The images and the exercise of measuring point on the circuits relative to where you have your meter black lead was designed to cause one to understand there is no "phase" in a single phase system. Well, there is "a" phase I guess....a single phase.
Consider this when looking at an AC sine wave power source......You sit down at the output leads of a single phase generator that is powered by a piston engine. The engine has an ideal throttle and you have it throttled to run at exactly 3600 RPM and the generator winding is such that at this RPM you get 60 cycles per second output. You can access the output on two wires, L1 and L2. The generator is not grounded to earth and no part of the generator output is grounded to the frame. No ground reference at all. You also have two "magic" controls. The controls are labeled, "stop time" and "slow time advance". The first will stop time so you can study what is going on. the second will allow you to "jog" time, moving it slightly forward. Connect to L1 and L2 are a couple of LED, connected opposite each other with resistors in series so they will not burn up. One LED outputs blue and the other one red. You also have a DC volt meter. The generator is running and it appears that blue and the red LEDs are on all of the time. You try to measure the voltage between L1 and L2 but get no reading.
Bob walks up and you tell him you are trying to study the output of this generator and are just getting started, but you really don't have the type of meter that you need. Bob see the Stop Time button and presses it. Now only the red LED is on. You measure the volts and now you have a reading of 47 volts. That is a bit odd. Bob likes to press buttons, so Bob presses the Jog button to see what will happen. You notice the voltage now says 16. Still this is not making clear sense. Bob again hits Jog. Now the blue LED is on and the red LED is off. The volts read -97. Somehow when the blue LED is on, the voltage is negative compared when the red LED is on. But if you switch your meter leads so that black is on L2, the voltage is negative when the red LED is on. You and Bob continue playing with Jog until you determine that the voltage is never more than 340v and that the red and blue LED are off and on in an alternating fashion. Then Steve shows up and has an oscilloscope. You connect it to L1 and L2, then release the Stop Time control and you see that you have a sine wave on the scope. Steve also has an AC volt meter and you discover AC RMS voltage is 240 volts. Now you have it all figured out.
Steve says, "Hey, I have a generator just like that. Do you think we can hook them up in tandem so they can output twice the power, working together?" Larry has been standing by enjoying the fun and adds, "You better make sure you get them in phase with each other before you connect them together!" So Steve gets his generator out and fires it up. Like yours it will hold 3600 RPM without waver. You know you need to get them in phase and if you can see both output waves on the scope, you can tell if they are in phase or not. So you connect Steve's L1 and L2 up on channel B of the scope, hoping that will work. Well you can see something but it makes no sense. Then you remember that for years you have been carrying this thing called a signal generator around and was never sure what to do with it, but you know it can output a 60 cycle square wave and you know that you can make the scope lock into that so the lines sweep across the scope screen at 60 cycles, independent of what is on either channel A or channel B. Now you can see two perfect sine waves but they are not lined up. Bob is still much interested in those buttons and hits the Jog again but that button will only affect your generator. You see the wave on channel slide over a bit. It is now actually closer to lining up with channel B than it was before. "Hit it again", Steve shouts. After several tries it looks like it is lined up as perfect as anyone could expect, with Bob at the controls. It looks good. The high points are in line. The low points are in line. The zero crossings are all in line.
So Steve grabs some wire nuts and a heavy cord with a 240v plug on the end. Steve is a man of men and is determined to connect up "hot". He joins the two L1 lines together and to his power cord, first wire with the wire nuts. He is just about to join the two L2 lines together and you have a thought. "What if we missed something somehow. This could be bad if done incorrectly." You have a new Iphone charging cable and the cable has one of those little wire twist ties on it. You get the twist tie, remove the plastic from it, leaving a thin bare wire. You tie the thin wire to Steve's L2, hand your safety glasses to Bob, and say, "Bob touch that little wire to that one on my generator marked L2 so we can get some more readings." Poof, in a flash of light the little wire is gone and Bob has a funny look on is face. What could be wrong?
