Single Phase

[ken44]

What is the specif theory behind why we refer to 2 poles as single phase?

 

[charlie b]

The current leaves via one pole, goes to and through the load, and returns via the other pole. If you look at the current's pattern on an o-scope, you will see only a single sine wave. This is as opposed to a three phase system, in which current leaves via one pole, and can take one of two possible paths through the load, and therefore can return to the source via either one of the two remaining poles. On an o-scope you would see three sine waves, each spaced one third of a cycle from the others.

 

[winnie]

It isn't really appropriate to call this 'theory' in the sense of a physics theory...this is a name assigned by people, and thus perhaps a sociological theory.

One way to think about it is to describe the similarities and differences between two systems.

So: If you have a 'two wire' AC system, then you can only have 1 voltage (measured between those two wires), and no matter what you only have single phase.

Once you have 3 wires, you have 3 possible voltages that you can measure (between any pair of wires). For an AC system you may ask 'what are the phase angles between the measured voltage'?

With single phase systems, the phase angle between all (3) of your measurements will be 0 (or 180). The 'zero crossing' of all of the voltages will be in the same place. There is no phase angle difference, and no ability to generate a rotating field. The supply comes off of a single transformer coil, with a center tap, or perhaps two coils wound on the same magnetic core.

With polyphase systems, the supply comes off of multiple coils, with non-zero phase angle measurements. You can have continuous delivery of power because the 'zero crossing' of the voltage measurements will happen at different times. Because of the timing difference a set of coils can generate a rotating field and motors can start without capacitors or other energy storage/phase change elements.

The exception is when you have two legs of a 208V wye supply, say in a single apartment of a large building. In this case you do have a real polyphase system, which has phase angle differences and can run motors...but it is only used for the sort of loads single phase loads used in a residence. Because it is _used_ like a single phase system, it is called a single phase system, even though it is really a polyphase system.

-Jon

 

[J.P.]

My oversimplified answer is because it comes from one phase of the utility power.

for a common 120/240 delta It goes into a transformer and they give opposite sides of that single phase. the null point in the windings is the neutral.

As far as you panel is concerned you have 2 phases.

I imagine they do this because it is much cheaper to run single power lines all though out an area.

 

[meternerd]

My oversimplified answer is because it comes from one phase of the utility power.

for a common 120/240 delta It goes into a transformer and they give opposite sides of that single phase. the null point in the windings is the neutral.

As far as you panel is concerned you have 2 phases.

I imagine they do this because it is much cheaper to run single power lines all though out an area.

I think your answer is close. The utillity usually does only supply one phase and neutral, and the secondary is 120/240 center tapped off of that transformer. But...there's always the exception. On a system where there is no primary neutral, the transformer is wired phase to phase, making it really a two phase source. Still 120/240 secondary...just a different primary voltage. Common voltages are 7,200 for line to neutral and 14,400 for phase to phase.

 

[zbang]

On a system where there is no primary neutral, the transformer is wired phase to phase, making it really a two phase source.

It may be a two pole source, but still only one phase.

 

[meternerd]

It may be a two pole source, but still only one phase.

Nope...most distribution is three phase out of the substation. 7200 is 12,470 WYE three phase measured phase to neutral. 14,400 phase to phase is usually three phase WYE center grounded at the substation but the neutral is not run to the distribution circuits from the sub. Three phase that uses phase to phase involves TWO phases, whether it's Delta or Wye. Transformer ratios are either 7200 to 240/120 (30:1)or 14,400 to 240/120 (60:1) center tapped secondary. Sub-transmission tends to be Delta. At least that's what I've seen in my experience. Not arguing, just commenting. Not sure what you mean by a "two pole source".

 

[jim dungar]

The utillity usually does only supply one phase and neutral...

Are you using the term Phase to describe a single physical conductor?

I have noticed that 'Utility industry' terminology is often different than IEEE/ANSI terminology, especially when it comes to describing voltage systems (e.g. using LV/HV versus HV/LV based on the system).

 

[meternerd]

Are you using the term Phase to describe a single physical conductor?

I have noticed that 'Utility industry' terminology is often different than IEEE/ANSI terminology, especially when it comes to describing voltage systems (e.g. using LV/HV versus HV/LV based on the system).

No...on a three phase four wire distribution system, a single phase tap involves one PHASE conductor and a NEUTRAL. Still two wires or there's no circuit. On a three phase three wire system, the circuit is between any two PHASES.

 

[jim dungar]

.... On a three phase three wire system, the circuit is between any two PHASES.

No it is between any two PHASE CONDUCTORS.

 

[meternerd]

No it is between any two PHASE CONDUCTORS.

Huh? A, B, C, N, one conductor for each....am I missing something?:?

