single vs. 3 phase

[jrclen]

But the real trick: Why is 120/208 single phase, which is actually two phases of a three phase system, called single phase? :grin: :roll:
-John

Think delta, you are only using 1 single winding. Then you center tap that single winding. Single phase.

 

[don_resqcapt19]

jrclen,

Think delta, you are only using 1 single winding. Then you center tap that single winding. Single phase.

With single phase 120/208 you are using two windings from a 208Y/120 volt three phase system.

 

[hardworkingstiff]

Question (actually 2)

Question (actually 2)

How many primary conductors does it take to drive the necessary transformer/s to make a 120/240-volt single-phase service?

How many primary conductors does it take to drive the necessary transformer/s to make a 120/208-volt single-phase service?

 

[mivey]

How many primary conductors does it take to drive the necessary transformer/s to make a 120/240-volt single-phase service?

How many primary conductors does it take to drive the necessary transformer/s to make a 120/208-volt single-phase service?

For the three phase bank, both can be done with three primary wires.

If you are talking about how many wires are connected to the highside of one transformer: two, in both cases (either two hots, or two hots and a neutral)

 

[chris kennedy]

How many primary conductors does it take to drive the necessary transformer/s to make a 120/240-volt single-phase service?

How many primary conductors does it take to drive the necessary transformer/s to make a 120/208-volt single-phase service?

A single primary will give the end user 120/240 with a grounded conductor. Yet you need 3 primarys to give the end user a 120/208 wye. Hence the label?

I'm with you Lou.

 

[jrclen]

jrclen,

With single phase 120/208 you are using two windings from a 208Y/120 volt three phase system.

I understand that don. I was attempting to show the 240 volt transformer in the yard or on the pole in easy to comprehend fashion. I was back on the 240 volt single phase concept. And why we call that single phase. Re-reading the question I was responding to, I see where I strayed.

Carry on the war. :smile:

 

[hardworkingstiff]

For the three phase bank, both can be done with three primary wires.

What 3-phase bank? I thought we were talking about two different types of single-phase services. (sarcasim intended).

 

[mivey]

What 3-phase bank? I thought we were talking about two different types of single-phase services. (sarcasim intended).

You can serve a single phase service off of a three phase bank.

 

[hardworkingstiff]

You can serve a single phase service off of a three phase bank.

Does that mean you are supply a single-phase service to the single-phase service? In my mind, it will always be 2 phases of a 3-phase service and will not ever be a single-phase service.

(It gets a little scary in my mind sometimes)

 

[mivey]

Does that mean you are supply a single-phase service to the single-phase service? In my mind, it will always be 2 phases of a 3-phase service and will not ever be a single-phase service.

(It gets a little scary in my mind sometimes)

Not sure what you mean but single-phase is single-phase, whether it comes from a single or three-phase bank. For the three phase bank, the pot's primary can have two primary "hots", or one primary "hot" and one primary neutral, for either the 120/240 volt case or the 120/208 case.

With only one pot, the primary can also have two primary "hots", or one primary "hot" and one primary neutral. For 240 volts, the one pot cases I've seen have all been either 240 or 120/240. I guess you could have a center-tapped 208 secondary, although I have not seen one and I can't imagine why you would need one.

 

[hardworkingstiff]

Not sure what you mean but single-phase is single-phase, whether it comes from a single or three-phase bank.

The definition of single-phase is.... ?

For the three phase bank, the pot's primary can have two primary "hots", or one primary "hot" and one primary neutral, for either the 120/240 volt case or the 120/208 case.

I think I know what you are saying, but the wording is a little confusing to me. I don't think my question was how many "hots" it took, but how many primary conductors. The neutral of a wye is a conductor

With only one pot, the primary can also have two primary "hots", or one primary "hot" and one primary neutral. For 240 volts, the one pot cases I've seen have all been either 240 or 120/240. I guess you could have a center-tapped 208 secondary, although I have not seen one and I can't imagine why you would need one.

When you feed a single-phase service with 2 phases of a 2-phase service, you essentially have a center-tapped 208 secondary.

