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

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!
>

This is why I now use an electronic control for this purpose. It ramps
up the voltage to the servo drives rather than trying to apply it
instantly. The main capacitor bank stays charged all the time.

Most relays are not rated to break DC, and may not be able to carry the
HUGE inrush surges the supply takes.
Where do you have this relay connected? Mariss' circuit doesn't look
really well thought-out. The N.O. start
button will have to carry the full surge when pushed, until the relay
pulls in. Most pushbuttons are not designed to carry several hundred
amp surges. I'm surprised that the button wasn;t the first thing to go,
but it will not last long.

Anyway, you need relay contacts that are rated to break 9 A at 60 V DC,
but you will have a real hard time finding an affordable one. A
double-break contactor would be the best choice, and will probably last
a while.

I make a device for this purpose, that goes between the DC supply and
the servo amps.
http://pico-systems.com/oscrc4/catalog/product_info.php?cPath=3&products_id=28
It is controlled by a 5 V logic signal that is grounded to power up the
amps. A solid state
relay can be used to switch it on and off. When the control input is
not grounded, then
the power is cut and the braking resistor is connected.

Jon