 

[ken44]

Charlie gave the answer that I was looking for, but thank you all for your input.

 

[electrofelon]

My oversimplified answer is because it comes from one phase of the utility power.

Around central NY, we have a lot of 2 wire lines where both are ungrounded (2 phases of a three phase system), a lot more than grounded - ungrounded. I know in some areas the MGN system is all people see.

 

[kwired]

Charlie gave the answer that I was looking for, but thank you all for your input.

I like that answer best myself, but beware this very topic has turned into threads hundreds of posts long just on debate over the definition of "phase".

 

[JRW 70]

Utility Terminology

Utility Terminology

I understand what was said above, but our line crews
call a two insulator (high side) two phase in the areas
where there is only three phase delta. Yes it could be
considered wrong but it is only semantics. If they get
it wired up right for our customers that's all that really
matters any way.
In my time I've been on both sides of this fence as to
what to call the high side when pulled from two of the
phase conductors, but on the other side of the transformer
it is still the 120/240v single phase system we would get
from one primary phase and the grounded system neutral.
Both installations are right and are reliable.
(Just my opinion)

JR

 

[kwired]

I understand what was said above, but our line crews
call a two insulator (high side) two phase in the areas
where there is only three phase delta. Yes it could be
considered wrong but it is only semantics. If they get
it wired up right for our customers that's all that really
matters any way.
In my time I've been on both sides of this fence as to
what to call the high side when pulled from two of the
phase conductors, but on the other side of the transformer
it is still the 120/240v single phase system we would get
from one primary phase and the grounded system neutral.
Both installations are right and are reliable.
(Just my opinion)

JR

You kind of said it in your post there - "pulled from two phase conductors"

In order to truly be "two phase" there about has to be at least three supply conductors, otherwise with only two input conductors there is only one current path and only will be one sine wave to measure, unless you try to connect more then one source to the same pair of conductors - then be careful as the one with more voltage had better be countered by higher impedance from the other one or smoke is likely being released from at least one of them. That setup is probably useless for power reasons but there are control/communications signals injected into higher voltage systems which can give you more then one wave to look at is why I bring it up, but those two waves are essentially two separate systems sharing same conductor and not a two phase system.

 

[meternerd]

OK...I finally get it.....the technically correct terminology for a Delta PHASE is A-B, A-C or B-C. As said above by JR 70, we in the POCO world call a conductor carrying line voltage a PHASE and the grounded conductor a NEUTRAL. So, if we go out on a line and find a wire laying on the ground, the first question is always "Is it a PHASE or a NEUTRAL?" Thanks to all for the clarification. Have a gidday.:thumbsup:

 

[winnie]

This goes back to naming, and though a tangent speaks to the original poster's question.

If you only have two conductors from the source, weather they be two phase conductors, a phase and a neutral, or even a phase and the earth (for high voltage 'single wire earth return') then you can only supply _single phase power_ to a single phase load.

The voltage _between_ a single pair of conductors on an AC system is always a single phase. You need more than 2 conductors to have multiple phases.

But from the point of view of the _source_ supplying these conductors, a single L-N load adds load to a single phase, whereas a single L-L load adds loading to _two_ phases.

In the situation that JRW 70 is describing, a single phase load is being served, but power is being drawn from 2 distribution phase conductors.

-Jon

 

[just the cowboy]

look on a scope

look on a scope

I grew up with all three, Single phase, Two phase, and Three phase.

When you look at them on a scope you will only see one sign wave for a single
phase system, it is referenced to ground so you get a positive RMS phase to
ground readind, a negative RMS phase to ground reading, or a positive to negative
RMS reading.


On a two phase system ( it was 5 wire I think, don't know if they are still used, most people never
heard of it) A scope shows two sign waves 180 degrees out of phase. And on a
three phase system you have 3 sign waves 120 degrees out of phase. Resolvers use two phase, 90 degrees out of phase


So if someone says two phase it is a sure bet they mean single phase. Bet them
it does not have two phases on a scope.

 

[kwired]

I grew up with all three, Single phase, Two phase, and Three phase.

When you look at them on a scope you will only see one sign wave for a single
phase system, it is referenced to ground so you get a positive RMS phase to
ground readind, a negative RMS phase to ground reading, or a positive to negative
RMS reading.


On a two phase system ( it was 5 wire I think, don't know if they are still used, most people never
heard of it) A scope shows two sign waves 180 degrees out of phase. And on a
three phase system you have 3 sign waves 120 degrees out of phase. Resolvers use two phase, 90 degrees out of phase


So if someone says two phase it is a sure bet they mean single phase. Bet them
it does not have two phases on a scope.

The scope needs the ability to connect to more then two leads as well or you will still only see one wave on the scope