Bottom line is, it takes 3 primary conductors to feed a single-phase service (whether it is 3 "hots" or 2 "hots and a neutral) consisting of 120/208-volts and only 2 primary conductors to feed a 120/240-volt single-phase service.

 

[hardworkingstiff]

Wikipedia

Wikipedia

This is an interesting way of looking at it.

A single-phase supply connected to an alternating current electric motor does not produce a revolving magnetic field;

Based on this definition, can we agree that 2-phases of a three-phase service would still have a "revolving magnetic field"? If yes (I know one leg is missing, but it is still revolving), then 2-phases of a three-phase service is not "single-phase".

 

[don_resqcapt19]

It can't revolve with only two hots...it only alternates.

 

[hardworkingstiff]

It can't revolve with only two hots...it only alternates.

Wouldn't alternate suggest a sort of flipping back and forth (the way a true single phase transformer secondary does)?

 

[hardworkingstiff]

Definition

Definition

Has anyone posted an accepted definition of "single-phase"?

 

[mivey]

The definition of single-phase is.... ?

The same as whatever you use for the 120/240 single phase service. It is the same animal, whether it comes from a single or three-phase bank.

Has anyone posted an accepted definition of "single-phase"?

How about IEEE:

single-phase circuit: An alternating-current circuit consisting of two or three intentionally interrelated conductors that enter (or leave) a delimited region at two or three terminals of entry. If the circuit consists of two conductors, it is intended to be so energized that, in the steady state, the voltage between the two terminals of entry is an alternating voltage. If the circuit consists of three conductors, it is intended to be so energized that, in steady state, the alternating voltages between any two terminals of entry have the same period and are in phase or in phase opposition.

two-phase circuit: A polyphase circuit of three, four, or five distinct conductors intended to be so energized that in the steady state the alternating voltages between two selected pairs of terminals of entry, other than the neutral terminal when one exists, have the same periods, are equal in amplitude, and have a phase difference of 90 degrees. When the circuit consists of five conductors, but not otherwise, one of them is a neutral conductor. NOTE—A two-phase circuit as defined here does not conform to the general pattern of polyphase circuits. Actually, a two-phase, four-wire or five-wire circuit could more properly be called a four-phase circuit, but the term two-phase is in common usage. A two-phase three-wire circuit is essentially a special case, as it does not conform to the general pattern of other polyphase circuits.

three-phase circuit: A three-phase circuit is a combination of circuits energized by alternating electromotive forces that differ in phase by one third of a cycle, that is, 120 degrees

six-phase circuit: A combination of circuits energized by alternating electromotive forces that differ in phase by one-sixth of a cycle, that is, 60 degrees.

 

[hardworkingstiff]

single-phase circuit: An alternating-current circuit consisting of two or three intentionally interrelated conductors that enter (or leave) a delimited region at two or three terminals of entry. If the circuit consists of two conductors, it is intended to be so energized that, in the steady state, the voltage between the two terminals of entry is an alternating voltage. If the circuit consists of three conductors, it is intended to be so energized that, in steady state, the alternating voltages between any two terminals of entry have the same period and are in phase or in phase opposition.

This seems to indicate 180 degrees out in reference to neutral.

three-phase circuit: A three-phase circuit is a combination of circuits energized by alternating electromotive forces that differ in phase by one third of a cycle, that is, 120 degrees

As referenced to neutral?

six-phase circuit: A combination of circuits energized by alternating electromotive forces that differ in phase by one-sixth of a cycle, that is, 60 degrees.

A referenced to neutral?

I seems like a neutral reference is required to determine "phase", and the definition of "single-phase" seems to require 180-degrees out of phase in reference to neutral. Therefore, 208/120 single-phase does not really exist. :roll:

 

[mivey]

This seems to indicate 180 degrees out in reference to neutral. As referenced to neutral? A referenced to neutral?

I seems like a neutral reference is required to determine "phase", and the definition of "single-phase" seems to require 180-degrees out of phase in reference to neutral. Therefore, 208/120 single-phase does not really exist. :roll:

I must admit, it sure read like that to me as well.

 

[quogueelectric]

I must admit, it sure read like that to me as well.

If I click my hooves three times will I go home now?

 

[mivey]

If I click my hooves three times will I go home now?

If you click three hooves at one time you might