Re: Short circuit/Control Panel - 220 V relay coils

Subject:      Re: Short circuit/Control Panel - 220 V relay coils
      Date:   Wed, 11 Jun 1997 01:47:36 GMT
      From:  jim.fosse-at-bjt-dot-net (Jim Fosse)
        To:  Tesla List <tesla-at-pupman-dot-com>
References:   1

If you apply DC to an AC coil the current draw will go through the
roof!  Those AC coils in the contractors rely in the impedance of the
coil to limit the current.  With DC, the current will go to V/R where
R is the DC resistance of the coil; which is low.

I just checked my 105A 660V contractor with a 120V coil.  The coil
measures 30 ohms.  Energized, it draws maybe 0.2A (my 5A ammeter is
hard to read at this low a level)  With 120VDC applied, 4A will be
drawn; smoking the coil.

If I wanted to run it on DC, I would use V=IR -> V= 0.2A * 30 = 6
volts! ...(lab time)...  WRONG! it doesn't even budge with 12VDC

Ding! (light on;) This is the difference between the "pull in" and
"hold" currents which can be tailored, for AC coils, with the use of
shorted turns that partially encompass the core.  For DC, the pull in
current must be sustained.  So for running an AC coil on DC a
compromise must be made between pulling in the contractor and over
heating it's coil.  I'm assuming here that the coil's wire gauge is to
small to handle the full pull in current.



>> just a comment - a coil rated 220 VAC will work fine if you build a
>> simple 
>> bridge rectifier and give it 110 DC - add a capacitor (filter) of 20 to
>> 100 
>> microfarads to keep the coil from buzzing and to give it a bit more
>> voltage.
>> snip  -----
>> . Be aware that most of
>> the contactors out there are using 220 volt relay coils. If that is
>> all you can find (and you need a 110 type), then get a 110 to 220
>> volt transformer to power the coil. The contacts can be used for
>> either 110 or 220.
>> snip
>That's a darned good idea!  Actually that bridge rectifier with a 
>capacitor will charge to some ~170 volts (1.414 times 120 volts RMS).  
>I'm thinking here that 180 volts on a 220 volt device is like me getting
>$180,000.00 settlement from a $220,000.00 lawsuit. Given that kind of 
>free money to spend on Tesla coiling you'd find me clicking up and 
>down just fine! : )