I know some people are going to say its the same thing but perhaps someone else will see what I'm trying to get at. Which is the more correct statement 120 volts is the result of 208 volts divided by 1.73 or 208 volts is the result of 120 volts multiplied by 1.73? I guess what I'm asking is which one is the base voltage?
wye voltage calculations
- Thread starter jimmie
- Start date
- Status
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: wye voltage calculations
The chicken or the egg.
If you pretend that a Wye 208V transformer was developed and they accidentally realized that if they measure from phase to X0 it is 120V, then you can believe that 120V is derived from 208V.
The argument the other way works too.
The chicken or the egg.
If you pretend that a Wye 208V transformer was developed and they accidentally realized that if they measure from phase to X0 it is 120V, then you can believe that 120V is derived from 208V.
The argument the other way works too.
wanderer20001us
Senior Member
Re: wye voltage calculations
Voltage systems are configured with specific purposes in mind. Which came first, 480 of 277? How about the high leg 240/120 systems with the 208 high leg? Once you've picked your predominant voltage and configuration, phase to phase or phase to neutral, geometry determines the rest.
That's my two cents
Sam A - PE
Voltage systems are configured with specific purposes in mind. Which came first, 480 of 277? How about the high leg 240/120 systems with the 208 high leg? Once you've picked your predominant voltage and configuration, phase to phase or phase to neutral, geometry determines the rest.
That's my two cents
Sam A - PE
Ed MacLaren
Senior Member
Re: wye voltage calculations
jimmie,
Both are correct, depending on whether you are calculating the phase voltage or line voltage.
Here's a little basic theory that I used to use with my students.
Ed
jimmie,
Both are correct, depending on whether you are calculating the phase voltage or line voltage.
Here's a little basic theory that I used to use with my students.
Ed
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: wye voltage calculations
If you are, then I have two answers for you. The first is that you are free to choose any two of the four terms listed above, and then you calculate the other two. If you were to choose ?base voltage,? you must make the further choice of ?line-to-line voltage? or ?line-to-ground voltage.? The correct answer to your question, then, is this, ?There is no ?base? quantity until you choose two of the four, after which you are no longer free to choose the others (i.e., they are to be derived from the first two.?
My other answer is that there is one set of quantities that is almost always chosen in any analysis. We look at any transformer in the system, and we choose its KVA rating (or MVA rating) as the ?base apparent power,? and we choose its secondary line-to-line voltage as the ?base voltage.? All else follows from this choice. In this case, the correct answer to your question would be ?208.?
- - - - - - - - - - - - - - - - - - - - - - - - - - -
If the above discussion does not help answer your question, then I will offer one more perspective. The ABSOLUTE TRUTH of the matter is that when we say ?120,? and when we say ?208,? we are being imprecise. I might even go so far as to assert that we are being sloppy. Both numbers are commonly used in conversational English. But both are incomplete, and both are sloppy. The right thing to say, the true thing, the complete thing, the precise thing, is to say ?120 volts line-to-ground? and to say ?208 volts line-to-line.? Then it becomes clear that the two phrases convey identical information. If one is true, the other is also true. If one is false, the other is false. Neither contains more truth than the other. But we seldom use these two complete phrases; we just say ?120? and ?208.? But if we at least think of the two numbers as being part of the two complete phrases, then we can avoid any confusion.
There is an analytical technique that applies to three-phase systems, and that makes frequent use of the terms, ?base voltage,? and ?base current,? and ?base apparent power,? and ?base impedance.? The technique is called ?per unit analysis.? Are you using the term ?base voltage? in this context? If not, then you may disregard the next two paragraphs.
If you are, then I have two answers for you. The first is that you are free to choose any two of the four terms listed above, and then you calculate the other two. If you were to choose ?base voltage,? you must make the further choice of ?line-to-line voltage? or ?line-to-ground voltage.? The correct answer to your question, then, is this, ?There is no ?base? quantity until you choose two of the four, after which you are no longer free to choose the others (i.e., they are to be derived from the first two.?
