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Hz of 2 Legs

  1. Aug 16, 2008 #1
    How does a DMM read the Hz of two hot legs, like in a 240v recep?

    It looks like it only reads one phase
  2. jcsd
  3. Aug 18, 2008 #2


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    Staff: Mentor

    The DMM has two connection leads. It just counts zero crossings of any AC waveform, and reports the frequency in Hz.
  4. Aug 18, 2008 #3


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    Also, keep in mind that the two hot leads of a 240Vrms home AC Mains supply are 180 degrees out of phase. So if you make a differential measurement between them (of voltage or current or frequency), you are still measuring a sine wave. That is how you get the 240Vrms -- each hot is 120Vrms with respect to neutral, and the two hots are of opposite phase.
  5. Aug 18, 2008 #4
    For all 60Hz multi-phase scenarios:

    You get a sine wave no matter what the non-zero phase angle between any two legs happens to be.

    Even when they are zero degrees apart, if they have different amplitudes, you still get a sine wave between them.
  6. Aug 18, 2008 #5
    so the multimeter combines both non zero phase angles of the legs inside the multimeter into "one" sine wave then gives you the HZ based on what the combined amplitude is????

    The legs are 120 degrees apart not 180 like berkeman suggests.
  7. Aug 18, 2008 #6


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    Er, no and no. The two hots of a 240Vrms home feed are 180 degrees apart. That's how 2x120Vrms is combined to make the 240Vrms. You may be thinking of a 3-phase system, where the three phases are 360/3 = 120 degrees apart.

    And no, the multimeter has only two leads. It is not doing any combining of anything. It just senses the differential signal that is applied to those two leads. It is a floating differential measurement, with no extra information.
  8. Aug 18, 2008 #7
    ok well, in an instance where they are 120 degrees apart how does it measure 60hz when the waves are peaking at diverent time intervals?

    I guess this is my real question...
  9. Aug 19, 2008 #8
    The meter is measuring the difference in potential between the two points to which the probes are applied. It makes no difference what the voltage of each phase above ground is. If you can imagine the two vectors of the two phases rotating counterclockwise about a zero point, with their voltage above ground being displayed on the vertical coordinate line, now imagine a line drawn between their ends...like spinning a rigid triangle counterclockwise about one of its corners. When the connecting vector (opposite the center of rotation) is horizontal and at the top, the potential between the two phases being examined is zero. That's when the meter goes through a zero reading.

    180 degrees later, the connecting vector is again horizontal at the bottom, again making the meter again detect a zero potential.

    Unless the two phases are exactly 180deg apart, when the connecting vector (the voltage being sensed by the meter) goes through its zero points, both of the phase voltages will be non-zero. All the meter "sees" is the difference between the two.
  10. Aug 22, 2008 #9
    Thank you
  11. Aug 22, 2008 #10


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    A picture, or java applet is worth a thousand words?

    (You need to hit 'enter' after putting data in any of the text boxes before the applet updates!)
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