need help on polarity of three phase

Discussion in 'The Projects Forum' started by obener, Jul 27, 2009.

  1. obener

    Thread Starter New Member

    Jul 27, 2009
    4
    0
    hi all,

    I know how to check the polarity of a single phase extension cable. I have never checked the polarity of a three phase extension cable. Is it possible someone can help me on this?

    thanks
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    AC has not a polarity actually. Maybe you mean to distinguish between the live and the neutral wires. In a three phase system you have only three phases (live wires) and not neutral.
     
  3. obener

    Thread Starter New Member

    Jul 27, 2009
    4
    0
    Yes I mean the L1,L2,L3 live wires in a 4 lead 3 phase extension cable. Also L1,L2,L3 live wires and N wire in a 5 lead 3 phase extension cable.

    I tested today a 15A single phase extension cable and I got 0.2ohms for Active-Active. I also got the same value for Neutral-Neutral.

    I also tested a 10A 3 phase (4 lead) extension cable and for L1-L1,L2-L2 and L3-L3 i got 1.6ohms and I wanted to know if this was acceptable values for a 3 phase extension cable?
     
  4. obener

    Thread Starter New Member

    Jul 27, 2009
    4
    0
    Is there any book or site where I can read more up on this?
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287
    The only sure way I know of doing this is seeing if any motors run the right direction. If a motor runs backwards, reverse ANY two lines.

    Eric
     
  6. Toughtool

    Member

    Aug 11, 2008
    10
    0
    I think the OP is meaning to check the resistance of each conductor, not polarity, since AC power presents an alternating polarity between phase to phase and phase to neutral.
    Polarized single phase extension cables usually refers to the physical wire connection that has the neutral (longer slot) identified and thus prevents connecting L1 (hot) to the chassis in devices like cheap transformer-less TV sets that ground one side of their chassis to the power neutral. In other words, the polarized plugs mechanically protects you from touching a hot chassis when you are grounded. Of course it only works if the cable is unmodified and you use polarized plugs.
    As Alberto mentioned, the longer the cable, the higher the resistance and voltage drop. Here is a table that may help.
    "Copper wire resistance table

    AWG Feet/Ohm Ohms/100ft
    10 490.2 .204
    12 308.7 .324
    14 193.8 .516
    16 122.3 .818
    18 76.8 1.30
    20 48.1 2.08
    22 30.3 3.30
    24 19.1 5.24
    26 12.0 8.32
    28 7.55 13.2
    These Ohms / Distance figures are for a round trip circuit. Specifications are for copper wire at 77 degrees Fahrenheit or 25 degrees Celsius. "
    Joe

    Source:http://www.radiolocman.com/shem/shem-cache.html?di=18899
     
    Last edited: Aug 7, 2009
  7. alim

    Senior Member

    Dec 27, 2005
    113
    1
    This is not accurate . In 3 phase wye systems it may or may not carry a neutral, that is you will get phase voltage --line to neutral and line voltage --line to line . line voltage is 1.732 * phase voltage.
     
  8. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    Swap any two HOT lines (L1, L2, or L3). Leave ground and neutral (if any) where they are. I know it's obvious, but I have seen people do it wrong...
     
  9. Toughtool

    Member

    Aug 11, 2008
    10
    0
    Not sure what you are referencing here. More likely you are computing the voltage phase to phase from the voltage measured phase to neutral in a 3 phase system. i.e. 120 volts phase to neutral times 1.732 equals 208 volts, which is the phase to phase voltage in most power distributions systems(in the US].


    From:http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm
    Wye Connections:
    In a wye system the voltage between any two wires will always give the same amount of voltage on a three phase system. However, the voltage between any one of the phase conductors (X1, X2, X3) and the neutral (X0) will be less than the power conductors. For example, if the voltage between the power conductors of any two phases of a three wire system is 208v, then the voltage from any phase conductor to ground will be 120v. This is due to the square root of three phase power. In a wye system, the voltage between any two power conductors will always be 1.732 (which is the square root of 3) times the voltage between the neutral and any one of the power phase conductors. The phase-to-ground voltage can be found by dividing the phase-to-phase voltage by 1.732 (see figure 1-4).

    Joe
     
    Last edited: Aug 8, 2009
  10. alim

    Senior Member

    Dec 27, 2005
    113
    1
    Hi Toughtool I was respoonding to mik3's post that I quoted which said "there are only three phases and NOT neutral."
     
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