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Amps for a coil!

Discussion in 'Electrical Engineering' started by CLT, Mar 20, 2006.

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  1. CLT

    CLT Guest

    How much amps can take a coil for a nema size 5 contactor. The coil is
    125 vdc! I am reading 3 amps and the coil gets hot, but i have no
    baterries, just a variac with a diode bridge to feed the coil!

    Thanks in advance.
    CLT.
     
  2. daestrom

    daestrom Guest

    If you're using a diode bridge to rectify AC to DC, how are you measuring
    the DC?

    Rectified DC fed into a highly inductive load (such as your coil), means you
    can't just use a simple voltmeter to get the voltage reading.

    Different types of meters work differently, but for example a meter using a
    D'Arsonval movement (old style analog meter) responds to the *average*
    voltage, not the RMS voltage. So adjusting your variac to get 125vdc (rms)
    is a bit trickier.

    daestrom
     
  3. Guest

    What voltage are you developing across the coil and how are you
    measuring it?
     
  4. CLT

    CLT Guest

    I am reading the 3 amps in the primary side from de variac, but it
    cant be so diferent in the coil. The question is, how many amps
    should i expect to read, lets say 0.25 amps? or less?
    Thanks for your answer daestrom!
     
  5. Bud--

    Bud-- Guest

    Measure the coil resistance. The normal coil current would be 125
    divided by the resistance (DC circuit and Ohm's law).

    Reading the current at the input of the vairac includes the current to
    energize the variac which may be a significnt part of your reading (and
    can't just be subtracted off because it is at a different phase angle).
    Note cautions about using an RMS reading meter.

    The coil will act as an inductor to smooth out the rectified AC supplied
    to it. In addition the diode bridge will act as a 'freewheeling diode'
    (conducting current from the coil when the supply pulses fall below the
    coil voltage) to further smooth the current. As a result you may have a
    fairly constant voltage across (and current through) the coil - I don't
    know how much smoothing results. Measuring the voltage across the coil,
    as in <gfretwell>'s post, is probably the best idea. You want this to be
    125V using a RMS reading meter (an average reading meter is probably
    quite close because of the smoothing).

    Significant ripple in the coil current can produce heating from eddy
    currents in the core unless the core is laminated.

    The 2 times I supplied a DC coil through a full wave bridge, the coil
    ran at its rated current when the AC supply voltage was very near the DC
    coil rating. I don't think the physics requires this.

    bud--
     
  6. CLT

    CLT Guest

    Thanks Bud!

     
  7. Bud--

    Bud-- Guest

    Interesting. I have never seen a contactor with a DC coil - or how it
    works. I looked at the SquareD catalog and all the NEMA 6 & 7 contactors
    have a DC coil. Do you know what is used to reduce the current, like
    shunted resistor?

    To the OP. If this seems odd, DC contactor coils behave somewhat
    differently form AC coils.

    In AC coils the current is limited by resistance and inductance. When
    the coil is energized, the magnetic path has an air gap because the
    armature is not pulled in. When the contactor closes, the armature
    closes the magnetic path, the inductance goes way up, and the current
    goes way down. This results in a desirable high current to close the
    contactor and low current to hold it. For a NEMA #3 SqD contactor the
    inrush is 700 VA and closed is 46 VA. Closed is 14 Watts indicating a
    relatively large inductance.

    In DC coils the current is limited only by resistance (ignoring some
    inductance with rectified AC) so the current is the same open and
    closed. Thus per BFoelsch, a means is used to reducing the holding
    current. The SqD catalog for a NEMA #6 contactor gives 1780VA inrush and
    48VA closed. Closed is 32 Watts compared to 48VA indicates low inductance.
    The SqD coil is about 120 VDC so 1780 VA inrush is about 14.8A. If the
    coil were totally resistive its resistance would be about 120/14.8 = 8
    ohms. If totally resistive the closed voltage would be about 19.7 V and
    current 2.4 A.

    The SqD catalog says the contactors I looked at are always supplied by a
    transformer with 120 VAC secondary, and are supplied by transformer even
    if the supply voltage is 120 VAC. The coil voltage is not actually given.

    bud--
     
  8. daestrom

    daestrom Guest

    Yep. Worked with a lot of such contactors. The older styles I've worked
    with have one large contact on each contactor, with arcing contacts, main
    contacts and often an arc-chute with blow-out coils. Then off the 'tail'
    side, there is a couple of pairs of 'auxilary' contacts. And one of those
    NC contacts is used to short out a resistor in series with the coil. The
    resistor is sized to provide just holding current when the NC contacts open
    up.

    Because they only have one large current carrying main/arcing contact, you
    often will see two of these beasts in the controller as a minimum. If its
    for a DC motor that has starting resistors, you may see more large
    contactors that are used to short the motor starting resistor as the motor
    accelerates.

    While an integral horsepower AC contactor may have all three phases routed
    through one contactor that has three/four pairs of contacts each the size of
    your fingernail, the equivalent horsepower DC starter would have two
    contactors, each having a pair of contacts the size of your entire thumb for
    arc interruption (picture holding up both thumbs facing each other and bring
    them together), and possibly smaller contacts underneath these for carrying
    the load current (these contacts break before the arcing contacts so there
    is no arc on these, then the 'arcing contacts' open up to interrupt the DC
    current).

    Such are the 'joys' and pitfalls of interrupting large DC currents.

    daestrom
     
  9. Bud--

    Bud-- Guest

    The OP has a NEMA 5 contactor with DC coil. SqD NEMA 6 & 7 contactors
    are only supplied with DC coils. Why switch to DC coils with large
    contactors?

    bud--
     
  10. daestrom

    daestrom Guest

    Well, I admit I may have rambled off topic a bit. But is this NEMA 5
    contactor with a DC coil meant for a DC motor or an AC motor? It occurs to
    me that the OP didn't specify the type of load. Interrupting DC load
    current takes quite a different contactor construction than AC.

    My 'rambling' was about the unusual size/construction of contactors used for
    DC motors versus those for AC motors. Obviously, the most often used coil
    for a DC motor controller is also DC and has auxilary contacts to put a
    resistor in series with the coil to limit 'holding current' while still
    providing the higher 'pickup current' needed.

    daestrom
     
  11. Bud--

    Bud-- Guest

    'Rambling' is often, including here, an interesting feature of threads.

    The OP didn't specify what is being interrupted. However the SqD NEMA 5
    & 6 contactors, the largest in that line, are for AC circuits. The coil
    has a required transformer feeding a "solid state rectifier circuit" to
    a DC coil. It is not obvious to me why SqD converts to DC for the
    largest AC circuit contactors. Could just be hum/armature vibration.

    bud--
     
  12. daestrom

    daestrom Guest

    Well, one thought is that 'amp for amp' you can get more mechanical force
    with DC. Any AC coil needs a 'shading coil' to even out the magnetic pull,
    otherwise they chatter at 120hz. This reduces the instantaneous magnetic
    flux available across the coil armature face. DC coils don't have that
    particular problem (although they do need external current limiting).

    Perhaps the larger contactors, with stronger springs for quick-opening, need
    more 'pull' to pick up?

    daestrom
     
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