Overview

  • Commercial electrical generators of any size generate what is called 3-phase AC power
  • Single-phase power is what you have in your house. Generally referred to as single-phase, 120-volt AC service.
  • The power plant produces three different phases of AC power, each offset 120 degrees from each other. There are four wires coming out of every power plant: the three phases plus a neutral or ground common to all three.
  • The three-phase power leaves the generator and enters a transmission substation at the power plant where large transformers which converts the voltage to extremely high voltages for long-distance transmission. Typical voltages here range from 155K to 754K volts. The transmission lines are carried by the huge steel towers you see.
  • Next is a power substation which steps the voltage down, generally less than 10K volts.
  • Typical residential has a set of poles with one phase of power (7.2 K volts) and a ground wire. Sometimes there will be two or three phases on the pole, depending on where the house is located in the distribution grid.
  • There is a grounding wire from each pole to the earth. The guy-wires are attached to the direct connection to the ground.
  • Two wires run from the utility pole transformer and three wires to the house. The two from the transformer are insulated and the bare one is the ground.

Fact Check: Please … those who are knowledgeable about this subject please look over what I say and if I am wrong then tell me .. when possible make the explanation so even my simple ‘lizard brain’ can understand. This applies throughout the article .. with the understanding that this is just enough information and ‘facts with pictures’ to enable those of us ‘Transformer challenged’ can make a believable model.

Context: Like it says in the title this does not reflect modern practices but instead the general era I model .. aka .. c1914

Single-Phase

In electrical engineering, single-phase electric power is the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors.
Applications
In North America, individual residences and small commercial buildings with services up to about 100 kVA (417 amperes at 240 volts) will usually have three-wire single-phase distribution, especially in rural areas where motor loads are small and uncommon.

Single-phase wire has three wires located within the insulation. Two hot wires and one neutral wire provide the power. Each hot wire provides 120 volts of electricity. The neutral is tapped off from the transformer. A two-phase circuit probably exists because most water heaters, stoves and clothes dryers require 240 volts to operate. These circuits are fed by both hot wires, but this is just a full phase circuit from a single-phase wire. Every other appliance is operated off of 120 volts of electricity, which is only using one hot wire and the neutral. The type of circuit using hot and neutral wires is why it is commonly called a split-phase circuit. The single-phase wire has the two hot wires surrounded by black and red insulation, the neutral is always white and there is a green grounding wire. 1

Two-Phase

Two-phase electrical power was an early 20th century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by 90 degrees. Usually circuits used four wires, two for each phase. Less frequently, three wires were used, with a common wire with a larger-diameter conductor.

I had a photo up but looking closer it seemed to be of a single-phase circuit. Tht was what .. I think .. Chris below was referring to. Not so sure now so researching.

Chris Atkins This is a 2-phase 440V service, which gives 220V phase to neutral. Since neutral is reference to ground, you don’t need it for metering, etc. Modern systems would have 2 or 3 separate transformers, but it appears in this view that this could be a 2-phase transformer, which I’ve never seen, but it theoretically possible. It’s also possible that the side view would show another one behind it. Most polyphase services supply 3 phases, but some equipment may work on just two phases, so this setup was evidently needed. I’d say this is an exception rather than the norm. Another interesting thing is that they refer to 440V and not 480V. At some time we jumped up 10/20/40V, but most devices say rated at 110/120V. The way it was explained to me 25 years ago was that it allows some voltage drop between the transformer and service. I don’t think it was always that way and at one time these voltages were referred to as 110/220/440, etc.

Three-Phase

Three-phase electric power is a common method of alternating current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by electrical grids worldwide to transfer power. It is also used to power large motors and other heavy loads.

Three-phase power is supplied by four wires. Three hot wires carrying 120 volts of electricity and one neutral. Two hot wires and the neutral run to a piece of machinery requiring 240 volts of power. Three-phase power is more efficient than single-phase power. Imagine one man pushing a car up a hill; this is an example of single-phase power. Three-phase power is like having three men of equal strength pushing that same car up the same hill. The three hot wires in a three-phase circuit are colored black, blue and red; a white wire is the neutral and a green wire is used for the ground. 1

Single-phase and Three-phase diagrams

Three-Phase 11000 volt

This diagram is taken from Electrical West, Volume 33 pub.1914.

I used this diagram as a guide for my own version. I liked the complexity but I wanted MORE .. so I added a workers platform. I am mostly concerned with how it looks .. not so much on how realistic .. although in this case it IS based on the diagram. 2

Here we go .. side bracket transformer of the left. I will be using two of these side by side and wired as in the diagram.

