# Old DC Contactor



## jmsmith (Sep 10, 2011)

Brian John-
The resistors were just a suppression circuit to limit the voltages developed whenever the relay is switched off. They are also done with reverse-voltage diodes and other forms of suppression these days. Using resistors is the recommended way of dealing with these transients, but only where heat generated would not be detrimental. Look at it this way.... a 12VDC relay coil when switched off can produce a voltage of up to 2000V... more than enough to fry a coil, or if breaking across a contact, weld a contact. Hoe I explained this right! Have a good day...
-Jim


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## jmsmith (Sep 10, 2011)

Sorry, John... Didn't mean to call you Brian John... You can come down and boot me any time you wish! :lol:


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## Big John (May 23, 2010)

_Jmsmith_, maybe I didn't look closely enough, but are you saying these are actually in parallel with the coil as a snubber?

Any particular reason they'd need two of them per coil? Thanks.

-John


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## jmsmith (Sep 10, 2011)

Big John said:


> Jmsmith, maybe I didn't look closely enough, but are you saying these are actually in parallel with the coil as a snubber?
> 
> Any particular reason they'd need two of them per coil? Thanks.
> 
> -John


What I described was a snubber circuit, and the only reason I could think of two resistors would be increased heat dissipation and taking up less real estate wherever they were mounted. I neglected to note a series connection. If it is series, the only thing I could see would be needing a voltage drop to keep from coil burnout, again using a smaller resistor in each line for the reason I stated above. Do you have a pic or drawing of this? Guess it's time to drag-out my books again! :lol:
- Jim


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## jmsmith (Sep 10, 2011)

Update here, John. On the older relays, an AC coil could be used on DC. But if used for longer periods of time, the coil would overheat and burn up. So resistors were added to not only reduce voltage impressed on the coil, but also to aid in heat dissipation. Hope this makes some sense.
-Jim


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## Zog (Apr 15, 2009)

A coil will have less resistance to DC than AC (Z) so resistors are often used for dual rated coils in a DC application. Or there could be an aux contact that inserts the resistors in series after the coil picks up. Few other senarios too, hard to tell without details of the whole control circuit.


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## oldtimer (Jun 10, 2010)

Zog said:


> A coil will have less resistance to DC than AC (Z) so resistors are often used for dual rated coils in a DC application. Or there could be an aux contact that inserts the resistors in series after the coil picks up. Few other senarios too, hard to tell without details of the whole control circuit.



This is interesting, I would love to see pictures, and more details.

What were these contactors controlling?


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## Big John (May 23, 2010)

Alright, I'm still leaning towards the dual-rated coils. These resistors aren't something that was cobbled in, it's definitely a GE part designed for use on these contactors.

And the whole added-heat-dissipation theory also makes sense, because they're installed vertically in free-air... almost like heat sinks.

I'll see if I can grab a picture.

-John


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## Frank Mc (Nov 7, 2010)

Zog said:


> A coil will have less resistance to DC than AC (Z) so resistors are often used for dual rated coils in a DC application. Or there could be an aux contact that inserts the resistors in series after the coil picks up. Few other senarios too, hard to tell without details of the whole control circuit.


Hi Zog

I do recall seeing resistors which were used via aux contacts connected to dc contactors....Since the hold in voltage can be a lot less than the pull in voltage...

Frank


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## jmsmith (Sep 10, 2011)

Big John said:


> Alright, I'm still leaning towards the dual-rated coils. These resistors aren't something that was cobbled in, it's definitely a GE part designed for use on these contactors.
> 
> And the whole added-heat-dissipation theory also makes sense, because they're installed vertically in free-air... almost like heat sinks.
> 
> ...


Lookin' forward to seeing a pic or two... It's sometimes a good thing, or could be bad. I've seen some pics of stuff here lately that sure does make me feel old.... "Man, I haven't even SEEN one of those in 30 years!!!"
 :lol:
Y'all have a good one.
-Jim


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## Wirenuting (Sep 12, 2010)

Big John said:


> Alright, I'm still leaning towards the dual-rated coils. These resistors aren't something that was cobbled in, it's definitely a GE part designed for use on these contactors.
> 
> And the whole added-heat-dissipation theory also makes sense, because they're installed vertically in free-air... almost like heat sinks.
> 
> ...


Heat sinks like these?
The open door on the left is the old 2 stage starter. The right one was gutted when I replaced a drive. 
The resistors are the white vertical stripes. They were ceramic and copper.. About 20# each.


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## JRaef (Mar 23, 2009)

Wirenuting said:


> Heat sinks like these?
> The open door on the left is the old 2 stage starter. The right one was gutted when I replaced a drive.
> The resistors are the white vertical stripes. They were ceramic and copper.. About 20# each.


If that was a "2 stage starter" it was likely a Primary Resistance starter and those were the resistors that would go in series with the motor connections during start-up.,It was an old form of Reduced Voltage starter that hardly anyone uses any more.


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## Wirenuting (Sep 12, 2010)

JRaef said:


> If that was a "2 stage starter" it was likely a Primary Resistance starter and those were the resistors that would go in series with the motor connections during start-up.,It was an old form of Reduced Voltage starter that hardly anyone uses any more.


Yup, that's them. 
Thanks for an explanation,, I couldn't remember off the top of my head how they were wired. 
I do know that the one pictured is due to come out. 
I'll take better pictures of it & the wiring diagram. 
It was installed about 30 years or so ago.


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## frenchelectrican (Mar 15, 2007)

Big John said:


> I've got some DC contactors that are probably twice my age. I can't find anything on the specific model, but they're part of the GE CR105 series.
> 
> They have two power resistors attached to the contactor body, in series with the coil, which seems like a strange design. Are these actually normally AC contactors designed to be field modified to tolerate DC?
> 
> ...


Big John.,

The CR105 series contractors I have see them before and yes the contractor coil itself is dual rated for both AC / DC ( not the music kind mind ya :whistling2 and they do have 2 snubbers in there to reduce the voltage after it pick up when it go to holding mode it will drop the voltage to prevent from overheating.

I have see them fail if the snubbers failed useally overheated core or some case they will drop out.


Merci,
Marc


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