My other answer is that there is one set of quantities that is almost always chosen in any analysis. We look at any transformer in the system, and we choose its KVA rating (or MVA rating) as the ?base apparent power,? and we choose its secondary line-to-line voltage as the ?base voltage.? All else follows from this choice. In this case, the correct answer to your question would be ?208.?
- - - - - - - - - - - - - - - - - - - - - - - - - - -
If the above discussion does not help answer your question, then I will offer one more perspective. The ABSOLUTE TRUTH of the matter is that when we say ?120,? and when we say ?208,? we are being imprecise. I might even go so far as to assert that we are being sloppy. Both numbers are commonly used in conversational English. But both are incomplete, and both are sloppy. The right thing to say, the true thing, the complete thing, the precise thing, is to say ?120 volts line-to-ground? and to say ?208 volts line-to-line.? Then it becomes clear that the two phrases convey identical information. If one is true, the other is also true. If one is false, the other is false. Neither contains more truth than the other. But we seldom use these two complete phrases; we just say ?120? and ?208.? But if we at least think of the two numbers as being part of the two complete phrases, then we can avoid any confusion.
scott thompson
Senior Member
- Location
- Anaheim, California
Re: wye voltage calculations
Would like to toss in some very basic and brief example "Answers" for the original person's question: "jimmie" - question re:
Wye Voltage Calcs.
For a "Real" Wye connected Transformer: The Transformer is comprised of three individual 2 wire coils on the secondary
side, which are connected together at one end - forming a "Star" or "Wye" looking situation.
For a 208Y/120 3?4W Wye, each individual coil is wound for 120 VAC.
Multiplying 120 VAC ? 1.732 results in an L-L Voltage of 208 VAC.
For a 480Y/277 3?4W Wye, each individual coil is wound for 277 VAC.
Multiplying 277 VAC ? 1.732 results in an L-L Voltage of 480 VAC.
For a 600Y/347 3?4W Wye, each individual coil is wound for 347 VAC.
Multiplying 347 VAC ? 1.732 results in an L-L Voltage of 600 VAC.
For Delta connected secondaries:
If the setup is a Closed Delta - using 3 individual coils:
Connecting one coil's end to another coil's end will eventually result in three coils connected in a parallel fashion, and the
setup will look like a triangle.
If the setup is an Open V Delta - using 2 individual coils:
Connection is similar to the above - just eliminate one coil.
Result is a "Vee" looking setup - or a triangle missing one side.
These setups above yield a 3 wire setup. To achieve a 4 wire setup, one coil gets center tapped.
Voltage on the Delta will be the voltage of one coil. If the output is 240 VAC 3? 3W, then each coil is wound to be 240
VAC. Same goes for 480 and 600 VAC setups.
On the 4 wire setup, the center tapped coil still is 240 VAC, only since there is a center tap, there will be 120 VAC from
either end of that coil to the center tap.
BTW, there will be around 208 VAC from the "Top" of the triangle to the center tap. This is known as the infamous
High Leg of the Delta 4 wire.
FYI: If the setup is an "Open T" Delta, 2 coils are used as with the "Vee" setup, only they are connected to form a "Tee". One
coil is connected via taps to the "Main" coil.
Also, per the reference of a "Real Wye" as noted in the beginning, small transformers of less than 15 KVA for
120/208 3? 4W output are typically an Open Delta T setup.
Additional info and schematics may be found at ECN - technical reference area.
Feel free to ask me for more information or links to these items.
E-mail if needed to: adst at pacbell dot net - just remove the spaces and use proper characters.
Scott
p.s. sorry for the odd text wrap, been trying to post this message and keep getting HTML errors per tags within brackets - so trimmed things down a bit!
Would like to toss in some very basic and brief example "Answers" for the original person's question: "jimmie" - question re:
Wye Voltage Calcs.
