true "two phase" service

[malachi constant]

I am the engineer on a job in the early design phase and I am looking for some thoughts.

Have a building almost 100 years old in rural Minnesota - about an hour outside of downtown Minneapolis. The gear inside is probably 50 years old and is labeled 208V three phase - I think it is a 400A service. There are only two pole mounted transformers (on the same pole) outside the building. Utility says it is NOT a three phase service. There is a three-phase generator, only a few years old, that is integrated into the system as an emergency backup for lighting, pumps, freezers, etc.

What is going on? Both the utility and the other engineer here speculate it is three-phase power derived from a true two-phase source, but are unable to go into depth beyond that.

I intend to upgrade the service to 208/120V 1000A 3-phase 4-wire. My primary concern is that existing motors may not being compatible with the new voltage. I'm also concerned any time I meddle with a system of which I don't have a solid understanding.

Read some history on wikipedia ("Two phase") but not sure that is the same thing I have here.

Any suggestions on what Qs I can ask, what I should plan against, or if I have their maintenance staff open up some equipment what I would be looking for, etc etc? Much appreciated.

 

[adavey]

Sounds like you have an open-delta configuration. Usually, one of the pole mounted transformer's is larger than the other. An open delta will give you 240V phase to phase and 240v from one of the phases (A,B or C) to neutral. (note: this phase mentioned above not to be used for 120V loads !!!!!) I've never seen an open delta w/ 208V between phases so you might want to ckeck the voltage w/ a digital multimeter or then again maybe my assumption is completly off !!!!

 

[adavey]

Questions to ask:

How many primary lines are on the pole ?????
Shall the upgrade be pole or pad mounted transformer ??????? (probably pad)
If there only two primary lines on the pole, will the power co. provide another primary line for a y configuration at no cost ??????
What type of config. will poco give you ????? open-delta, delta or Y. Usually i tell them what I want, but i don't know your area

 

[al hildenbrand]

I think you might be confusing yourself by thinking of "two phase".

Two phase is in 90? relationship.

Three phase is in 120? relationship.

With the power company balking at calling the available service as "not three phase" is probably with respect to the service size increase to a 1000 Amps, and not a definitive statement of what is actually there now.

I'll just bet the PoCo primary only has two phase legs coming to the transformer bank, and that the transformers are connected open-wye primary, open-delta secondary.

Questions to ask, would be, simply, what is the current service voltage. . .is it really 208V or 240V, and how much equipment that is one voltage has to be replaced with new equipment at the other voltage. Also, what will the PoCo require to extend their primary, thus providing "primary" 3? (if they have to go far, this could be expensive).

 

[Bob NH]

Here is a link to an earlier thread that might be relevant. Two ungrounded connductors and a grounded conductor on the pole producing 240 and 120 Volts.

http://www.mikeholt.com/code_forum/showthread.php?t=81207

 

[sandsnow]

Do a search here for two phase under the theory section.

http://electrical-contractor.net/cgi-bin/Ultimate.cgi?action=intro

Also there is a member there "scott35" who is very savy about these types of systems and how you can get two pahse from three pahse or maybe vice versa

 

[mdshunk]

I think the last two phase generation went out of service at Niagra Falls in the late 80's. Even there, they were converting their two phase generation to three phase for transmission and distribution.

 

[sandsnow]

bump

Just curious what the result was?????

 

[beanland]

V-phase, 2-phase, 3-phase

V-phase, 2-phase, 3-phase

Two transformers means either open-delta 240/120V 3-phase 4-wire service (208 high leg) or 208/120V 3-wire network service. Look at the primary on the pole, three wires on top generally means 3-phase not V-phase.

You can take true 3-phase and convert it to true 90 degree 2-phase using a Scott T transformer connection. I saw an old motel where the elevator was 2-phase fed from two transformers fed from 3-phase.

 

[jim dungar]

Two transformers means either open-delta 240/120V 3-phase 4-wire service (208 high leg) or 208/120V 3-wire network service. Look at the primary on the pole, three wires on top generally means 3-phase not V-phase.

You can take true 3-phase and convert it to true 90 degree 2-phase using a Scott T transformer connection. I saw an old motel where the elevator was 2-phase fed from two transformers fed from 3-phase.

Two transformers does not always mean 240/120 open delta, it could be only 240V 3W. Often the two transformers in an open delta are different sizes when supplying 240/120 3Ph 4W.

What voltages (L-L and L-G) do you have in your facility?

I know of no Minnesota utility distributing 2-phase power. And yes there are several methods to move between 2-phase and 3-phase systems.

I would like to start a movement to not call "line" conductors "phases". It takes one line-to-line voltage to create a single phase (if you use line-neutral then it becomes difficult to describe delta systems).

 

[John Valdes]

If you are going to incorporate motors in this facility I would not opt for the 208. 480/240 3 phase would be my choice if this is industrial. Look at some motor nameplate ratings for voltage. The 208 and the 240 VAC connection is the same. Also, if the motor is rated for 240/480 you may not be very efficient with a 208 supply. You will have to find motors that specifically state
"can be used with 208 VAC". For the warranty. I have seen to many motors running on 208 that don't last long and run hot. Also most motor controllers (ac & dc drives) are rated for 480. But you can get 240 volt controllers at a higher price. (Larger components). This is your chance to get it right the first time.

 

[Dnkldorf]

I think the last two phase generation went out of service at Niagra Falls in the late 80's. Even there, they were converting their two phase generation to three phase for transmission and distribution.

Marc, from what I know, some places in Philly still use the 2 phase.
About a yr or so ago, a guy called me looking for a 2 phase, to 3 phase converter.
I assumed then, it was still being used.