Bill;
Enjoyed reading your further input to this terminology conflict. Please be warned ahead of time that this message is...long!! If you don't want to read it all, just scroll down to the end where the "Punch Line" is
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You are right, when a white or gray colored conductor is seen by many people, they automatically call it a Neutral. That isn't so bad, just calling it a name [I prefer to use more correct terms also :)], however when the grounded common conductor for a 4 wire Wye system is thought to be the same as a true "Neutral" conductor found on a 1 phase 3 wire circuit with low Harmonics, it's a different story. The two different grounded conductors work nothing at all similar to each other. The Neutral conductor is just that - A conductor derived from the center tap connection of a single transformer coil which can be used for a circuit [or two] that will be 1/2 the voltage of the complete coil. Since this conductor is connected to the middle of the transformer's secondary coil [or the series jumper connection between split coil secondaries, where the end of the first coil is connected to the beginning of the second one, in proper phase polarity as to cause the connection to be series additive], it will have 1/2 the voltage between either of the ends of the coil to the center tap. The current is created across the complete coil and is in phase time with the magnetic current flowing in the core, which is produced from the primary winding. It's only when there is an inbalance of current flow from the end conductors of the coil to the center tap that causes the center tap to carry current, since the current is produced only to the center tap from the magnetic current in the core. The level of current that is even, or balanced, will flow between the ends since it is primarily created from the magnetic current in the core at these two points. Harmonic distortion is the same as inbalanced current, so it has the same inbalance characteristics. These examples are only for 1 phase 3 wire multiwire circuits [or 2 - 120 volt 2 wire circuits connected to opposite phases with the grounded conductors connected to the "Neutral Bus"]. 2 wire 240 volt circuits [connections to the coil's ends only] do not have this characteristic.
The frequency and phase on the center tap Neutral is exactly the same as would be on the other opposite phase. The 1 phase 3 wire transformer should not be confused, or mistakenly thought of as being a "Two Phase" system. No matter where you look at the Sine wave, it is only one. The system cannot have a single machine / load connected to all three conductors of a multiwire circuit, such as how a polyphase [3 phase, for example] motor would. The transformer's primary is only connected to 2 conductors, which even if they are off of a poly phase system, it only produces one single phasor into the core, which alternates direction of flow of magnetic flow in the core at 180 degrees [when one side is flowing forward ,the other is flowing back, and vice verse, but they both peak at the same time].
I included this mumbo-jumbo because I have heard lots of people say it, even demand it works like a 2 phase system! A two phase system is just that - it has two independent phases, which are 90 degree off. I leave this explaination out to end the topic now, but would be glad to explain further if you like.
The Wye system's grounded conductor is common to three - 2 wire transformer coils, causing them to be connected in a Star, or series additive connection. The common connection can be, and is, used for a circuit conductor when a lower voltage than the phase to phase voltage is needed. Since the common is connected to each coil's end, each phase to common circuit works as if it was only that - a 2 wire circuit. Sometimes, current will balance across the phases, but not normally. Once again, I'll leave off here, but feel free to ask more if needed.
I prefer to use the term "Grounded Conductor" when describing them, unless it's just to be quick, that's when I use "Neutral" to describe the grounded circuit conductor used on a 1 phase 3 wire multiwire circuit, either from just a 1 phase 3 wire transformer, or from the transformer coil in which the 1 phase 3 wire circuitry is derived from a 3 phase 4 wire Delta [center transformer or coil on closed deltas, 3 wire output transformer or coil on open delta Vee connected setups - usually the transformer to the left side]. For a Wye system, I use the term "Common". Don't discuss 2 phase systems very much, but if a grounded circuit conductor is used with a 2 phase 5 wire system, it works similar to the 1 phase 3 wire "Neutral".
As far as a grounded circuit conductor on other systems, such as: 1 phase 2 wire, 3 phase 3 wire [corner grounded delta] and, once again, 2 phase - 3 wire or 4 wire, this conductor is definitely not a neutral! It could be used as a common when 3 phase 3 wire deltas are using 1 phase 2 wire circuits, along with 2 phase 3 wire doing the same. An example would be using the grounded circuit conductor [grounded phase] to connect to the "Common" leads in HID lighting, where the screw shell should be connected to a grounded conductor for safety. We will make phase B the grounded phase. Two 1 phase 2 wire branch circuits running HPS fixtures [2 pole breakers]. Circuit 1 = phase A and B, circuit 2 = phase B and C. The grounded conductor would be common to these circuits, but would not balance the current. Phase B will have the highest load, due to it's use across 2 separate circuits.
To sum it up; The only place that the word Neutral should be used to describe a grounded circuit conductor is when a 1 phase 3 wire multiwire circuit is being described, which derives only from a 1 phase 3 wire transformer coil [1 phase transformer or 4 wire delta].
A Wye system's grounded conductor, when used for a multiwire circuit, should be only thought of as a Common conductor, when either a 3 wire or 4 wire multiwire circuit is being described.
When a grounded conductor is used on a 2 wire circuit, the circuit description that describes it the best would be 1 hot, 1 white [or gray]. For a 3 phase 3 wire circuit, it would be best described as a 3 phase 3 wire circuit [except the colors would be black, white - or gray, and red or blue].
The fact that many people in the trade use the term Neutral to describe the grounded conductor is of no harm or offence, since it's a commonly understood term and takes a lot less mouth movement to say than Grounded conductor. I prefer to use the actual, more technically correct terms, but sometimes it confuses people! That's when it's time to either explain it all or just call it a .....Neutral....
Hope you didn't get too bored from my endless message! I have a habit of running off sometimes on certain subjects. Hopefully it doesn't put you to sleep.
Reply with your thoughts or comments.
Scott "S.E.T."