Some of the photos I have found of older can type transformers were ribbed to aid cooling .. so I did that with these. The side/bracket mount was just to add interest.

.. and .. here we go. I followed the diagram pretty much but added a platform, ladder and foot pegs.

I also added telephone/telegraph insulators down lower .. again .. to add interest.

Hey .. if you can’t have fun why do it.

When/if I ever run wire to these it will be quite .. call it interesting!

Have all the wiring figured out except for the ground. In the wiring diagram above it shows the center lug grounded. Nothing is shown connected to the right center lug. I can *assume* that this is also grounded but .. gonna ask the experts …

Well. I did get answers to this .. from electrical engineers that have worked this industry and the answer .. best I can remember from the ‘Techno Babble’ is .. it depends .. depend on how the transformers are internally would that is. So you can hook up one or both to the same neutral line and all is good.

I think that .. and come on .. I understand this really doesn’t matter except to those of us with OCD .. but the same type of transformer should be wired the same everywhere. I suppose a case could be made that other transformers could be wired within each ‘class’ differently but it seems to me that .. given the choice and supposing it would make no difference that a common pattern of wiring .. where applicable .. would make sense for the worker bees installing the things.

Notes

Cross-arms for telephone lines are usually 10 or 6 pin, the wires adjacent to the pole being 16 inches apart and others 12 inches apart. Cross-arms are mounted two feet apart. Poles are usually set to give an average span of 130 feet. 3

On 40 ft. poles the transformer arms are placed 4 ft. below the 11,000 volts arm; on 45 ft. poles, 7ft; and on 50 ft. poles, 10 ft. 4

Diagrams

So .. use whatever CAD program that works for you. Me .. I use Sketchup. The cool thing about using such a CAD program is that you have a virtual model. You can hide components as you wish and create diagrams as needed.

I copied the crossarms to one side so I could get a good view of the locations for the bolts and pins.The left side is the view from the front. These are all bolt locations .. in fact too many. The ones in the center actually have a vertical 4×4 at that location .. but it does show the center where those vertical members will go.

The right side is the same crossarms as viewed from the top. These are all pin locations (pins for insulators) except where noted.

I just placed the stripwood 4×4 below each of the diagrams and marked the locations of the holes. The bolts will be Tichy#8035 .. O scale – 3″ washer, 1.5″ sq. bolt. The pins will be Dritz “Extra-Fine Glasshead PIns” from the sewing section at WalMart .. 0.018″ dia.

I copied the structure to one side and deleted everything but the poles and crossarms. I will then be able to use this to locate the cross arms on the poles.

This will be simple ‘eyeballing’ the locations and should be “good nuff” .. hopefully since that is how I will do it.

Jig

At first I just taped the poles to the ‘Hanging’ diagram but quickly discovered that would not work so I created a jig. Two 1/4″ bolts, 3/8″ sq. wooden clamps and a couple of thumbscrews. The wooden clamps have a radius filed where they clamp to the poles (picked up a pack of two medium sized round files from Lowes, used for sharpening chain saw blades). This should work for both notching the poles (lower one already done on the “it didn’t work taped jig” .. and for gluing together.

Mounted

Everything mounted .. just need wiring up. The large insulators on top will get the high-voltage lines at some point.

Having finished transcribing the from my old website to this one I realized that at some point the top cross-arms and insulators are MIA. The cross-arm replacement is of no matter but .. not sure if I have any appropriate insulators for the top – or assuming I do .. if I can find them. That’s something I need to rectify .. need to stock up on various insulators.

Notes

Cross-arms for telephone lines are usually 10 or 6 pin, the wires adjacent to the pole being 16 inches apart and others 12 inches apart. Cross-arms are mounted two feet apart. Poles are usually set to give an average span of 130 feet. 3

On 40 ft. poles the transformer arms are placed 4 ft. below the 11,000 volts arm; on 45 ft. poles, 7ft; and on 50 ft. poles, 10 ft. 5

O and HO scale Utility Pole Transformers


O Utility Pole Transformer
HO Utility Pole Transformer

Source
  1. Sciencing : Electrical Wiring[][]
  2. Electrical West, Volume 33, page 340[]
  3. Electrical Engineer’s Pocket-book: A Hand-book of Useful data for Electricians and Electrical Engineers. pub. 1908[][]
  4. Electrical West, Volume 33 pub. 1914[]
  5. Electrical West, Volume 33 pub. 1914 []

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