Question about the neutral wire in an electrical system...


by dannymacpunx
Tags: electrical, neutral, wire
dannymacpunx
dannymacpunx is offline
#1
Apr27-09, 02:37 PM
P: 2
IF a phase touches another phase in a 3 phase AC supply it goes bang due to them being 120 degrees apart and therefore being at different voltages when they touch (this, i understand), so WHY, after passing through an electrical component, can all three phases be joined onto a 0 potential wire (the neutral)? Surely they are still out of phase and should still go bang?

all text books seem to be really poor at explaining this and im yet to meet anyone that has got anything other than a half arsed answer that strays from the point... any help appreciated.
Phys.Org News Partner Engineering news on Phys.org
A smart prosthetic knee with in-vivo diagnoses
Old tires become material for new and improved roads
Students take clot-buster for a spin
MATLABdude
MATLABdude is offline
#2
Apr27-09, 03:11 PM
Sci Advisor
P: 1,724
Please don't cross-post across multiple forums:
http://www.physicsforums.com/showthread.php?t=310150

(And welcome to Physicsforums!)
vk6kro
vk6kro is offline
#3
Apr28-09, 07:19 PM
Sci Advisor
P: 4,003
It is relative voltages that matter.

If you take 3 car batteries, can you see that it doesn't matter if you connect one terminal of each of them to the same point? No current will flow if you do that because each battery is still open circuited.

The output of a transformer that has 3 phases going into it is 3 independent windings which you can connect any way you like as long as you don't short out the windings themselves.

skeptic2
skeptic2 is offline
#4
Apr28-09, 11:09 PM
P: 1,784

Question about the neutral wire in an electrical system...


I'm not sure I understand your question. The 3 phases are not joined to create the neutral. There are 2 types of 3 phase circuits and you may be confusing them. For power transmission 3 wires are used because a 4th wire for neutral would be expensive. This type of 3 phase circuit can be stepped up and stepped down with a delta transformer. The core of the transformer is in a triangular or delta shape and the windings for each phase are around each side of the triangle.

The other type of 3 phase circuit uses 4 wires, the 3 phases and a neutral and this is what goes into businesses and houses. Why, because it's safer and the cost of the extra wire is minimal. The transformers used for 3 phase with neutral are in the shape of a "Y" often called "wye" transformers. The neutral is connected to the center of the "Y". Three phase without neutral can be converted to three phase with neutral with a delta-wye transformer. The neutral is never created by connecting the three phases together.
vk6kro
vk6kro is offline
#5
Apr29-09, 04:40 AM
Sci Advisor
P: 4,003
This forum goes world wide and the system in some countries, like Australia, is that 3 phase IS brought into some houses and is used for water heating and electric ovens and other applications that use a lot of power. But it is also the source of all electric power in the house as outlets are 230 volts single phase.

There are 4 wires plus an earth.
One neutral and 3 phases which are all 230 volts relative to Neutral and to Earth (which are usually joined together at the meter box) and 400 volts relative to each other.

This seems to be the system the question was about.

Each phase is developed in a step down transformer with three cores and then linked at the neutral inside the transformer and sent out as 4 wires to houses.

.
zgozvrm
zgozvrm is offline
#6
Aug28-10, 10:57 AM
P: 754
Quote Quote by dannymacpunx View Post
WHY, after passing through an electrical component, can all three phases be joined onto a 0 potential wire (the neutral)? Surely they are still out of phase and should still go bang?
This is no different than a 120V single phase scenario: If you connect a 120V line directly to neutral, it will go "bang." But, if instead you connect the 120V line to a load and connect the "other side" of the load to neutral (i.e. pass the phase through an electrical component to a zero potential wire), all is okay.

The reason there is a "bang" (in either situation) really has nothing to do with phase angles, rather it has to do with voltage potentials. If there exists a potential between any 2 wires of more than 0 volts, connecting the wires directly to each other creates a short-circuit (i.e. a no-load situation). This means that the resistance in the circuit is 0 ohms and, by Ohm's Law, this causes an extremely high current, thus the "bang."
zomgineer
zomgineer is offline
#7
Aug28-10, 11:43 AM
P: 21
Each phase is never shorted to another phase. In a wye configuration, where you have a neutral wire ideally at 0V, each phase is connected to the neutral through some load (resistance). Like zgoz said.

To add a little more, if the 3 loads are balanced, no current will flow through the neutral line. In AC, the current will flow back and forth across a wire. In a balanced wye system, the forward current in one phase will be exactly equal to the return currents through the other phases and vice versa. The sum of currents flowing through the neutral is zero. This is desirable. It prevents resistive losses in the neutral line and it also prevents the generator from taking damage due to vibrations.
sophiecentaur
sophiecentaur is offline
#8
Aug28-10, 03:49 PM
Sci Advisor
PF Gold
sophiecentaur's Avatar
P: 11,383
The short answer to the OP is that you can connect between phases or from one phase to neutral with a load and you will get the appropriate current (V/R).
If you actually have a neutral wire coming into your house then the transformer which feeds you is a star configuration - giving three possible 'phase - neutral' connections. This is the most likely situation. A Delta wired transformer will give you just three wires and you can get power by connecting between two of the three. This would just not be convenient for house wiring, though and introduces more possible hazards due to possible mis- connection (and a higher voltage too, which you would have to take into account)..


Register to reply

Related Discussions
Question about the neutral wire in an electrical system... Classical Physics 4
Neutral wire? Electrical Engineering 1
Wire Reverse between Life and Neutral Electrical Engineering 30
Neutral Wire General Physics 16
question about USA electrical system Electrical Engineering 13