For a "Real" Wye connected Transformer: The Transformer is comprised of three individual 2 wire coils on the secondary
side, which are connected together at one end - forming a "Star" or "Wye" looking situation.
For a 208Y/120 3?4W Wye, each individual coil is wound for 120 VAC.
Multiplying 120 VAC ? 1.732 results in an L-L Voltage of 208 VAC.
For a 480Y/277 3?4W Wye, each individual coil is wound for 277 VAC.
Multiplying 277 VAC ? 1.732 results in an L-L Voltage of 480 VAC.
For a 600Y/347 3?4W Wye, each individual coil is wound for 347 VAC.
Multiplying 347 VAC ? 1.732 results in an L-L Voltage of 600 VAC.
For Delta connected secondaries:
If the setup is a Closed Delta - using 3 individual coils:
Connecting one coil's end to another coil's end will eventually result in three coils connected in a parallel fashion, and the
setup will look like a triangle.
If the setup is an Open V Delta - using 2 individual coils:
Connection is similar to the above - just eliminate one coil.
Result is a "Vee" looking setup - or a triangle missing one side.
These setups above yield a 3 wire setup. To achieve a 4 wire setup, one coil gets center tapped.
Voltage on the Delta will be the voltage of one coil. If the output is 240 VAC 3? 3W, then each coil is wound to be 240
VAC. Same goes for 480 and 600 VAC setups.
On the 4 wire setup, the center tapped coil still is 240 VAC, only since there is a center tap, there will be 120 VAC from
either end of that coil to the center tap.
BTW, there will be around 208 VAC from the "Top" of the triangle to the center tap. This is known as the infamous
High Leg of the Delta 4 wire.
FYI: If the setup is an "Open T" Delta, 2 coils are used as with the "Vee" setup, only they are connected to form a "Tee". One
coil is connected via taps to the "Main" coil.
Also, per the reference of a "Real Wye" as noted in the beginning, small transformers of less than 15 KVA for
120/208 3? 4W output are typically an Open Delta T setup.
Additional info and schematics may be found at ECN - technical reference area.
Feel free to ask me for more information or links to these items.
E-mail if needed to: adst at pacbell dot net - just remove the spaces and use proper characters.
Scott
p.s. sorry for the odd text wrap, been trying to post this message and keep getting HTML errors per tags within brackets - so trimmed things down a bit!
Re: wye voltage calculations
Thank for the replies. As mentioned on another post Ed your drawing are quite helpful. Also Charlie, I might not be using the term base correctly. My question stemed from a small lab board I set up to show the neutral current on a three phase wye system. The board is very basic using one switch and three 200 watt bulbs (in parallel) per phase. Turning on each load and measuring the neutral current. Everything was fine. But then out of curiosity I disconnected the neutral. With no neutral. Turning on one switch did nothing of coarse. Turning on two switches created a 208 circuit with the voltage midway (where the neutral would be) to be 104. Turning on the third switch (completing the wye) the voltage (to the center) went up to 120. A noticeable increase in the light output. Too bad I can't draw this out.
I can't fully explain the 104 volt portion of the circuit without getting the class and myself confused.
I believe Charlie has me close to my explanation but I'm still coming up short. When only using phase "A&B". Why only 104 volts without a neutral and 120 volts with? Also,when using phase "A&B". Why 104 volts (to center)without "C" and 120 volts (to center)with "C". Without a neutral.
Understanding this myself is a small problem. Explaining this to a third year class is a big problem. Recommended text reading would also be appreciated.
Thank for the replies. As mentioned on another post Ed your drawing are quite helpful. Also Charlie, I might not be using the term base correctly. My question stemed from a small lab board I set up to show the neutral current on a three phase wye system. The board is very basic using one switch and three 200 watt bulbs (in parallel) per phase. Turning on each load and measuring the neutral current. Everything was fine. But then out of curiosity I disconnected the neutral. With no neutral. Turning on one switch did nothing of coarse. Turning on two switches created a 208 circuit with the voltage midway (where the neutral would be) to be 104. Turning on the third switch (completing the wye) the voltage (to the center) went up to 120. A noticeable increase in the light output. Too bad I can't draw this out.
