




#76023  10/19/00 05:47 PM
Re: What is a Neutral?

Broom Pusher and
Member
Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA

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 :).. 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 mumbojumbo 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."
_________________________
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!

Top













#76025  10/21/00 02:50 AM
Re: What is a Neutral?

Broom Pusher and
Member
Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA

Bill; I'll keep this message's content low and the answers more consise than indepth, but even the most brief examples will still have a lot of reading involved, so please do not be discouraged or offended. A quick and simple cover on the Grounded Conductors: For a single phase 3 wire system, such as the ones used in Residential areas [one transformer], the grounded conductor will work as a "Neutral" because it will carry the unbalanced amount of current in a 120 volt 3 wire multiwire circuit. An example circuit: Phase A has a load of 10 amps at 120 volts, Phase B has a load of 15 amps at 120 volts. The "Neutral" [center tapped conductor] will carry 5 amps, which is the left over [unbalanced] load. 10 Amps is common to Phase A and Phase B, so they will carry this "balanced" load current between them. The remainder of current flows on the center tap [Neutral]. If the load on Phase A is 10 amps and the load on Phase B also is 10 amps, there will be no current flowing on the Neutral and the system [circuit] is said to be balanced. This is why it's called a Neutral. It carries the unbalanced current from which ever Phase has the highest load. As you can see from this example, if the loads are split up evenly between the two Phases, the neutral will have a very light load on it between the service and the power transformer. The above example also works for multiwire circuits from a 3 phase 4 wire Delta system, however if the "High Leg" is used for a 208 volt 2 wire circuit, this current will not balance out the neutral. Harmonic distortion on LN circuits / loads causes the neutral to carry the THD current as unbalanced current. This can overload the neutral conductor[s] if excessive. Now for the 4 wire Wye system: The grounded conductor for this system will typically carry the highest load. Each Phase coil is connected at a common point, which is where the 4th wire is derived from. This makes it possible to have a dual voltage system [multiwire circuits]. Loads can be connected LL for 208 volt 1 phase loads, or LG [Line to Grounded conductor] for 120 volt loads, along with 3 phase LLL 208 volt loads. Same goes for 277/480, 347/600, 2400/4160 and the like. The common conductor works with either 3 or 4 wire LG multiwire circuits as follows: Each LG circuit is a somewhat direct connection to that transformer coil. There is no center tapped winding like the 1 phase system. This makes the grounded conductor common to all three phases [phase coils]. The common conductor will carry the highest load from a multiwire circuit. An example circuit: Phase A = 3 amps, Phase B = 5 amps, Phase C = 1 amp. The load on the common [grounded conductor] will be 5 amps, which is the higher load on the ungrounded conductors. Another circuit example: Phase A = 10 amps, Phase B = 10 amps, Phase C = 10 amps. Common [grounded conductor] = 10 amps. 10 amps is the level of current flowing. These loads are, of course, connected LG. As you can see, the common does not offer a Neutral state, as did the 1 phase system. There is an exception to this. When the loads are completely Resistive [Incandescent lamps, electric ovens, heaters with no motors and water heaters], the common will become a Neutral. This would be if the multiwire circuits were connected to only the resistance loads, with no Reactive loads included. One other way to look at that is to say loads with unity power factor [100% PF] could apply here. In this case, the common will carry the unbalanced portion of current. If the currents are equal, there will be no current flowing on the common. This could be determined to be a "Perfectly Balanced Circuit". On the Technical side, even pure resistance loads still contain some Inductive Reactance [Xl] and some Capacitive Reactance [Xc]. Heaters with large  long coils are definitely Xl, along with Quartz Halogen lamps. All other types of loads which are not pure resistance [Incandescent lamps, or electric resisance heating elements] are Reactive loads. They have a power factor and create a lead or lag current in the AC sine wave. These Reactive loads do not balance across the common for LG loads. The way these levels are figured is by using Vectors. Draw Vector lines of a certain scale size [length], separate at 120 degree angles, then connect the intensity of the LG currents together via a trapazoid. To explain anymore on the Vectors, or the theories behind the current flow, would make the last message seem like a 2 paragraph leaflette! The above examples and explainations might look long, but they are the shortest, most brief, simplest to understand, lowest tech terms and still consise way I can possibly explain this to anyone. It's much easier to understand face to face where example circuits can be drawn. I understand that this is completely opposite from what you were told before, so I understand fully why it might be difficult to grasp. Besides, this is Electrophysics / Electrical Engineering! They just don't print this stuff on the back of a box of Coco Puffs :).. My suggestion [as always] is to find reference materials, such as Electrical Engineering books or Electrical Circuits / components analysis and theory for AC type books. They can explain a lot better than me :(. If you have the chance[s] to, make an on site analysis of some existing multiwire circuits to determine the loads on each conductor. Using a clamp on type ammeter, measure the current on different multiwire circuits at the panel / subpanel at as many different projects as you can and to 1 phase + 3 phase systems. Gather and collate your data to base a conclusion for your self of how different system types, load types, etc. work out. If you wish to contact me directly by Email, feel free to do so. Click the link below to send me a message: Email me at adst@SoCA.com Good luck!! Scott.
_________________________
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!

