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Thread: 3-Phase Delta (240 VAC)

  1. #1
    Join Date
    May 2010
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    3-Phase Delta (240 VAC)

    There are 3 basic types of 3-Phase Delta (240 VAC) ... Floating, Center Tap, and Corner Tap. Can any two legs of any type be used for a 240 VAC Split Phase Motor without effecting its performance? In particular, if the non-grounded two legs of a Delta Corner Tap are used, can this cause an unbalance of some sort and in particular effect the quality of the ground by inducing current in the ground? The reason for the question is that although the Phase to Phase voltage of all three are 240 VAC their phasor diagrams are much different. For example, the Center Tap Delta has two 120V Phases that are 180 degrees out of phase whereas the Corner Tap has two 240V Phases that are 60 degrees out of phase. So, even though a test meter displays a nice 240 VAC display, at instances in time the two phases (legs) of the Corner Tap are "fighting" each other.

  2. #2
    Join Date
    Feb 2003
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    Wisconsin
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    You are shifting reference points.

    If you are only using a 2-wire 240V L-L connection, you can totally disregard the center tap, the result is a standard set of 3 different L-L voltages.
    Just because you can, doesn't mean you should.

  3. #3
    Join Date
    Apr 2006
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    Portland, OR
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    What Jim said.

    The only reason you can say you have different phase angles is that you are shifting your reference points. If you select as your reference point one of the terminals connected to the motor, and measure the voltage to the other terminal, you will always have a single 240V phase. The apparent 60 degree phase angle difference that you see when you select the third (not connected) delta terminal as your reference point is not relevant as far as the current through the coil and the magnetic field created by the coil. As far as all of the _intentional_ current paths all of the grounding flavors are the same.

    Where you _might_ see a difference is in any circuit path from the coil to the frame of the motor, since the frame of the motor is grounded. The voltage that is trying to 'break down' the winding insulation, leakage through the insulation, capacitive coupling between the coils and the frame, etc. All of these are circuit paths that bypass the _intentional_ current paths. Depending upon the grounding of the delta and the connection of the motor to that delta, you will put different voltages across these parasitic circuits, and thus see different current through them.

    For conventional low voltage motors at 60Hz, I doubt that this is an issue, but it certainly is an issue at higher voltages and frequencies.

    Also note that in your list of 'delta flavors' you did not mention the option of 'open' versus 'closed' delta. The voltage across the 'open' side of an open delta will be less stable than the voltage across the closed side, and for most open delta services, the voltage across the grounded side will have different stability characteristics than that across the stinger side, because different size transformers are used. This can certainly affect motor performance.

    -Jon

  4. #4
    Join Date
    May 2005
    Location
    Richmond, Virginia
    Posts
    21,838
    Quote Originally Posted by blazep View Post
    Can any two legs of any type be used for a 240 VAC Split Phase Motor without effecting its performance?
    Yes, even across the open side of an open Delta. With rare exception, a load doesn't care which, if any conductor is grounded, nor the relative phase angles.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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