240V from a 3 phase main ?

Don,

That is great.. Thanks for the URL. Much appreciated. I went through NY Trade School in 1957 and have been trying my very best to remember all of the 'stuff' I learned back then. Everything from motor winding to pulling and hooking up raw power. Now, at a retarded (eh.. retired?) state of being, it is fun, but sometimes hard to remember. It's kind of like the ole' dog, you know, he chases cars but cannot figure out what to do with them once caught? lol..

PJ

Oh - 4 phase is nothing more than 2 phase with a center tap on the generator winding. (Plus a polarity change) Each phase is a 90º displacement on the rotor/stator. (3 phase is 120º)

208 is what I just calculated. This is the configuration you get when you use 240V split phase through a rotary phase converter.

Speaking of phase converters, I have a question about them. I read somewhere that the newer, "high efficiency" three phase motors aren't good for use in phase converters because they'll overheat. So I was wondering what it is about them that would cause that, and if it's not simply a matter of them being rated less conservatively than older motors, would using a larger motor solve the problem?

allan

Good information, Don. Thanks!

Yea me too!

I especially liked the part where he said "he [meaning me] was correct."

Just thought I'd repeat it.

George.

NO, it is ~208, for a 240 V delta center ground system. It is the "Hot" leg because that is the highest voltage to ground in the system.

For a 480 V delta center tap system it would be ~416.

jim rozen wrote:

jk

Someone else commented in the thread that 2 phase power didn't exist. It does, as has been explained by "e". There is also 4 phase power.

Four-phase is a ficticious special case of two phase.

Just as six-phase and twelve-phase are ficticious special cases of three phase.

"Four-phase" may be made from two-phase using a simple, center-tapped transformer in each phase.

Similarly, "six-phase" may be made from three-phase using a simple, center-tapped transformer in each phase.

Finally, "twelve-phase" may be made from three-phase using a simple, center-tapped transformer in each phase, *plus* so-called "zig-zag" transformers in each of the ficticious six phases.

. .

. .

A note on DC- As of teo years ago, New York transit was still running DC rotarary converters that are about 100 years old-AC in, DC out, but not quite a motor generator like a welder. I believe the last went out of service recently, but I may be wrong. The hookups use have been thyristor converters (all electronic) since the 50's.

I read recently that the last of the Edison DC service was finally discontinued.

e

Ok, thanks - I keep confusing the wild leg for that with 277.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

If anybody's interested in this stuff, take a look at

Also, "Networks of Power"

"If you're a history buff, and appreciate the technology that surrounds us all, you'll love reading "Networks of Power: Electrification in Western Society, 1880-1930" by Tom Hughes. Hughes takes us back to the days of fierce rivalry between Edison and Westinghouse; the early era of electric power generation and consumption where the battle of DC vs. AC consumer power was born and decided.

Hughes doesn't stop there. Also included in this well-footnoted edition are in-depth narratives of the evolution of commercial power systems in England and Germany through 1930. A well written, readable snapshot in time.

Compelling historical reading for the non-technologist as well as the student of electrical power commercialization."

At the transformer, as per the diagram in that excellent link. When you do the actual service lateral, you pull three primary cables and each one has an aluminum (usually) center conductor and a copper concentric grounded conductor. The grounded conductors are joined at the transformer and also bonded to the transformer case and ground rods.

No, it's 208. Drop by and check it out. I have a three phase delta system in the shop.

Harold

Ok, thanks - I keep confusing the wild leg for that with 277.

277/480 is a Wye system, as is 120/208.

However, the 120/240 Delta system, with one phase center tapped and grounded, also produces 208 from the high leg to neutral.

This is only case where 208 includes a grounded conductor ... the other 208 is phase-to-phase.

Those 'fictitious size phases' - are poly-phases - many versions.

The Y is twisted naturally - it is intended for motors - very special shading and torque urging type. Can and Box companies use them. I first saw them in action in the 70's at CanCo IIRC in Arlington Tx. A German can machine that was a real monster but turned out cans like nothing else at the time.

Martin

The concentric on the primary cable along with the transformer grounds are not always a metallic return to the substation. Primary neutral and common neutral (common to primary and secondary, run in the secondary position) are metallic returns to the substation. You find primary neutrals in the older 4KV where they've used 2400V transformers phase to neutral (star or wye). We also run a common neutral system in our 21KV that we use 12000V transformers on.

A side note to the underground cable, is they are not getting the life expectancy out of it that they thought when they put it in. One of the problems that we've run into is some soil conditions dissolve the copper neutral on the older installations where it was direct buried. We've since gone to a jacketted cable and require everything to be in conduit now. Another problem they find is when the cable is faulting it's through hairline cracks through the poly. We've replaced a considerable amount of this at great expense, and have recently been working with a company that injects silicone in the end of the strands that fills these hairline cracks. At 5 dollars a foot, it's about half the price of cable replacement. This year we have some 40000' coming up for cable replacement and another 20000' that we are going to do the silicone injection on.

Don

Do they mention anything in the book, "Networks of Power" about the Folsom Powerhouse? I was privileged enough in my career to work with a man that operated it. He operated it from 1948 til they closed it in

1955. I worked with him in the '70's when he was Chief Dispatcher with the company I work for. I took a tour of the powerhouse with him and I took my video camera. I have him telling a lot of wonderful stories of things like the collectors flying off the generators, sparks everywhere and him diving out the window.

Today when you talk about a network in the electrical utility industry it usually refers to a system in the downtown area of large cities. This is different from the transformers in the suburbs, in that, the secondaries are all in parallel. So if you lose a transformer you don't drop any customers, you just lose KVA. But you have to run your network with more reserve than the largest transformer.

Don

The county took over an Air Force base that closed and we inherited the electrical system from the AF. We removed a lot of the antiquated equipment and then the county decided they wanted to power up some of the hangars so they could be rented. They had 416V drypacks in them for the lighting. So we hung a bank with 3 240V transformers, starred the secondary side; that gives them 3 240V legs and 416V 3-phase. You don't see stuff like this all the time, but there is a lot of oddball stuff out there.

Don

You're way above my head, are you talking about upstream of the customer-owned cable? In the typical case, the concentric grounds are the only return, right?

That's probably the way to go- I once pulled an old lead-jacketed primary run through about 400' of conduit from a man-hole cover to an underground vault. It was a real PITA but pulling the new poly-jacketed stuff in was easy and saved a lot of trenching and restoration. Still, the majority of installations in this area are direct-bury. The only advantage is that sometimes you can Biddle the cable and make a quick repair without pulling the whole line out.

Another problem they find is when the cable is faulting

They inject silicone in the center strand which fills the gaps in the white poly insulation between the center strand and the semiconducting layer? That's a pretty neat trick. Is it only practical for utilities? Do they have to redo the potheads/elbows?

>

It's been a few years since I read it, but I don't remember that. A lot of the book dealed with rationalization (standardization and the rationalization movement) of the power industry, the balance between competition and the need for a unified grid. On the technical side, much of what we take for granted, such as the concept of a feed with two hots and a neutral, (vs. one hot and one neutral) was once a bright new idea. With your background, I think you would enjoy at least some of the book.