AC current flow

[Monkeyblaine]

Im having a little trouble understanding AC current flow... My understanding is that the electrons jiggle backwards and forwards due to the waveform of AC, so I don't understand why there is a need for a neutral as the electrons don't need a return path as they don't actually move anywhere unlike in a DC circuit where they move round and round... If someone could correct my understanding of this I'd muchly appreciate it.

 

[Marvo]

Firstly I would forget about the difference between AC and DC. If you look at a very small portion of an AC waveform it looks the same as DC.

Current doesn't flow in an AC circuit without a neutral for the same reason it doesn't flow in a DC circuit if you disconnect one side of the battery.

We only get into Elf/Santa theory in very advanced electrical study levels.

 

[Monkeyblaine]

Ok so now I understand that on a positive half cycle the current will be flowing through the line conductor, through the load and through the neutral and then in 1/100th of a second or the negative half cycle of the wave it will flow in the opposite direction from the neutral through the load and finally through the line... and visa versa every 1/100th of a second... am I getting it yet???

 

[Darkwood]

Ok so now I understand that on a positive half cycle the current will be flowing through the line conductor, through the load and through the neutral and then in 1/100th of a second or the negative half cycle of the wave it will flow in the opposite direction from the neutral through the load and finally through the line... and visa versa every 1/100th of a second... am I getting it yet???

If your supply is 50hz then in your example then yes.

 

[Darkwood]

Do remember though; without me trying to confuse you more, that neutral as we call it isn't always needed to have a circuit, you can have 2phase and 3phase circuits AC that use the potential difference between the phases and dont always need a neutral (contactor coils, transformers and 3ph motors), all you actually need is a constant potential difference between 2 wires and a load thats designed for the voltage present.

 

[Monkeyblaine]

Would I be also right in saying that the reason we don't have protective devices on our neutrals then is because it alternates so quickly that in the case of a short circuit it only needs to protect Half the cycle.

And if there is a fault to earth will the fault current alternate between travelling one way through the earth then back from the earth in the opposite direction 50 times a second until the protective device operates.

 

[Darkwood]

Would I be also right in saying that the reason we don't have protective devices on our neutrals then is because it alternates so quickly that in the case of a short circuit it only needs to protect Half the cycle.

And if there is a fault to earth will the fault current alternate between travelling one way through the earth then back from the earth in the opposite direction 50 times a second until the protective device operates.

No!! its for safety reasons, because the neutral is derived as is the earth from the centre tapping of the sub'-transformer it has the same potential and up until it is devided it effectively the same thing, so in normal conditions touching the neutral wont give you a shock, if you had a fuse break the neutral but leave live connected then that neutral would be 230v potential to earth, but show no volts to live as its floating at the same voltage. Also to note is if anything which had a broken neutral had a N/E fault then it would use earth as a return path leaving many hazards and can go undetected. If like we do you fuse the live then when operated it should 'if all wiring is ok' remove all potentials between conductors and to earth on isolated circuit making it safe to work on.

 

[Monkeyblaine]

Ok I get that. So the 230v would continue up the line through the load, down the neutral until the break thus making that particular neutral at 230v to earth and if that neutral was ever to come in contact with an earth it would use that as it's return and may make other things live and very dangerous.

 

[Guest111]

Would I be also right in saying that the reason we don't have protective devices on our neutrals then is because it alternates so quickly that in the case of a short circuit it only needs to protect Half the cycle.

And if there is a fault to earth will the fault current alternate between travelling one way through the earth then back from the earth in the opposite direction 50 times a second until the protective device operates.

no years ago we had fuses in both live and neutral but if the fuse in the neutral blew and the one in the live didnt the faulty equipment was still live and very dangerous

 

[Monkeyblaine]

On the negative half cycle when the current is flowing in the opposite direction I.e through the neutral. There is no voltage going through the neutral is there. It's just the current that alternates. Not the voltage??

 

[Marvo]

The 'voltage' is what pushes and pulls the current around the circuit. The AC voltage alternates between a positive value and a negative one which causes current to flow in alternate directions. You don't get current flow unless there's a voltage present.
Check out these books linked in this thread, there's one of them covers basic AC theory and I'm sure it will explain the principals better than I can.

 

[Darkwood]

yes as Marvo says you need to get a grasp of the basics first, remember the Neutral is derived from the centre tap of a 3ph transformer and so is the earth, this is your confusion you assume because it shows 0volts to earth it dosn't have a voltage but Voltage is relative so relative to live it is 230v.
If you have some standard probe testers their is usually 2 neons which detect whether its an AC supply or DC supply one neon lit will detect DC as it only see's the voltage in one half of the cycle if both light then its AC as its sees voltage in both positive and negative cycles the neons are just basic with each having a diode in reverse to the other so only been capable of lighting when the waveform matches its diode direction.

 

[Engineer54]

so is the earth

Not quite right, but i know what you mean...lol!!

 

[Monkeyblaine]

AC confusion

On the negative half cycle when the current is flowing in the opposite direction I.e through the neutral. There is no voltage going through the neutral is there. It's just the current that alternates. Not the voltage??

 

[telectrix]

Re: AC confusion

the voltage alternates between + and - relative to 0. 0 being the neutral, so there is no voltage in the neutral.

 

[Knobhead]

Re: AC confusion

The voltage and current alternate above and below 0V.
0V being referenced to earth AKA neutral.