why is it called 1 phase when its 110/220v. but 3 phase when its 3 "hots"

[Don Randall]

The dots do that by indicating winding starts.

Oh well, when I studied basic ac circuits, we had a lot of problems to solve using instantaneous ac voltages and currents and the polarities were always used in these problems. It is certainly not inappropriate to indicate polarities in an ac circuit and has been done for decades. The dots represent the phase relationship of the windings, but a student may need more information to understand precisely how the voltages in the separate conductors interact. For instance, he may not know that the center tap would be negative with respect to one end conductor, while at the same time it would be positive with respect ot the other end. Don

 

[K8MHZ]

The simple answer is:
The reference to "phase" is short for "phase angle differrence".
Describing the 60 Cycle/Second waveform timing of the voltage or current between sources. In a single loop with 2 wires the waveform phase angle difference will always be the same. Add more wires to the loop, a phase angle difference is possible.
With 4 wires it is possible to have a 2 phase system.
The RMS Voltage between wires is dependent on the phase angle difference between them and how they are connected.

Actually, there can be no simple answer in writing.

To really 'get the picture' of the difference between single and poly phase sources, a pattern on an oscilloscope helps like nothing else. You can actually see that both the 120 and 240 volt sources are in phase, just of different amplitudes. You can also see the different wave forms of poly phase sources and which wave is produced by which pair of conductors and why one phase needs two conductors and both two and three phase need three conductors.

I saw a video of this during my apprenticeship. I wish I could remember the title.

 

[Don Randall]

Yeah

Yeah

Marky the Sparky---I'll second that!---Don

 

[mivey]

In a three-phase system there are three different zero crossing points.

And in a six-phase you have how many?

I think the zero crossings comment should have stated a zero crossing when going from the negative half-cycle to positive half-cycle, or vice-versa.

 

[Besoeker]

I think the zero crossings comment should have stated a zero crossing when going from the negative half-cycle to positive half-cycle, or vice-versa.

My point wasn't about how many zero crossings in total, just that the three phases cross zero at different times. This is in contrast to the centre-tapped single-phase where the zero crossings are at the same point in time.

 

[K8MHZ]

And in a six-phase you have how many?

I think the zero crossings comment should have stated a zero crossing when going from the negative half-cycle to positive half-cycle, or vice-versa.

There would be six in a six phase system if determined in the same manner in which we say there are three in a three phase system.

The 'crossing' is the point in the center of the wave that crosses the zero line. The beginning and the end are not considered crossings even though in an instant before or after there would be a crossing.

Usually the wave starts on a positive rise so the crossings are usually from positive to negative.

FWIW, Tesla experimented with more than six phases. It was Tesla's work that determined that after three phases there exists a point of diminishing return so three phase became on of our standards.

 

[Besoeker]

Oh well, when I studied basic ac circuits, we had a lot of problems to solve using instantaneous ac voltages and currents and the polarities were always used in these problems.

TBH, it's something I neither used nor seen except on this forum.
The dot notation, yes. And we currently use it on gate driver transformers (for IGBTs and SCRs) to show how the input, output, and reset windings are to be connected. The dots tell you what you need to know.
Power transformers, we generally describe by vector group e.g. Dyn11 would be a typical distribution transformer.

 

[mivey]

There would be six in a six phase system if determined in the same manner in which we say there are three in a three phase system.

Then I think you and I see it differently. A six phase system can be derived by splitting the windings of a three-phase system.

The system that has traditionally been called a two-phase system (two single phase systems with a 90 degree separation) is an irregular system. It is correctly recognized as a subset of a four-phase system (or two 90-degree separated single-phase systems with a neutral point).

This fact is recognized in current and historic engineering texts and has been discussed in prior threads.

Historic naming conventions and the use of those historic naming conventions as an all-encompassing definition is what causes some of the trouble with a more complete understanding of the general definitions.

 

[weressl]

I always wondered this.

Thanks
Joe Sweeney

It's is called single phase in error. It is accurate to call it two phases, but of course it will never happen.

 

[mivey]

It's is called single phase in error. It is accurate to call it two phases, but of course it will never happen.

Two phase. Two phase. Two phase. Two phase.

Now it is a historic fact. :grin:

 

[weressl]

Two phase. Two phase. Two phase. Two phase.

Now it is a historic fact. :grin:

You mean histeric?

 

[mivey]

You mean histeric?

Probably.:grin:

 

[rbalex]

I always wondered this.

Thanks
Joe Sweeney

Joe,

phase of a periodic phenomenon f(t), for a particular value of t) The fractional part t/P of the period P through which f has advanced relative to an arbitrary origin.
Note: The origin is usually taken at the last previous passage through zero from the negative to the positive direction.

One problem is the word phase has at least 14 definitions. But I find the one above is the easiest to start from – if the recipient understands the mathematics.

The first thing to understand is we are discussing a “periodic phenomenon f(t);” in our case, it is a sine wave. We will call it “sine wave α” or just “α.” We will assume α has a regular period P. Note the amplitude of α is not relevant.

We will now pick our arbitrary origin, using the one suggested above; i.e., when α last passed through zero from the negative to the positive direction.

Any sine wave that has the same period P and passes through zero from the negative to the positive direction at the same time as α is said to be “in phase” with α. All such sine waves are considered to be “single phase” since the “fractional part t/P of the period P through which t has advanced relative to an arbitrary origin” is the same for all of them.

Any other sine wave that has the same period P but passes through zero from the negative to the positive direction at some other time than α is another phase.

Using any two conductors of a properly installed 120/240 system as an arbitrary reference and assuming the system is stable; with the proper instruments, measuring any combination of the three conductors would establish there is a common period P. Four (4) will be 120V and two (2) will be 240V. Since we can force the “passage through zero from the negative to the positive direction,” by simply reversing the instrument leads, of the six possible measurements, three will always be “in phase” with the original reference including itself. We will arbitrarily select them to be the “single phase” system. Two will be 120V and one will be 240V.

Using any two conductors of a properly installed 240D system as an arbitrary reference and assuming the system is stable; with the proper instruments, measuring any combination of the conductors would establish there is a common period P. All six (6) be 240V. Since we can force the “passage through zero from the negative to the positive direction,” by simply reversing the instrument leads, of the six possible measurements, one will always have a negative to positive zero crossing displaced in time by 1/(3P) and one by 2/(3P) from the original reference. We will arbitrarily select those two along with the original reference to be the “three phase” system.

A similar analysis for 120Y-208 system would result in twelve (12) measured voltages. Six (6) would be 120V and six (6) would be 208V. Using the six measurements that had the same voltage as the original reference we would find similar 1/(3P) and 2/(3P) time displacements in two of them. If we only considered passing through zero and ignored amplitudes and the crossing direction, we would find this is actually a six-phase system. This is one of the reasons it’s so friggin’ hard to calculate a combination of L-L and L-N loads. .

 

[mivey]

Joe,

One problem is the...

The first thing to understand...

We will now pick our arbitrary origin...

Any sine wave that has the same period...

Any other sine wave...

Using any two conductors... Since we can force...We will arbitrarily select...

Using any two conductors... Since we can force...We will arbitrarily select...

A similar analysis...Using the six measurements...time displacements...we would find this is actually a six-phase system...

I'm going to have to diagram this post to follow it. My ADD must be elevated today.:grin:

 

[rbalex]

I'm going to have to diagram this post to follow it. My ADD must be elevated today.:grin:

If it gets too hard, drop the last three paragraphs - they were only examples.