You look at your generator closely and then look at Steve's generator. You see that Steve's L1 exits the housing right where your L2 exits, like his wires are backwards from yours. We had the waveforms lined up, so how can they be different? Not sure but let's switch one of them and try again. Now looking at the scope, the waves are still in sync but the top and the bottom are opposite each other. The generator leads were marked wrong on Steve's generator. We switched them and NOW the waves are messed up. So we swap the scope channel B's ground and probe leads on Steve's L1 and L2, and all looks good again. So swapping the scope leads around, made the waveform move 180 degrees? After digging around in the floor of Steve's truck, Steve finds a bread twist tie and says, "Lets use this". Again Bob is called on for his special skills. This time the wire does nothing, like nothing is going on. Steve brings his AC meter over and measures between both L2 lines from the two generators and reads zero volts. Yes, that make sense. We wouldn't want anything happening there. The work needs to be connect to L1(combined) and L2(combined) ....and we should get twice the power.
The end.
The story above demonstrates two phases, one from each generator. While they were brought into phase, a polarity issue cause confusion both with what was seen on the scope and in the connections made. At a given frequency (cycles per second) being out of phase means a time shift between phases.
In a single phase power source, there is one phase and no part has a time shift. What looks like 180 degrees "out of phase" is a polarity issue in the way the scope is connected. The 4 images that I posted are to allow a person to see this if the mental exersize is done. Set your black meter lead at ground and in the first battery image you should get 0v, 12v and 24v as you measure 3 points in the open circuit. If you do the same on the second battery image you will get -12v, 0v, 12v. That does not mean that the battery on the left is reversed (or out of phase), but means the meter leads are reversed. Do the same thing with an AC volt meter and you will get 0, V and 2 times V on the first transformer image. Repeat on the second transformer image and you will get V, 0, V. No negative because you don't see polarity with the meter. Repeat with a scope and you can see polarity, the lower half and the upper half are flip-flopped because you flipped the direction of your metering according to the direction of current (at any instant in time).
Again one phase in single phase. Polirity on a scope shows one half of the transformer output upside down if you keep your reference connection on the center tap and move both up and down the coil from that point. If you have a scope without any common connection between both probe ground leads and the scope is isolated from ground, you should be able to do this for real. But use extreme care. Even with 12volt systems you can melt your scope leads if you don't have your ducks in a row.
Dan
Consider this when looking at an AC sine wave power source......You sit down at the output leads of a single phase generator that is powered by a piston engine. The engine has an ideal throttle and you have it throttled to run at exactly 3600 RPM and the generator winding is such that at this RPM you get 60 cycles per second output. You can access the output on two wires, L1 and L2. The generator is not grounded to earth and no part of the generator output is grounded to the frame. No ground reference at all. You also have two "magic" controls. The controls are labeled, "stop time" and "slow time advance". The first will stop time so you can study what is going on. the second will allow you to "jog" time, moving it slightly forward. Connect to L1 and L2 are a couple of LED, connected opposite each other with resistors in series so they will not burn up. One LED outputs blue and the other one red. You also have a DC volt meter. The generator is running and it appears that blue and the red LEDs are on all of the time. You try to measure the voltage between L1 and L2 but get no reading.
Bob walks up and you tell him you are trying to study the output of this generator and are just getting started, but you really don't have the type of meter that you need. Bob see the Stop Time button and presses it. Now only the red LED is on. You measure the volts and now you have a reading of 47 volts. That is a bit odd. Bob likes to press buttons, so Bob presses the Jog button to see what will happen. You notice the voltage now says 16. Still this is not making clear sense. Bob again hits Jog. Now the blue LED is on and the red LED is off. The volts read -97. Somehow when the blue LED is on, the voltage is negative compared when the red LED is on. But if you switch your meter leads so that black is on L2, the voltage is negative when the red LED is on. You and Bob continue playing with Jog until you determine that the voltage is never more than 340v and that the red and blue LED are off and on in an alternating fashion. Then Steve shows up and has an oscilloscope. You connect it to L1 and L2, then release the Stop Time control and you see that you have a sine wave on the scope. Steve also has an AC volt meter and you discover AC RMS voltage is 240 volts. Now you have it all figured out.