Top













#76028  10/28/00 08:06 AM
Re: What is a Neutral?

Junior Member
Registered: 10/22/00
Posts: 2
Loc: Portsmouth, RI, USA

Bill,
I don't think I have ever tried to explain the current flow in a multiwire circuit without the aid of chalk and a blackboard, but I'll give it a try.
First of all, an electrical current cannot flow in two different directions in a conductor at the same time. You may have opposing voltages, but the higher voltage will win and the resultant current will flow in that direction. For example, an automobile alternator must have a charging voltage higher than battery voltage in order for current to flow back through the battery. This voltage should be somewhere around 14.5 volts for a 12volt system. The net charging voltage would only be between 2 to 2.5 volts.
In a 120/240 volt multiwire circuit, we have two parallel circuits sharing a common wire so current can return to whatever half of the transformer it was derived from. If the load is balanced, current will flow in through both loads and through the entire winding of the transformer in a 240 volt series circuit. There will be no current in the common wire of the parallel circuits. In this state, the neutral is not needed and could be removed from the circuit with out any ill effects. Yes, the voltage has doubled in the circuit, but so has the load. Current will be the same throughout the circuit. Voltage drop across each load will still be 120 volts and equipment will not be damaged.
Let's look at a circuit with some resistance and current values. I am assuming that the readers are familiar with Ohm's Law calculations and the behavior of resistances connected in series and parallel.
If we take two 60 watt light bulbs and connect one to each line and neutral, we will have a circuit like the one I described above. The resistance of each load will be 240 ohms and current will be .5 amperes (we will disregard change in resistance because of heating of the filaments, etc.) The voltage drop across each load will be 120 volts.
If we now look at the point in the circuit where the two resistances and the neutral conductor are joined, and apply Kirchhoff's First Law, things may become a little clearer. Kirchhoff's First Law states that algebraic sum of the currents at this junction must equal zero (or that the current entering this point must equal the current leaving, which ever you prefer). We will have .5 amperes entering this point from one resistance and .5 amperes leaving the point to the other resistance. No current is entering or leaving on the neutral conductor and Kirchhoff's First Law is satisfied.
Let's now turn on one more 60 watt light bulb. This would connect, in parallel, another 240 ohms of resistance to one side of our circuit. The result would be a resistance drop to 120 ohms and a current increase to 1 ampere in that branch of our circuit. We have not changed anything in the other branch. Back at our junction we now have 1 ampere entering from the side with the increased load, .5 amperes leaving to the side where the load stayed the same and .5 amperes leaving on the neutral conductor to go back the side of the transformer from which it was derived. Again the equation is balanced and all the gods are happy. The voltage drop is still 120 volts across each load because of the neutral connection to the transformer midpoint. What we actually have is part of the current from the side with the higher load travelling in the 240 volt series circuit (the balanced portion) and the remainder in the 120 volt parallel circuit.
Not to beat this thing to death, but let's look at the circuit in one more way. Let's connect the additional 60 watt light bulb to the other parallel branch and look again at the neutral junction point. We now have .5 amperes of current entering the junction from the smaller load, .5 amperes of current entering on the neutral conductor, and 1 ampere of current leaving the point to the side that now has the larger load. The .5 ampere neutral current is now coming from the side of the transformer where it was derived. The neutral current is now traveling in the opposite direction.
It might as well be mentioned that in the circuit with the unbalanced load, if the neutral connection is removed and we revert back to a 240 volt series circuit and the voltage drop across the loads would not be equal. The current in the circuit will be .66 amperes, and the voltage drop across the loads will now be 160 volts across the single 60 watt bulb and 80 volts across the two other bulbs. The 60 watt bulb just might survive this, but we all know what happens to the VCR.
One more thing for those with insomnia. Reading this might help. It seems to have this effect on some people in my classrooms.
R Dimery

Top









