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

```Subject:       Re: Short circuit/Control Panel - 220 V relay coils
Date:   Thu, 12 Jun 1997 08:49:28 +1200
From:   "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization:  Wellington Polytechnic, NZ
To:   tesla-at-pupman-dot-com

Jim Bert, all,

>        From: Bert Hickman <bert.hickman-at-aquila-dot-com>
> Organization: Stoneridge Engineering
>          To:  Tesla List <tesla-at-pupman-dot-com>
>  References:
>             1
>
>
> Tesla List wrote:
> >
> > 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
> > applied.
> >
> > 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.
> >
> >         Regards,
> >
> >         jim
> >
> > >> 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
> > >>
> > >
> > >Bill,
> > >
> > >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
> > >a
> > >\$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! : )
> > >
> > >rwstephens
> > >
>
> Jim,
>
> I agree with your analysis. Sooo....
> How about using an electrolytic capacitor charged through a power
> resistor from the DC source. The cap would initially be charged to the
> full AC Peak value to supply the initial surge of pull-in current
> required. Once the initial current surge has energized the contactor,
> the holding current would be set by the sum of the power resistor and
> contactor coil resistances (R1+Rc). Might take a bit of trail and error,
> but should be a usable approach.
>
>                                    |
>                R1               -------
>    ---------/\/\/\//\------------o   o--------O
>    +                      |                   O
>                         + |                   O
>                         -----                 0  Rc
>   DC                    -----                 0
>                           |                   O
>    -                      |                   O
>    --------------------------------------------
>
> We used to use a similar approach to rapidly brake AC motors by
> disconnecting the motor from the AC and switching it over to a charged
> capacitor through a contactor to bring it to a _screeching_ halt.

The same technique of sorts was used in one of the last electro-
mechanical telephone exchanges I worked in to pull in relays with
a huge number of wire-spring contacts, the idea being to minimize
operating power. I think we used a v-doubling scheme a la connecting
charged caps in series.

Malcolm

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