Commercial power questions

[mjr6550]

I inspected a warehouse/office building today that was built in 1963. At the time the building was apparently supplied with 2 phase power. Two transformers were later added, apparently to convert the 2 phase to three phase.

I think that I understand what I am looking at, but thought that the photos may be of interest (and maybe teach me something).

With 2 phase you have two hots and two neutrals and end up with three hots and a neutral? How does the transformer create the third phase?

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[kurt]

There's single and 3 phase....no 2 phase. One can purchase a phase converter to get from one to 3, but I think it's not true 3 phase, or something like that.

I ramped up my woodshop years ago when I got a 3 phase table saw; I remember getting a phase converter to make the transition, but it's about 35 years ago and I forget the details.....

[Marc]

Two phase power never made it past the runoff when AC power first advanced past the single phase configuration. Other configurations, like 5 phase, never even made it past the drawing board. They all quickly yielded to the 3 phase configuration which was shown to transmit more power per given amount of conductor material than anything else. There were other reasons for the superiority of 3 phase, such as simpler electric motor design.

In 63', I doubt any 2 phase systems remained for the simple reason that you can't obtain decent 2 phase (0 degree and 90 degree vectors) service from either single or three phase (0 degree, 120 degree and 240 degree vectors) sources without a motor-generator.

Likely what you saw was the voltage transformation of a three phase system by using an 'open-delta' connection. The 'open-delta' connection requires only two single phase transformers (the common terminal is the third leg). It's an inefficient and wasteful use of transformer capacity but it saves you the trouble of needing three of them. It's very common among utilities who need to get three phase power to small-fry locations like Kurt's wood shop.

I remember reading of 'converters' that would yield 3 phase from a single phase source but they're only crude approximations of true 3 phase power.

Marc

[inspector57]

I can't see anything resembling electric equipment in your photos. Am I missing something in the darker background?

[mjr6550]

Sorry, posted the wrong photos.

Philadelphia is one of the strange places that did use 2 phase power.

I know people that use rotary phase converters to run 3 phase machine shop equipment in their houses. I also saw one recently in a house where the owner had a 22" planer. They look more like a motor than a transformer.

[Marc]

Sorry, posted the wrong photos.

Philadelphia is one of the strange places that did use 2 phase power.

I know people that use rotary phase converters to run 3 phase machine shop equipment in their houses. I also saw one recently in a house where the owner had a 22" planer. They look more like a motor than a transformer.

Sounds like a motor/generator. It's a single machine that turns like a motor but the shaft doesn't come out. One winding is motor, the other is generator.

I once worked in a forge plant where induction machines heated metal ingots white hot before being forged into high pressure pipe fittings for petro/nuclear industry. The 800 volt, 3,000 single-phase power for the induction coils was furnished by 250 hp motor/generators that ran on 480 volt, three phase utility power.

Marc

[Jim Katen]

Sorry, Mark. I've read a little bit about 2-phase power, but I've never seen it and I really haven't made much of an effort to understand it.

I did once make a single-phase to 3-phase converter to run an old 3-phase table saw. It consisted of some capacitors and an ancient 10 hp 3-phase motor which, as I recall, we had to kick start the spindle to get going. I left it in Berkeley in an old shop that, I later learned, burned down.

[Douglas Hansen]

I've never personally seen a two-phase system, though the diagrams I've seen for them are typically 5-wire, with a neutral and 4 phase conductors at 90 degree vectors. I have heard of these systems being used in railroads in Pennsylvania, though again haven't seen it myself.

I think the issue in the inspection in question would be to alert the client that the system might not provide the type of power needed for standard induction motors and other equipment. While this could all be a head-scratcher, in the big picture it is probably not that relevant to your client. If the space is to be used as a warehouse/office, odds are their only needs will be for single-phase power. If they are going to install new machinery, they will be replacing the electrical power distribution system anyway.

[mjr6550]

From what I understand I don't believe that two-phase is supplied anymore. I believe it is now a three-phase system. One of the transformers fed a three-phase trash compactor, but the fuses upstream of the three-phase circuit breaker had been removed so it appeared that it was no longer used. I did not locate anything else operating on three phase.

Everything was from 1963, including a couple Federal Pacific circuit breaker panels. It needs major work.