Is it Single or Two Phase?

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IMHO single phase center tap is _single phase_ service. rick hart made the essential point in the third post of this thread: the offset in the zero crossings (and by implication the offset of the entire sine wave).

The power delivered by a single phase system must of necessity drop to zero twice per AC cycle. This is true no matter how many taps you have on the system; with a properly wired transformer you could have 5 wire 480/360/240/120 single phase; but the power would still drop to zero twice per AC cycle. When you have a polyphase system, you can always draw power from at least some of the phases.

Part of the confusion comes from the fact that we use the term 'phase' to describe both the time displacement of the AC cycle, and also to describe the energized conductor itself.

In a residential 120/240V system, you have two legs, and by common usage each leg can be referred to as a 'phase'. But if you do this then you really should describe these as phase A and phase A' (read that as "A-prime"), since there is no offset in the zero crossings of the AC. (And in the silly 480...120 system above you would have A, A', A'', A'''' and the neutral :) )

If you have a three phase center tapped delta, but look at only the two legs on the center transformer, then you still have _single_ phase power. There is no time offset between the two legs _as referenced to each other or the neutral_. The power delivered by these three terminals (the two hots and the neutral of the center tapped transformer) must fall to zero twice per cycle. It is only when you look at the voltages between all four of the taps on this delta transformer set that you have polyphase power.

If you have two legs and the neutral of a wye system, then even though you only have two legs and are using single phase equipment, this _is_ polyphase power. With a suitable load, connected to both legs and neutral, you could have power continuously delivered to the load; the zero crossings are offset between the various voltage pairs. You could even use these two legs with a suitable transformer set to re-derive the full three phase set.

-Jon
 
winnie said:
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If you have a three phase center tapped delta, but look at only the two legs on the center transformer, then you still have _single_ phase power. There is no time offset between the two legs _as referenced to each other or the neutral_. The power delivered by these three terminals (the two hots and the neutral of the center tapped transformer) must fall to zero twice per cycle. It is only when you look at the voltages between all four of the taps on this delta transformer set that you have polyphase power.

...
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That is incorrect. There is still a time offset: 240? lag between A and C [EDIT: full-cycle] zero volt crossings. The neutral, if it were not connected to ground, would have a 300? lag to A and precede the next cycle of A by 60?. Since it is bonded to ground, there is generally no discussion about it. In the case of 240/120 from a 240Δ system, voltage is the RMS value. This part of the system could accurately be described as a "two-phase 240? system". The original and so-called "two phase" is a 90? system.

[I stand corrected... see later post. My sincerest apologies, Jon]
 
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Wow!

Wow!

This part of the system could accurately be described as a "two-phase 240? system". The original and so-called "two phase" is a 90? system.
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WOW! I have never seen this in any technical books. Please provide some documentation as to your world of physics :eek:

I will be surprised if Jon is incorrect.
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I will be surprised and shocked!
 
Smart $ said:
That is incorrect. There is still a time offset: 240? lag between A and C [EDIT: full-cycle] zero volt crossings. The neutral, if it were not connected to ground, would have a 300? lag to A and precede the next cycle of A by 60?. Since it is bonded to ground, there is generally no discussion about it. In the case of 240/120 from a 240Δ system, voltage is the RMS value. This part of the system could accurately be described as a "two-phase 240? system". The original and so-called "two phase" is a 90? system.
Click to expand...

smart, that has got to undoubtedly be the best example of the term "Baffle them with B.S" ever.
appl.gif


You need to think "winding and legs" for the center tapped portion of the delta, not windings and phases, the other winding or windings, is or are not part of the 120/240 portion of this system. As far usability, it must be looked at as as a stand alone system, the same as a typical single winding 120/240 volt service would be.

One question regarding this center tap (I know, another question and with your past history of answering questions I don't expect an answer, but I'm a glutton for punishment) why would bonding/grounding the center tap of a winding matter as far as changing it's relationship electrically to legs 1 and 2 of the winding?

Roger
 
websparky said:
Smart $ said:
This part of the system could accurately be described as a "two-phase 240? system". The original and so-called "two phase" is a 90? system.
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WOW! I have never seen this in any technical books. Please provide some documentation as to your world of physics :eek:
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Ooops! I made a mistake. Technically it would be called a "two-phase 120? sytem" using the original two phase sytem as a reference. More later...

[Edit ADD] After further evaluation I may not have been wrong. Nevertheless, it isn't worth the effort to consider which is actually correct. [END Edit]
 
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As promised earlier...

See attached images (courtesy of American Electricians' Handbook, Thirteenth Edition, DIVISION 1: FUNDAMENTALS).

View attachment 52

View attachment 53

View attachment 54

Study them well. When you get to three phase, remember that for this discussion you will be using only Legs 1 and 3 and a center tap, 3{C}1, between them, so mentally remove Leg 2 from the diagram, graph, connections. If you need an actual diagram and graph I suppose I could draw 'em up... but I really don't want to put that much time into it. Additionally and technically, Leg 2 could be considered a center tap on the other side of the diagram. This is why Jon could make the statement about using two legs to generate the third. What you will use remains the same.

