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Question about contactor ratings.

Discussion in 'Electronic Basics' started by Jon Danniken, Jul 28, 2005.

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  1. Jon Danniken

    Jon Danniken Guest

    Hello, I have a question about contactor ratings. On a contactor I am
    looking at, it is rated at 60A (for 2240VAC), but also at only 10 HP (for
    240VAC). From my understanding, ten horesepower is ~7460VA, which is only
    ~31A at 240V.

    Why the discrepancy between the HP and A ratings? I thought it could be due
    to resistive vs. inductive load, but the same contactor is also rated 75A

    Thanks for any insight into this,

  2. JazzMan

    JazzMan Guest

    I suspect that it has something to do with the amount of
    current the contacts will break, as well as what they
    will carry closed. Someone with more knowledge should be
    along shortly.

    Please reply to jsavage"at"
    Curse those darned bulk e-mailers!
    "Rats and roaches live by competition under the laws of
    supply and demand. It is the privilege of human beings to
    live under the laws of justice and mercy." - Wendell Berry
  3. Keep in mind that no motor is 100 % efficient, but look up the locked
    rotor current for a few 10 HP 240 VAC motors. (Hint: depending on the
    class of motor, this can be something between 4 and 20 times full load
    rated current.) Each time you start one of these, they draw this
    current for at least a fraction of a second as they accelerate from a
    standstill. If they are actually locked, they draw this current till
    some protective device opens the line or turns off the relay. So if
    driving a 10 HP motor, not only do the contacts have to handle this
    current at the moment of contact closure, they may have to break this
    high and inductive current if the motor is ever overloaded.
  4. Jon Danniken

    Jon Danniken Guest

    Thanks, John, that makes absolute perfect sense; thanks for the information.

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