

You might want to read through the wiki article on these things: http://en.wikipedia.org/wiki/Distribution_transformer

We tend to think of transformers as having a primary and secondary winding with two wires each and a straightforward V IN : XV OUT arrangement. But things get more complex in the power distribution business with not only Voltage changes but phase changes as well.
• Solar Expert Posts: 259✭✭
Re: Transformer Explanation (not really solar)

Ok I read that, I also read through something similar in HowStuffWorks. What Im confused about is why does a transformer have an entry from the top and an entry from the side wires into the terminals.

A) Why is there a red line entering through the top-right (which I understand is the HV line with about 7200V and its the one that should be stepped down to 240V) as well as the blue line coming in from the right? Is it because it needs to somehow make sure to take any incoming leftover voltage otherwise ending at the transformer and add it to whatever it steps down from the top as well?

Is there a reason why the right entry is taken, say from L2, and the outgoing left exit is to the other hot, L1 line?

C) Why is the center line exiting the transformer the neutral? I understand the neutral exits the transformer from the middle and taps into the wire neutral via the purple line, but is it the ground neutral that runs down the pole into the ground?

• Solar Expert Posts: 3,009✭✭✭✭
Re: Transformer Explanation (not really solar)

Sir, you're trying to make it way too complicated. It's quite simple really, HV in and 120/240 out. The workers installing it know which wire goes where.
• Banned Posts: 17,615✭✭✭
Re: Transformer Explanation (not really solar)

Oh boy. Why did they design it like that?

Well there's these things called "wiring conventions": in general trying to make the physical package resemble the electrical aspects. Since the output is L1 N L2 (standard configuration for 240 VAC split-phase) the wire in the middle of the output is Neutral and the ones on either side of it are L1 and L2. Center-tapped windings, see? It is not necessary to align them this way it is just a convention which makes it easier to understand. You could have it L1 L2 N or N L1 L2 if you like. They could be stacked vertically. The little image you posted should show the three gray lines continuing behind the transformer: they are the 'carries' for L1 N and L2.

The input for high Voltage is on top due to another convention which Niel mentioned before: higher up the pole = higher Voltage wiring. This helps keep things straight when poles are use for multiple purposes. The HV input has a second line of course, which is often connected to the case and ground which may also be connected to the center N on the output. It can be located on the side (often rearwards; close to the pole for the line to run to ground) because technically it is zero Volts in respect to ground.

Sometimes the HV lines will be 3 phase and they will pull from each of these 'in turn' for succeeding transformers to keep the loads on the 3 phase circuit as balanced as possible (instead of stack 100 transformers on one phase and nothing on the other two). Sometimes the transformers are 3 phase and have 3 HV inputs on top and 3 separate outputs which may be at 208 VAC 'Y' or simply an interim Voltage along the way.

I have to ask: why are you interested in this?