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

KZF,

It sounds like you should have a higher resistance value for the dump
resistor. It might take a bit longer for the voltage to drop, but a
higher value would limit the peak current through the dump contacts.

It wouldn't hurt to use a higher rated contactor for the dump relay,
either. The larger the contact area, the less likely they are to weld
with a current pulse. A big old open frame relay with 3/8" silver
graphite diameter contacts is less likely to weld closed than a compact
ice cube relay with 1/10" contacts.

If you can find a big double pole relay for the dump circuit, then you
can split the current between the two contact sets. The only thing is
that you have to use a separate power resistor for each contact set.
For instance, I'd use two 10 - 15 ohm 50 watt resistors. Connect the
power cap to one end of each resistor (in parallel). The other end of
each resistor goes to one end of a contact set on the dump relay. The
opposite ends of the dump contacts go to ground.

You can't just parallel the dump relay contacts without individual
resistors, 'cause one contact will inevitably close a few milliseconds
earlier than the other, and end up carrying all, or the majority, of the
current.

It may also be advisable to use a power thermistor or a power resistor
bypassed by a short delay cutout relay to limit the inrush current to
your supply. That saves me from blowing breakers with my big VFD
circuits, and should stop you from dimming your shop lights at system
turn-on.

Dave

On 1/29/2011 3:49 PM, kzf817 wrote:
> Good afternoon guys. I am just completing a Bridgeport series 1 CNC Stepper to Servo retrofit. I am using the servo E-stop circuit that Mariss recommends on his site. It is the "Electromechanical E-stop" schematic shown at the top of this page: http://geckodrive.com/support.aspx?q=10003
>
> Whenever I clear the E-stop button and press "Reset" the lights in the workshop blink, clearly from high instantaneous load as the filter capacitor charges. After my initial power up tests, I blew the load dump resistor after about the 5th or 6th E-stop cycle over a period of a couple of hours. When I inspected the contacts that carry the load between the 50 watt, 7 ohm wirewound resistor, and ground, I saw that the contacts were welded together, clearly from high load. This, of course, made the resistor carry current continuously, instead of only as a bleed resistor immediately after an E-stop event. My transformer toroid is 1000va, and my filter capacitor is 56,000uf, and DC out is a nominal 60 volts. I know that this CAP is a little bigger than is strictly necessary, but it is what I had available. My E-stop relay contacts are rated for 15 amps, so it should be able to carry this load. I clearly must be arcing the contacts HUGE every time the relay cycles. Can anyone tell if I am overlooking anything obvious in how I have things sized and configured? Thanks!