t williw
- Thread starter t willie
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- Chapel Hill, NC
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- Retired Electrical Contractor
Two 120V legs will give you 240V if it is coming from 2 phases but that does not make it a 2- phase system. Someone else will need to explain that.
You can have 240v on a single phase 3 wire system --- 2hots and one neutral, you can have 240V or 208V on a 3 phase system if you connect to just 2 phases.
Hopefully someone else will explain what a 2 phase system is but I am not sure that is what you want to know.
You can have 240v on a single phase 3 wire system --- 2hots and one neutral, you can have 240V or 208V on a 3 phase system if you connect to just 2 phases.
Hopefully someone else will explain what a 2 phase system is but I am not sure that is what you want to know.
- Location
- Lockport, IL
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- Retired Electrical Engineer
Here is a simple way to look at it. Voltage must be measured between two points. If you take a 120/240V standard single phase household panel, and measure between the neutral point and one of the phases, and then look at the results on an oscilloscope, you will see a sine wave. If you measure between the neutral point and the other phase, you will also see a sine wave, and it will look pretty much the same as the first one. If you instead measure from one phase to the other, you will still see a sine wave, but the peak to peak size will be double what you had seen before. The fact that you still see a sine wave, and not a set of two or three separate sine waves, tells you that you are still seeing a single phase system.
mivey
Senior Member
To simplify what charlie said:
Any two-terminal source is single-phase. It is that by default. One voltage, one current. End of story.
To elaborate on what you might see:
The issue can be complicated by considering multiple two-terminal sources, but to make it simple, we have a standard naming convention for the systems you will mostly see (using a 120v nominal reference):
A 3-wire two-phase system has two voltages that are in quadrature phase (displaced by 90?). You would get two 120v line-to-neutral voltages and one 169.7v line-to-line voltage. The system name "two-phase" is reserved for the quadrature phase system.
A 4-wire three-phase wye system has three voltages displaced by 120?. You get three 120v line-to-neutral voltages and three 207.8v line-to-line voltages.
If we drop one of the lines of the wye, we get a 3-wire 120/208v system called a network system. The voltages almost always are used as single-phase voltages (i.e., two terminals at a time for the end loads) so is also called a single-phase system by some.
A 3-wire three-phase delta does not have the neutral like the wye and you just have line-line voltages. The voltage is usually higher and you have 240v line-line voltages with 120? displacements.
The delta can have the middle of one of the windings grounded and used as a neutral point for smaller single-phase loads. This adds two 120v line-neutral voltages. You also add one 207.8v line-neutral voltage but it is not normally used (the "high leg").
The 120/240v system is a single-phase system. It is like the delta with the neutral point but with the "high leg" removed. Even though we have both 120v and 240v available, and the two lines can be the two lines from a three-phase system, we call it single-phase.
There are many pics of these available here if you want to see them more clearly.
Any two-terminal source is single-phase. It is that by default. One voltage, one current. End of story.
To elaborate on what you might see:
The issue can be complicated by considering multiple two-terminal sources, but to make it simple, we have a standard naming convention for the systems you will mostly see (using a 120v nominal reference):
A 3-wire two-phase system has two voltages that are in quadrature phase (displaced by 90?). You would get two 120v line-to-neutral voltages and one 169.7v line-to-line voltage. The system name "two-phase" is reserved for the quadrature phase system.
A 4-wire three-phase wye system has three voltages displaced by 120?. You get three 120v line-to-neutral voltages and three 207.8v line-to-line voltages.
If we drop one of the lines of the wye, we get a 3-wire 120/208v system called a network system. The voltages almost always are used as single-phase voltages (i.e., two terminals at a time for the end loads) so is also called a single-phase system by some.
A 3-wire three-phase delta does not have the neutral like the wye and you just have line-line voltages. The voltage is usually higher and you have 240v line-line voltages with 120? displacements.
The delta can have the middle of one of the windings grounded and used as a neutral point for smaller single-phase loads. This adds two 120v line-neutral voltages. You also add one 207.8v line-neutral voltage but it is not normally used (the "high leg").
The 120/240v system is a single-phase system. It is like the delta with the neutral point but with the "high leg" removed. Even though we have both 120v and 240v available, and the two lines can be the two lines from a three-phase system, we call it single-phase.
There are many pics of these available here if you want to see them more clearly.
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