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1.) Delta Or Wye?

Discussion in 'Electronic Basics' started by Randy Gross, Nov 18, 2003.

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  1. Randy Gross

    Randy Gross Guest

    Greetings,

    This question concerns the configuration of an AC genny stator. I like the
    Idea of a single common lead and I understand Wye. Delta, on the other
    hand, I'm a little unclear about especially causing it to short by improper
    hookup. What are the advantages?

    An Inquiring mind,
    Randy Gross
     
  2. Don Kelly

    Don Kelly Guest

    ---------
    For most cases the Y has the advantage in that the output is not floating as
    in a delta and it is also possible to draw single phase loads - i.e. 120V
    phase to neutral and 208 phase to phase for 3 phase loads. Most large
    generators are Y as the insulation needs are a bit less and one can detect
    ground faults.
    A delta is essentially an ungrounded system and for low voltage situations
    one phase can go to ground accidentally with no problem. Single phase loads
    have to be at the voltage from phase to phase (i.e. a 240V delta -allows
    240V single phase on any pair of terminals) If you want a 120V/240V load,
    then one phase must be center tapped and that tap is usually grounded. that
    means you can't do the same on the other phases.
    The delta winding has lower currents in the phase windings but the voltage
    of the winding is higher (both by a factor of root(3). There is then a
    balance between insulation, turns per winding and conductor size. For higher
    voltages, this favours Y.
    There is a difference in the case of third harmonics. The delta provides a
    path for third harmonics but this is of more importance in transformers than
    generators.
    As far as shorting is concerned: With a proper connection, the voltages of
    the three phases cancel when one looks at the loop formed by the connection.
    If one of the windings is reversed, then this cancellation doesn't occur and
    the voltage source seen by the loop (triangle) of the windings will be twice
    the phase voltage causing high short circuit currents to flow in the loop.
    Normal : Va +Vb +Vc = 1 +(-0.5 -j0.866) +(-0.5 +j0.866) = 0
    reversed: Va +Vb -Vc =1 +(-0.5-j0.866) -(-05+j0.866) =1-j1.73 (magnitude
    =2)
    Draw the phasors.
     
  3. Randy Gross

    Randy Gross Guest

    Thanks Don,

    From your descriptions (very well done), it seems that Delta is a
    specialized config where that type of output would be the advantage. I
    can't think of a use at this time but, It's good to have options.

    Randy























    <[email protected]>...
    :
    :
    :
    : : > Greetings,
    : >
    : > This question concerns the configuration of an AC genny stator. I like
    the
    : > Idea of a single common lead and I understand Wye. Delta, on the other
    : > hand, I'm a little unclear about especially causing it to short by
    : improper
    : > hookup. What are the advantages?
    : >
    : > An Inquiring mind,
    : > Randy Gross
    : >
    : ---------
    : For most cases the Y has the advantage in that the output is not floating
    as
    : in a delta and it is also possible to draw single phase loads - i.e. 120V
    : phase to neutral and 208 phase to phase for 3 phase loads. Most large
    : generators are Y as the insulation needs are a bit less and one can
    detect
    : ground faults.
    : A delta is essentially an ungrounded system and for low voltage
    situations
    : one phase can go to ground accidentally with no problem. Single phase
    loads
    : have to be at the voltage from phase to phase (i.e. a 240V delta -allows
    : 240V single phase on any pair of terminals) If you want a 120V/240V load,
    : then one phase must be center tapped and that tap is usually grounded.
    that
    : means you can't do the same on the other phases.
    : The delta winding has lower currents in the phase windings but the
    voltage
    : of the winding is higher (both by a factor of root(3). There is then a
    : balance between insulation, turns per winding and conductor size. For
    higher
    : voltages, this favours Y.
    : There is a difference in the case of third harmonics. The delta provides
    a
    : path for third harmonics but this is of more importance in transformers
    than
    : generators.
    : As far as shorting is concerned: With a proper connection, the voltages
    of
    : the three phases cancel when one looks at the loop formed by the
    connection.
    : If one of the windings is reversed, then this cancellation doesn't occur
    and
    : the voltage source seen by the loop (triangle) of the windings will be
    twice
    : the phase voltage causing high short circuit currents to flow in the
    loop.
    : Normal : Va +Vb +Vc = 1 +(-0.5 -j0.866) +(-0.5 +j0.866) = 0
    : reversed: Va +Vb -Vc =1 +(-0.5-j0.866) -(-05+j0.866) =1-j1.73 (magnitude
    : =2)
    : Draw the phasors.
    : --
    : Don Kelly
    :
    : remove the urine to answer
    :
    :
    :
     
  4. Steve

    Steve Guest

    US Navy ships are always 440 VAC 3 phase Delta. I'm not an electrical type
    but as a marine engineer I had to be aware of this and deal with the
    limitations for 30+ years.

    For lower voltage, such as lighting, you always had to have a bank of 3
    transformers.. To mention one of a few limitations..

    Don did a better explaination that anyone ever offered me in the navy.
    (always got a 'take it or leave it').

    Steve
    s/v Good Intentions
     
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