I'm sure someone will come along and comment on the images are regarding generators. But that is the major point. The emfs impressed on line circuits, including typical transformers and their secondaries, are the same phasing as the generated emfs. Simply using only two out of three does not change the emf relationships. Yes special transformers can change phasing, such as the ones depicted in the article Roger linked to, as can the use of capacitors and inductors, but that is not the case in this branch of the discussion (TTBOMK, anyway).
 
And what does any of that show?

It shows that there are three distinct and separate systems depicted on the page.

Single Phase

Two Phase

Three Phase.

But hey keep going don't let reality slow you down. :)
 
smart, that has got to undoubtedly be the best example of the term "Baffle them with B.S" ever.

-diddo

I personally think has to do with e to the minus j theta. :)

-Matt
 
I try to speak for myself

I try to speak for myself

Additionally and technically, Leg 2 could be considered a center tap on the other side of the diagram. This is why Jon could make the statement about using two legs to generate the third. What you will use remains the same. -smart $
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are you speaking for Jon now?


also... leg 2 could not be considered a center tap for 3 & 1.
 
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"Single phase" 60 Hz (typical residential power) can be represented as two separate sine waves, relative to the neutral.

One of them is:

L1 Voltage = A*sine(2*Pi*60*time) and the other is
L2 Voltage = -A*sine(2*Pi*60*time)

Where A is the peak voltage relative to the neutral. In a 120 Volt RMS system, A is about 170 Volts and the RMS voltage is Peak voltage divided by (Square root of 2).
The sine term varies from 0 to +1 to 0 to -1 to 0 during each cycle of 1/60th of a second.

Because L1 and L2 are always the same amplitude and opposite sign, when you take the difference between the two:

(L1 - L2) = 2A*sine (2*Pi*60*time) which produces 240 Volts RMS or about 340 Volts peak to peak.
 
Just for the record, the center-tapped transformer secondary of a 240/120v Delta or open-Delta (hi-leg) system is identical, and I mean identical, to a 240/120 single-phase secondary.
 
Exactly

Exactly

Larry,

You are absolutley correct. :cool:

The point to this thread (I think) is a residential 240/120V system is single phase with no angle relationships to other coils (if any). In a 3 phase delta with a center tap on one coil, the maximum recommended load is 5% of the total load. This is used on industrial locations that can use the delta and also need to run a few lights! Never is it used as a residential service configuration and never is it a 2 phase system.
 
Smart,
The pdfdoesn't show anything that is relevant to the question. If you have two hots from any AC system and connect it to a scope you will only see a single sine wave. That makes it a single phase system.
Don
 
Education from the DOE

Education from the DOE

The only approved method of wiring single-phase power is the scheme commonly referred to as the 3-wire, single-phase Edison system. The illustration depicts the use of a centertapped transformer, with the center tap grounded, providing half voltage (120 V) connections on either side or full voltage (240 V) across both sides.

3wire.jpg


The physical connections to the transformer secondary involve two insulated conductors and one bare conductor. If the conductor is a current-carrying leg or neutral leg, the conductor will be insulated. The remaining uninsulated conductor will serve as a safety ground and will be bonded to the ground point of the system. In all cases, 3 wires will be presented to the load terminals, and the safety ground will be bonded to each junction box, or device, in the distribution system. In the case of half voltage (120 V) use, the intended path of the current is from the supply leg through the load and back to the source on the neutral leg. No current would be carried on the ground unless a fault occurred in the system, in which case the current would flow safely to ground.

In the full voltage system (240 V), the insulated conductors are connected across the full winding of the transformer, and the uninsulated conductor is again bonded to the grounded center tap. In a balanced system, all currents will flow on the insulated conductors, and the grounded neutral will carry no current, acting only in a ground capacity. In the case of either an unbalanced load or a fault in the system, the bare conductor will carry current, but the potential will remain at zero volts because it is tied to the ground point. As in the case of the half voltage system, the uninsulated conductor will be bonded to each device in the system for safety.
 
don_resqcapt19 said:
Smart,
The pdfdoesn't show anything that is relevant to the question. If you have two hots from any AC system and connect it to a scope you will only see a single sine wave. That makes it a single phase system.
Don
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OK. I see where you're coming from (and Jon). However, the same is true if you hooked up an oscilloscope in the same manner to the original two phase hots (three-wire), which goes with the point I was really trying to make way back when, that 240/120 off of a center tap delta system is really a two-phase source, and residential power is single phase, derived in my neck of the woods from one leg of the poco three-phase distribution grid (See this image). Using one leg of a three phase power system is the only way to get true single phase.
 
roger said:
...

One question regarding this center tap (I know, another question and with your past history of answering questions I don't expect an answer, but I'm a glutton for punishment) why would bonding/grounding the center tap of a winding matter as far as changing it's relationship electrically to legs 1 and 2 of the winding?
Click to expand...
If you don't bond the center tap, it's output would be the same as a fourth phase conductor at an angle of +300? with respect to A phase at 0?.
 
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