neutral wire

[Gixxerdan]

Hi Guys,

I am a third year apprentice training to be an electrical technician.
I have been asked to build a circuit with a reverse contactor in to run a three phase motor and an oil heater with only one of them being able to run at a time.
I have been told that the three phase motor does not need a neutral connection but the heater does but I don't understand why?
My understanding was that devices needed a neutral for the current to flow back to the supply. Have I got the wrong end of the stick?
Any help would be much appreciated.
Thanks, Dan

 

[ghost]

The heater will be single phase and the motor 3 phase. The motor is a balanced 3 phase load, therefore does not require a neutral connection.
Have a look at these pages for some 3 phase motor theory.
http://www.nfphampden.com/3phmotor.pdf

Induction motor - Wikipedia, the free encyclopedia

Reliance - Basic Motor Theory
Hope this helps

 

[James Sutton]

Do not need a neutral on the 3phase motor as its a balanced load

 

[Gixxerdan]

What does the neutral do with regards to a three phase load? does it take away excess current if it is unbalanced?
The heater is a three phase supply being fed via a 4 pole contactor so very confused

 

[ghost]

Ahh, in the case of the heater the elements will be split across the 3 phases, but terminated down to neutral, therefore each element will have a potential of 240V across them rather than 415V between phases. Remember the neutral will be carrying the current from all 3 phases, therefore all cable csa should be rated to this.
I will try to find a diagram for this for you.

 

[James Sutton]

In a three-phase system, three circuit conductors carry three AC (of the same frequency) which rea
In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electrical current. This delay between phases has the effect of giving constant power transfer over each cycle of the current, and also makes it possible to produce a rotating magnetic field in an electric motor.
ch their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electrical current. This delay between phases has the effect of giving constant power transfer over each cycle of the current, and also makes it possible to produce a rotating magnetic field in an electric motor.

 

[ian.settle1]

What does the neutral do with regards to a three phase load? does it take away excess current if it is unbalanced?
The heater is a three phase supply being fed via a 4 pole contactor so very confused

Are you sure your a third year apprentice asking these questions this is first year stuff

 

[Gixxerdan]

In a three-phase system, three circuit conductors carry three AC (of the same frequency) which rea
In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electrical current. This delay between phases has the effect of giving constant power transfer over each cycle of the current, and also makes it possible to produce a rotating magnetic field in an electric motor.
ch their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electrical current. This delay between phases has the effect of giving constant power transfer over each cycle of the current, and also makes it possible to produce a rotating magnetic field in an electric motor.

I know this thanks. Its the neutral and why it is used in some applications and not others which confuses me

 

[Gixxerdan]

Are you sure your a third year apprentice asking these questions this is first year stuff

If your replies are not going to be useful then please don't comment.

At work we work on systems and build/repair/maintain things and don't really do the theory side and at college we do the 3 phase motors and how power is distributed and then more technical stuff but we have never done things like about the neutral and how it works. Sorry if it is a daft question.

 

[Guest123]

Constructive rather than demeaning replies please gents.

 

[Digster]

Just read my message back to myself and saw that i refered to you as the boy.
Sorry, Just a figure of speach down here. Daarn Saarf.
I've been sparking for 15 yrs now and don't know anything about motors,
Just something else that i've forgotten from college,had no need for it;each to his own you see.
Different sector of the industry.We all know what we need.
just pee's me off that people take the p.

 

[volt-dropping]

What does the neutral do with regards to a three phase load? does it take away excess current if it is unbalanced?
The heater is a three phase supply being fed via a 4 pole contactor so very confused

IF a 3 phase load has an excessive imballance then the neutral can carry high currents in a backfeed process mainly in 3ph distribution boards and note this type of 3ph+N supply can become numerous single ph and 3 ph and 3ph+N supplies , so the main neutral is needed to distribute any single ph or 3ph+N supply from this db (unlike a pump or motor load there are no constants )

If the 3 ph load is deemed ballanced then any neutral in the supply cable is most likely to be needed to supply a single phase device integral to the device startup procedure or monitoring I.e most 3 phase motors dont need neutrals but the contactor coil in the motor starter box is single ph and does.

IF the oil heater aint single phase then its internal thermal cutout probably is and needs a neutral