3phase and neutral

Discussion in 'Electrics UK' started by Christhescott, 25 Nov 2010.

  1. Christhescott

    Joined:
    21 Feb 2010
    Messages:
    51
    Thanks Received:
    2
    Location:
    Aberdeen
    Country:
    United Kingdom
    Hi, Im looking for some connection diagrams/pictures of 3phase and neutral motor start/stop connections, would anyone in here have any links I could have a look at? I have tried google with little avail.

    Thankyou!
     
  2. ban-all-sheds

    Joined:
    27 Aug 2003
    Messages:
    52,103
    Thanks Received:
    2,033
    Location:
    London
    Country:
    United Kingdom
    I don't think that 3-Ph motors have a neutral do they?

    There might be one to the controls, but not to the actual motor.
     
  3. Spark123

    Joined:
    2 Jun 2005
    Messages:
    14,386
    Thanks Received:
    424
    Location:
    Cumbria
    Country:
    United Kingdom
    As the motor is a ballanced 3 phase load there is no need for a neutral, however a neutral may be used in the control side if it is 230v. The control side may be any voltage sufficient to drive a contactor i.e. common size contactors I come across are 24v, 48v, 110v, 230v and 400v.
    What type of motor control are you talking about, direct on line (DOL)? If so there is a similar diagram here:
    As you don't need the neutral ignore the neutral contact at the bottom, the normally closed contacts on the left hand side are stop buttons.
     
  4. IJWS15

    Joined:
    18 Oct 2006
    Messages:
    1,345
    Thanks Received:
    48
    Location:
    Staffordshire
    Country:
    United Kingdom
    Try googling delta star wiring
     
  5. bernardgreen

    Joined:
    3 Nov 2006
    Messages:
    12,377
    Thanks Received:
    1,049
    Location:
    Bedfordshire
    Country:
    United Kingdom
    Some motors that run in Star mode do have a "neutral" connection to the star point. Its purposes more often than not is provide a means to detect faults and /or unbalances in the three phase windings while the motor is running.
     
  6. Christhescott

    Joined:
    21 Feb 2010
    Messages:
    51
    Thanks Received:
    2
    Location:
    Aberdeen
    Country:
    United Kingdom
    Yes its a DOl starter. The manual says It needs a neutral, I can't remember off the top of my head what its for but I think its an air brake. Aswell as the 3 cables going away to the motor it has a two core flex. Its a second hand machine and it looks like the starter is not standard. I wanted a picture as Im not sure if the neutral should go into the top and through the contactor aswell as the 3 phase supply.
     
  7. PrinceofDarkness

    Joined:
    1 Jan 2006
    Messages:
    611
    Thanks Received:
    62
    Location:
    London
    Country:
    United Kingdom
    "Some motors that run in Star mode do have a "neutral" connection to the star point"...................




    I'd like you to give me an example of such a motor, 'bernard', where the star link is connected to the supply neutral. I've been dealing with rotating machines since the early 60's and I've never come across such a ridiculous situation as that. Don't you realise the implications of doing such a thing?

    It's certainly true in some circumstances that a lead is taken from the star position of the motor terminal block for monitoring purposes - but that type of connection is never connected to the supply neutral.

    Getting back to the O/P: I'd say that the DOL starter needs a neutral simply because the hold-in coil is 230 Volt - as they often are. But this doesn't mean that that neutral has to be taken to the motor itself......


    Lucia.
     
  8. ELZ4742

    Joined:
    22 Jan 2007
    Messages:
    557
    Thanks Received:
    16
    Location:
    Oxford
    Country:
    United Kingdom
    A three phase motor does not need a neutral because it has two 'phase' leads running it at any given point in time. The electricity comes in on one phase lead and goes out on the other phase lead.
     
  9. PrinceofDarkness

    Joined:
    1 Jan 2006
    Messages:
    611
    Thanks Received:
    62
    Location:
    London
    Country:
    United Kingdom
    The O/P, Chris-the-Scot, hasn't actually implied that the motor itself needs a neutral. He's surely on about the control circuit of the starter and perhaps, the wiring of remote start/stop buttons.

    This seems to me to be a bog-standard arrangement, and I'm rather surprised that he can't find any 'Googlese' for such a plain task.


    Lucia.
     
  10. Christhescott

    Joined:
    21 Feb 2010
    Messages:
    51
    Thanks Received:
    2
    Location:
    Aberdeen
    Country:
    United Kingdom
    Thanks for your help. As you can tell Im not as familier as I would like to be with motor control. I dont think the neutral is for the coil, I think the neutral is for something to do with dust extraction and I think the saw motor will work without the neutral connected. The neutral does not go to the motor itself but it is terminated inside the starter.
     
  11. ban-all-sheds

    Joined:
    27 Aug 2003
    Messages:
    52,103
    Thanks Received:
    2,033
    Location:
    London
    Country:
    United Kingdom
    Could be a single-phase extractor.

    I believe that sometimes motors have single-phase anti-condensation heaters.

    Or as others have also said, single-phase components in the control systems.
     
  12. bernardgreen

    Joined:
    3 Nov 2006
    Messages:
    12,377
    Thanks Received:
    1,049
    Location:
    Bedfordshire
    Country:
    United Kingdom
    I cannot remember the manufacturers part numbers of the motors I have seen with the star point bought out as a monitoring access for the control system. Hence the " and " round the word neutral.

    I didn't say it was.

    I can recall one production line where the 4 or 5 main conveyor drive motors did have their star points commoned up but not connected to the main neutral supply. Something to do with reducing slip to an acceptable level.
     
  13. mikhailfaradayski

    Joined:
    21 Jul 2009
    Messages:
    1,677
    Thanks Received:
    86
    Location:
    Derby
    Country:
    United Kingdom
    Chris-T-S,

    as others have said, the neutral is most probably required for the coil in the start/stop gubbins.

    It may also be the case that the 2 core flex you refer to is a thermistor or other thermal device. If it is (which you need to find out) then it should be included in the control wiring
     
  14. sparkyy

    Joined:
    10 May 2008
    Messages:
    24
    Thanks Received:
    3
    Location:
    Aberdeen
    Country:
    United Kingdom
    The two core flex is probably a thermistor which should be incorporated into the stop circuit. It's purpose is to stop the motor if it overheats.
     
  15. ricicle

    Joined:
    14 Sep 2006
    Messages:
    6,347
    Thanks Received:
    383
    Location:
    Gloucestershire
    Country:
    United Kingdom
    Via a suitable thermistor relay.....
     

Share This Page