Is the ugly's book wrong?

[drbond24]

Do I remember correctly that 2-PH are 180 Deg. out of phase with each other?

Did you do that on purpose, or do you not know any better? :grin:

You just made sure that this thread will go several hundred posts.

 

[rattus]

If anyone has a drawing of this two phase system and the voltages that you would read line to line and line to ground, that would be helpful. The uglys book says a 2-phase 230 Volt system in it's example. I take it that would be line to ground? and 460 across both phases?

Imagine a 3-wire, 230V, 2-ph system. The phase voltages are 90 degrees apart, therefore , the line to line voltage would be 1.414 x 230Vrms = 325Vrms--not 460V!

Rattus,

Could you explain the "secondary" and "line" voltages/currents a little more? If I put my amprobe on one leg of a 3 phase motor do I have a line current reading or secondary current reading?

If I read 480v across two lines is that line voltage? and 277V to ground/neutral is that secondary voltage?

Your amprobe would measure line current.

Line current is provided by the secondary windings of the transformer. In this case you are describing a 277V wye, therefore the line current is the same as the phase (coil) current.

If the secondary is a delta, the line current would be 1.73X the phase (coil) current (in a balanced system).

 

[Sharpie]

Did you do that on purpose, or do you not know any better? :grin:

You just made sure that this thread will go several hundred posts.

I didn't do that on purpose (or did I? ).

I actually meant 90 Deg., but my brain wasn't kicking in. Sorry

 

[quogueelectric]

Picture two of these placed at 90? with the center taps connected together. Now you have four hots and a single neutral.


It would be more accutare to say 90? than 180?.

The second post was a response to Jraefs post about a sing phase 2 wire system with 2 hots but french electrician posted right in between the time it took me to type so it seemed like it wasnt a response to Jraef. OOPS. I just asked if he meant 3wire being he said 2 hots. I am just waiting for the big gunslingers to return.

 

[Ranch]

I have heard that there are still a few true 2-phase systems still in use around the country

Late 80's I recall it was time to forget the term 2 phase ....

If you want to satisfy your curiosity, do a search on a "Scott-tee transformer"

It was Charles F. Scott

I'm of the opinion that information on it should be footnoted in the NEC (and its derivatives such as Ugly's) because it causes more confusion than anything else.

I second your opinion. For others may I add that a phase can only exist between two points. Draw a line between two points and you get one phase, Draw lines between three points and you get three. You cannot make two unless you combine two independent power sources. Then you have at least a four wire circuit, more likely five if you want to turn lights on connected to each source and run a polyphase motor simultaneously.
Three phase is much less confusing today, we just didn?t know 100 years ago. We were too concerned about the effects of one phase on another.

 

[mivey]

Draw a line between two points and you get one phase, Draw lines between three points and you get three.

Please forgive me, I can't stop myself:

That description needs work. Maybe you should say three points with at least one non-linear point.

The center-tapped leg of a delta has three points and seems to be a contributor in the other threads because the neutral lies between the two line conductors and produces two voltage vectors with a 0 or 180 degree difference. The neutral between the two line conductors of the wye bank is non-linear and produces two voltage vectors with a 60 or 150 degree difference.

 

[Ranch]

Please forgive me, I can't stop myself

All is forgiven, but you can't go tapping things on me. I have only ever used a zig zag to establish a neutral on delta.

On the WYE, can you explain the "neutral between the two line conductors"

Pardon the pun but you have tapped into something I have to figure out. Can a two phase system exist from one source? (one generator perhaps)

Mostly I'm just making DC, but need to know more about how not to wreak havoc on the utilities.

 

[LarryFine]

Can a two phase system exist from one source? (one generator perhaps)

Yes, as long as the single generator is wired that way, with four line conductors with 90? timing.

 

[mivey]

All is forgiven, but you can't go tapping things on me. I have only ever used a zig zag to establish a neutral on delta.

On the WYE, can you explain the "neutral between the two line conductors"

Pardon the pun but you have tapped into something I have to figure out. Can a two phase system exist from one source? (one generator perhaps)

Mostly I'm just making DC, but need to know more about how not to wreak havoc on the utilities.

The center tap on one leg of a delta serves as a neutral point for the single phase loads it can serve. As you might suspect, it is not the real neutral point for the three phase system because it is not "neutral" to the three line conductors. This center tapped neutral lies along the line drawn between the graph points representing the voltage of the two line conductors.

If you use a wye bank to serve single-phase loads (sometimes called "network service") the neutral does not lie on the line drawn between the two line conductors. This offset neutral allows the production of a higher order of phases from the three-wire service because it has the phase displacement needed. The three wire center-tapped delta service does not have the ability to create a higher order of phases without adding some phase displacement capabilities.

I would say that most people do not call the "two phase system" a two phase system because they see the voltage at the two line conductors (when referenced to a center-tapped neutral) merely as a voltage inversion. They would call this a single-phase system with a center-tap and this seems to be the most common usage. You can find reference material with either usage.

 

[SegDog]

Philadelphia Electric (PECO)

Philadelphia Electric (PECO)

I was, also, told that you will never see 2-phase. In 2004, I saw it twice in downtown Philadelphia, in the old section. Last week, I met a guy who just worked on a system on the extreme other end of town. PECO still supports 2-phase, and there is some info on the internet regarding their equipment.

 

[mivey]

I would say that most people do not call the "two phase system" a two phase system because they see the voltage at the two line conductors (when referenced to a center-tapped neutral) merely as a voltage inversion. They would call this a single-phase system with a center-tap and this seems to be the most common usage. You can find reference material with either usage.

I hope it was clear that I was talking about the voltages with a neutral reference and 180 degree displacement. This is not the common system most people are talking about when they refer to 2-phase.

The common, historical 2-phase is a system that can be "missing" something in the true polyphase sense in that the voltages may not be uniformly displaced. In the canonical definition, an "n-phase" system will have the voltages displaced by 360/n degrees. The whole system of terminology has been made a mess over the decades.