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Thread: Wye start, Delta run...

  1. #1
    Join Date
    Jan 2007
    Location
    Chattanooga, TN
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    118

    Wye start, Delta run...

    Was reading a closed thread that had this discussion and I would like to bring it back up again.


    Some had mentioned that in a wye configuration the torque of the motor would be less due to the fact that the windings are seeing less voltage than in a Delta config ( ie soft start equip)

    My understanding is that In a Wye config..the torque would be greater, since in this configuration the current would be greater due to the Wye config, then when motor is up to speed it is then changed to a delta since less power is now needed during running conditions, would this be correct?

    Thanks
    Luke
    Luke

    Electrical Planner for TVA

  2. #2
    Join Date
    Feb 2003
    Location
    Illinois
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    18,977
    The purpose of a wye/delta start/run system is to reduce the starting current. The current is less when you have the motor connected in wye.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  3. #3
    Join Date
    May 2005
    Location
    Redmond, WA
    Posts
    778
    Luke- You're thinking of a motor as a constant power device: V x I = constant. If volts go down; amps go up.
    That is true for a running motor. But during starting the motor is not a constant power device. It follows ohms law (more or less). If voltage is lower, current is lower. Lower current = less torque.
    Bob Wilson

  4. #4
    Join Date
    Sep 2008
    Location
    UK
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    6,153
    Quote Originally Posted by Luketrician View Post
    Was reading a closed thread that had this discussion and I would like to bring it back up again.


    Some had mentioned that in a wye configuration the torque of the motor would be less due to the fact that the windings are seeing less voltage than in a Delta config ( ie soft start equip)

    My understanding is that In a Wye config..the torque would be greater, since in this configuration the current would be greater due to the Wye config, then when motor is up to speed it is then changed to a delta since less power is now needed during running conditions, would this be correct?

    Thanks
    Luke
    For a cage induction motor, torque is approximately proportional to the square of the applied voltage.
    If you connect the three windings in WYE they will each see 1/SQRT(3) of the delta voltage - about 277V if the line to line is 480V.
    It will thus produce about (1/SQRT(3))^2 or 1/3 of the torque in delta.

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