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True or False ?

Discussion in 'Electronic Basics' started by [email protected], Nov 23, 2005.

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  1. Guest

    Hello and good morning to all members. A retired electrician told me
    that to prolong the life span on the contact point of the contactor,
    you can add one capacitor between the two contact point. By theory it
    sound acceptable, but i had not try it out,because this is to control a
    ac 230 volts 1600 watts heater. Does any member can advise in this
    issuse. Thanks.
    Regards
     
  2. A capacitive arc snubber connected across a contact can reduce the
    flash each time the contact opens. By diverting the load current to
    the capacitor for a brief moment, it allows the contact to get enough
    air between the points that and arc is not drawn between them.
    However, that same capacitor can dump a big current into those points
    when the contacts close, causing them to pit and possibly weld
    together. For this reason, such snubbers also include a resistor in
    series with the capacitor.

    This sort of thing works best with inductive loads, like motors, where
    the inductance causes the current to decrease more slowly than the
    voltage does. Resistive loads, like heaters do not produce the extra
    voltage at the moment contacts in series with them open, so a snubber
    may not help much. You may just need a bigger contactor.

    But if you want to try it you will need a 1 microfarad capacitor rated
    for use with 240 volts AC or more (or 1000 volts DC) and a 47 ohm, 2
    watt resistor. Connect them in series and across the contact points.

    Here is a data sheet for an example of a capacitor type rated for
    across the line, AC operation (class X), available from Digikey:
    http://www.panasonic.com/industrial/components/pdf/abd0000ce30.pdf
     
  3. Guest

    Thanks John.I had a question on your capacitor rated voltage,why does
    the dc voltage need to be 4 time higher then the ac rated vale and what
    is the pupose on the 47 ohm resistor ?
    Regards.
     
  4. Pooh Bear

    Pooh Bear Guest

    The 4x is misleading. The cap is subjected to pulse operation which makes
    rather strenuous demands of it. Voltage 'derating' is helpful for pulse
    operation.
    To limit the current. Re-read what John said.

    Using a cap without that R can result in some 'interesting' results.

    Graham
     
  5. Line voltage can have some nasty spikes riding on it, occasionally.
    Class X, line rated capacitors are made with generous over voltage
    ratings and safe failure modes (the metalizations tends to evaporate
    around a hole burned through the film, rather than short and explode).

    You can get some idea of the real voltage capability of a Class X1
    capacitor by looking at some of the details on the data sheet I
    referenced.

    DC rated capacitors are not generally made with such over voltage in
    mind, so you have to get the safety factor by using a much higher
    voltage rating than the normal peak line voltage, and you are still
    not necessarily getting the graceful failure modes.

    The resistor is a current limit mechanism for when the contacts close
    near peak line voltage (1.414 times 240 VAC RMS = 339 volts). There
    will be a very large current pulse as the capacitor dumps all that
    voltage while the contacts are bouncing together, and that inrush can
    damage the contacts and/or the capacitor. The 47 ohm resistor will
    limit that peak current to 339/47=7.2 amps (in addition to the current
    of the heater).
     
  6. Guest

    Thanks John and Pooh for the reply and explaination in this issuse.
     
  7. Dan Hollands

    Dan Hollands Guest

    The capacitor or capacitor / resistor will have no effect on a resistive
    (heater) load

    --
    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
    585-872-2606

    www.QuickScoreRace.com
     
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