Re: Short circuit/Control Panel - 220 V relay coils
Subject: Re: Short circuit/Control Panel - 220 V relay coils
Date: Wed, 11 Jun 1997 06:29:05 -0700
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
Organization: Stoneridge Engineering
To: Tesla List <tesla-at-pupman-dot-com>
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
> 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! : )
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.
+ | O
+ | O
----- 0 Rc
DC ----- 0
- | 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.
Safe contactin' to ya!
-- Bert --