I can't fully explain the 104 volt portion of the circuit without getting the class and myself confused.
I believe Charlie has me close to my explanation but I'm still coming up short. When only using phase "A&B". Why only 104 volts without a neutral and 120 volts with? Also,when using phase "A&B". Why 104 volts (to center)without "C" and 120 volts (to center)with "C". Without a neutral.
Understanding this myself is a small problem. Explaining this to a third year class is a big problem. Recommended text reading would also be appreciated.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: wye voltage calculations
The simple answer is that you are adding 120 apples to 120 oranges, and there is no reason to expect the answer to be 240 of anything. The more complicated answer is that you are really subtracting ?120 volts at an angle of 0 degrees? MINUS ?120 volts at an angle of 120 degrees.? The result is ?208 volts at an angle of negative 30 degrees.? This is a subtraction, not an addition, because current is flowing out one winding (from center to hot) and into the other (from hot to center). The 104 volts you see on each set of light bulbs is simply half of the 208 volts.
To give a more thorough (and easier to follow) explanation would require me to be able to draw and point, while talking. I don?t know how to do that on this type of forum.
I can?t draw pictures and post them either. But let me describe how I think your bench circuit will work. You have a 3-phase transformer with a 120/208V secondary that is connected in a WYE configuration. When you say you disconnected the neutral the threw two switches, here is what I think you have: From the center point of the transformer, current will flow through one secondary winding, through one switch, through one set of lights, and to a common tie point. From that point, current will flow ?backwards? through the other set of lights, the other switch, the other secondary winding, and finally return to the center point of the transformer. I infer that you are measuring the 104 volts across each set of light bulbs. You did not say this, but I believe that if you were to measure across one of the two secondary windings (pick either one, the result should be the same), you would read 120 volts. So the question is, ?How does 120 plus 120 (across the two secondary windings) become 104 plus 104 (across the two sets of light bulbs?? Do I have your question right?
The simple answer is that you are adding 120 apples to 120 oranges, and there is no reason to expect the answer to be 240 of anything. The more complicated answer is that you are really subtracting ?120 volts at an angle of 0 degrees? MINUS ?120 volts at an angle of 120 degrees.? The result is ?208 volts at an angle of negative 30 degrees.? This is a subtraction, not an addition, because current is flowing out one winding (from center to hot) and into the other (from hot to center). The 104 volts you see on each set of light bulbs is simply half of the 208 volts.
To give a more thorough (and easier to follow) explanation would require me to be able to draw and point, while talking. I don?t know how to do that on this type of forum.
Ed MacLaren
Senior Member
Re: wye voltage calculations
Jimmie, Charlie B was correct when he said
Think of one load at a time (in isolation from the other two). With the neutral connected, that load is connected directly across (in parallel with) one 120 volt phase of your power supply transformer.
With the neutral not connected, that load is connected in series with another load, across two phases of your power supply transformer. (208 volts)
Ed
Jimmie, Charlie B was correct when he said
120 + 120 = 208 when they are 120 degrees "out of phase". (See my sketch #4 above.)
It is the neutral conductor that maintains the 120 volts across each individual load.
Think of one load at a time (in isolation from the other two). With the neutral connected, that load is connected directly across (in parallel with) one 120 volt phase of your power supply transformer.
With the neutral not connected, that load is connected in series with another load, across two phases of your power supply transformer. (208 volts)
This happened because your loads were balanced. They all had the same resistance, therefore all drew the same number of amps of current, so that their voltage drops were all equal, with or without the neutral. There was no "unbalanced load" current to flow on a neutral, even if one existed.
Ed
- Status