Re: [CAD_CAM_EDM_DRO] Re: E-stop relay welds contacts for load dump resistor

Dan,

You can indeed weld the contacts upon contact make, if there is a big
enough capacitive charge dumped through them. That's how spot welders
work. At the instant of closure, only a tiny area of the contact is
carrying current. If you can heat up a small enough area fast enough,
the metal faces will liquefy and fuse, and the contacts will be welded
shut. The design of a relay contact has to take into account this
phenomenon, and is a bit of a black art.

The whole trick is to limit the peak currents per contact.

A big open frame contactor should handle it.

Be sure that you power circuitry is set up so that the line power input
to the transformer breaks before the dump relay engages. You don't want
the dump relay trying to carry both the stored energy in the cap, plus
whatever the line can provide through the transformer and bridge. Not
healthy for any of the components involved.

I'm a little puzzled about the whole issue. The dump circuit should
really be reserved for rare emergency situations, not for routine
shutdown operations. Perhaps there is a bit of a learning curve being
addressed?

Dave

On 1/30/2011 3:53 PM, kzf817 wrote:
> Ahh, Haah: From your description of arcing conditions, I think that I know what is happening now. Although my relay is rated at 15 amps, I bet that I am killing the contacts with a "break" arc. When I push the "reset" button I have probably already charged the power supply by the time the relay opens the contacts for the load dump resistor, arcing the life out of the contacts and eventually welding them closed. This would all make sense, as the load dump resistor has blown both times while the machine was powered up, not immediately after an E-stop. I had to cut the power to stop the load dump resistor from smoking. Sounds like it is impossible to have a substantial arc on initial close (make). I do have some big monster open frame relays lying around, and will probably try one of those. At least I will be able to directly observe any arcing that takes place during break. Thanks,
>
> Dan.
> =================================================================
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, David Speck<Dave@...> wrote:
>> Dan,
>>
>> TBQH, I'm not sure how this would be beneficial for you.
>>
>> However, at 60 volts, there will be no appreciable arcing on contact
>> "make" of the dump circuit. There may be some arcing at the contact if
>> it bounces, but a well designed and spec'd relay, in conjunction with a
>> proper dump resistor should prevent any arcing.
>>
>> Relay contacts routinely arc upon release if they are carrying
>> appreciable current.


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