Steve says, "Hey, I have a generator just like that. Do you think we can hook them up in tandem so they can output twice the power, working together?" Larry has been standing by enjoying the fun and adds, "You better make sure you get them in phase with each other before you connect them together!" So Steve gets his generator out and fires it up. Like yours it will hold 3600 RPM without waver. You know you need to get them in phase and if you can see both output waves on the scope, you can tell if they are in phase or not. So you connect Steve's L1 and L2 up on channel B of the scope, hoping that will work. Well you can see something but it makes no sense. Then you remember that for years you have been carrying this thing called a signal generator around and was never sure what to do with it, but you know it can output a 60 cycle square wave and you know that you can make the scope lock into that so the lines sweep across the scope screen at 60 cycles, independent of what is on either channel A or channel B. Now you can see two perfect sine waves but they are not lined up. Bob is still much interested in those buttons and hits the Jog again but that button will only affect your generator. You see the wave on channel slide over a bit. It is now actually closer to lining up with channel B than it was before. "Hit it again", Steve shouts. After several tries it looks like it is lined up as perfect as anyone could expect, with Bob at the controls. It looks good. The high points are in line. The low points are in line. The zero crossings are all in line.
So Steve grabs some wire nuts and a heavy cord with a 240v plug on the end. Steve is a man of men and is determined to connect up "hot". He joins the two L1 lines together and to his power cord, first wire with the wire nuts. He is just about to join the two L2 lines together and you have a thought. "What if we missed something somehow. This could be bad if done incorrectly." You have a new Iphone charging cable and the cable has one of those little wire twist ties on it. You get the twist tie, remove the plastic from it, leaving a thin bare wire. You tie the thin wire to Steve's L2, hand your safety glasses to Bob, and say, "Bob touch that little wire to that one on my generator marked L2 so we can get some more readings." Poof, in a flash of light the little wire is gone and Bob has a funny look on is face. What could be wrong?
You look at your generator closely and then look at Steve's generator. You see that Steve's L1 exits the housing right where your L2 exits, like his wires are backwards from yours. We had the waveforms lined up, so how can they be different? Not sure but let's switch one of them and try again. Now looking at the scope, the waves are still in sync but the top and the bottom are opposite each other. The generator leads were marked wrong on Steve's generator. We switched them and NOW the waves are messed up. So we swap the scope channel B's ground and probe leads on Steve's L1 and L2, and all looks good again. So swapping the scope leads around, made the waveform move 180 degrees? After digging around in the floor of Steve's truck, Steve finds a bread twist tie and says, "Lets use this". Again Bob is called on for his special skills. This time the wire does nothing, like nothing is going on. Steve brings his AC meter over and measures between both L2 lines from the two generators and reads zero volts. Yes, that make sense. We wouldn't want anything happening there. The work needs to be connect to L1(combined) and L2(combined) ....and we should get twice the power.
The end.
The story above demonstrates two phases, one from each generator. While they were brought into phase, a polarity issue cause confusion both with what was seen on the scope and in the connections made. At a given frequency (cycles per second) being out of phase means a time shift between phases.
In a single phase power source, there is one phase and no part has a time shift. What looks like 180 degrees "out of phase" is a polarity issue in the way the scope is connected. The 4 images that I posted are to allow a person to see this if the mental exersize is done. Set your black meter lead at ground and in the first battery image you should get 0v, 12v and 24v as you measure 3 points in the open circuit. If you do the same on the second battery image you will get -12v, 0v, 12v. That does not mean that the battery on the left is reversed (or out of phase), but means the meter leads are reversed. Do the same thing with an AC volt meter and you will get 0, V and 2 times V on the first transformer image. Repeat on the second transformer image and you will get V, 0, V. No negative because you don't see polarity with the meter. Repeat with a scope and you can see polarity, the lower half and the upper half are flip-flopped because you flipped the direction of your metering according to the direction of current (at any instant in time).
Again one phase in single phase. Polirity on a scope shows one half of the transformer output upside down if you keep your reference connection on the center tap and move both up and down the coil from that point. If you have a scope without any common connection between both probe ground leads and the scope is isolated from ground, you should be able to do this for real. But use extreme care. Even with 12volt systems you can melt your scope leads if you don't have your ducks in a row.
Dan
