Please explain this

Discussion in 'Electricians' Talk' started by Lokkars Daisy, Nov 19, 2012.

  1. Lokkars Daisy

    Lokkars Daisy New Member

    [​IMG]This set up convinced me that I know flip all. The steel enclosure contains all the electronics to convert raw ac to a 230v sinewave ac. I haven't drawn the gubbins
    just the final output which is via an isolating transformer, this supplies a standard 1.5kW oil filled radiator. The manufacturers have marked the output L and N .
    I am a tad confused by the connection to one side of the tansformer windings to the earth of the equipment and also to the DNO earth
    How does one side of the winding become neutral ? if the DNO earth were not connected then you would neither L or N just ac on the winding.
    But how does the DNO earth alter matters as there is no  circuit between the DNO earth and the isolated transformer windings ?
    Is it me, shall I retire, dya think
     
  2. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Is neutral just a misnomer to denote the actual return path which is Earth?

    Mr. HandyAndy - Really
     
  3. Lokkars Daisy

    Lokkars Daisy New Member

    The thing is Handy that the secondary winding is ISOLATED there is no path to dno earth . The only path for the current is in and out of the secondary winding.
    If the dno earth were disconnected then the body of the heater would be at 230v ac
    I think I've lost the plot over this one :'(
     
  4. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    I agree. I also think you've lost the plot :)

    Mr. HandyAndy - Really
     
  5. spinlondon

    spinlondon Screwfix Select

    They've had to connect to the DNO's earth because the metal enclosure is an exposed-conductive-part.
    If an earth fault develops on the input side of the transformer, the enclosure would become live.
    They've then had to earth between the rad and the enclosure, in case the red develops an earth fault, and can be simultaneously touched along with the enclosure.
     
  6. Lectrician

    Lectrician Screwfix Select

    It shows one side of the winding being earthed not just to earth, but to the case too.

    The case is earthed to the 'DNO' earth according to the drawing.

    This makes the transformer no longer isolating.  If you touch their 'live', you would recieve a shock via earth, as you would with any substation/transformer derived supply.  The only way to keep the isolating transformer as isolated would be to not earth either side of it.

    What is the story behind this?  Why is the oil radiator fed via 'an isolating' transformer?
     
  7. Lokkars Daisy

    Lokkars Daisy New Member

    I have picked up on this because this is the third version of a technical diagram, the first showed the transformer winding connected to an earth rod ( two types of earth existing in a domestic location-no good)
    The second drawing had the dno earth removed and the connection marked as (Local Earth Only  Do Not Connect to Mains(DNO) earth)

    And the third drawing is how you see it now.

    The power is derived from a turbine ,the metal cabinet contains all the gubbins for conversion to AC the output is via an isolating transformer, I don't know why
    Because of recent grid constraints 3.6kW of the turbine power can be used or fed into the grid but any excess must be dumped in this case an oil filled heater uses the extra 1.4 kW

    So there are infact two sets of control gear , but that's irrelevant.
    Because of the two previous strange diagrams I am wondering if the designers have it right. I mean they are no doubt great at the electronics side but perhaps the practicable electrical system is new to them, I dunno
    There is something niggling me , the fact that the 230v ac is connected to the case of the heater an to dno earth

    As I recall the connection to the neutral winding of the transformer has been made internally during the manufacture of the transformer, a green/yellow lead leaves the transformer
    The transformer is marked ISOLATING
     
  8. spinlondon

    spinlondon Screwfix Select

    It would appear that the transformer is incorrectly labeled.
    The transformer also appears to be rather redundant.
    I would expect that the output from the turbine to be isolated, so there would be no need for an isolating transformer.
    I see that the output appears to be fed through a rectifier/inverter.
    Why is this, is the output d.c.?
     
  9. Lokkars Daisy

    Lokkars Daisy New Member

    That's how it's done, turbine generates 3phase rough ac, this is converted to dc and then the inverter converts the dc to standard 230v ac

    The transformer is an isolating transformer, but only as long as the intrinsic earth wire is left disconnected.

    The same transformer set up is used for PELV  but that of course is Extra Low Voltage, whereas this is 230v AC .
    I guess it's similar to a 110V safety transformer which is tapped in the centre of the windings, not connected to dno earth though
     
  10. Bazza-spark

    Bazza-spark Screwfix Select

    Hi LD

    Just a thought, isn't it because the transformer is being used to isolate the turbine from the mains?  The primary side being isolated from the DNO earth and mains supply.  The secondary being earthed to give normal protection in the event of an earth fault in the mains side?

    Kind regards

    BS
     
  11. Lokkars Daisy

    Lokkars Daisy New Member

    Hi Bs, It could well be for that reason, I'm not that interested in it's purpose .
    What intrigues me is the fact that the body of the heater and I guess anything connected to the dno earth is sitting at 230 v with respect to the other side of the transformer winding.
    The earth connection is both at Earth potential and 230v ac
     
  12. Bazza-spark

    Bazza-spark Screwfix Select

    Hi LD

    Not true.  The connection of the winding to earth stabilises the system so that the live side of the coil is held at 230v with respect to earth and the neutral side of the coil is held at earth potential.

    Kind regards

    BS
     
  13. Lectrician

    Lectrician Screwfix Select

    Site transformers have the centre tap earthed to the supply 'dno' earth, and the 110v earth terminals.

    Is the metal chassis earthed to the DNO earth elsewhere?
     
  14. Lokkars Daisy

    Lokkars Daisy New Member

    Sorry Bs I can't see it :'(  I can see the current flowing out of both ends of the transformer winding reversing direction at the rate of 50cps   Hz
    I can't see the dno earth having any effect whatsoever.
    Earth stabilises the system ? can that be true , I don't understand how it can ?
     
  15. Bazza-spark

    Bazza-spark Screwfix Select

    Not quite LD.  It flows out from the live and back on the neutral, but changes from a positive half cycle to a negative half cycle.  Like a radio wave doesn't o to the radio then back to the transmitter, neither does AC.

    Have a look at http://en.wikipedia.org/wiki/Ground_and_neutral and see if that helps explain it.

    Kind regards

    BS
     
  16. Lectrician

    Lectrician Screwfix Select

    Earthing one side of a transformer will pull one side down to 0v, the otherside being 240v.  Just the same way as a DNO transformer mounted up a pole.
     
  17. Lokkars Daisy

    Lokkars Daisy New Member

    Bs, when an ac current changes from a positive half cycle to a negative half cycle it changes direction, so in the case of my sketch the current is alternating back and fore through the output winding and the load resistance. A half cycle clockwise and the next half cycle anti clockwise .
    Lec, connecting one side of the output winding to the dno earth 'again as in my sketch above' will have no effect whatsoever.
    I put it to you once more that in the example above , the earths are at 230v potential with respect to the transformer winding.
    You cant have current flowing without a circuit to conduct it and for that reason the dno earth does nowt , it just sits there smiling with 230 v sitting on it, likewise all the earthed conductive parts.
     
  18. wally

    wally Screwfix Select

    If 1 side is tied down to a good dno earth how can it have 23ov on it.  The only way that could happen is if the earth becomes defective ie disconnected in some way.     
     
  19. Lokkars Daisy

    Lokkars Daisy New Member

    Because the winding is isolated Wally, just like a shaver supply transformer, there are no actual L or N points on the winding. The dno earth can't possibly tie one end down, the current has to alternate, what are you suggesting , 0 volts at one end of the winding and 230v at the other!
     
  20. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    If you connect live to a load and on to earth(no neutral) the earth would have 230v on it, wouldn't it?

    Mr. HandyAndy - Really
     

Share This Page