Re: Currect size relay for E-STOP

I agree with Jon. Another point:

DC arcing will tend to pit the contacts with microscopic "dimples".
After a while, the two surfaces of the contact are not flush with
each other and begin to overheat under normal conditions. This can
cause a number of problems including fusing closed and/or losing the
annealing on the copper (in small realays which use copper leaves for
tension) leaves which will prevent the contact from receiving proper
closing tension in the future (causes them to arc more and condition
is cyclical to destruction of contacts).

Being conservative costs money up front. In the long run, being
conservative saves money.

Jeff

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> Michael Gamber wrote:
>
> >I intend to clamp a wire-wound resistor across the filter cap on
my
> >68.7v DC PS.
> >
> >I have 3 75v 2A servos running. Assume the purpose of this is to
> >absorb energy when the e-stop is pressed and the AC is cut to the
PS.
> >
> >If I use a relay rated at 25A @ 28v DC does this mean that it is
> >good for 10A @ 70v DC by using OHM's law?
> >
> >
> No, absolutely not. The ratings of relays, especially for DC
current
> breaking applications, is not anything like Ohm's law. A 28 V relay
> cannot be used at all at 70 V DC. It may work fine, but you can't
> KNOW that it will work reliably to break the arcs that can be
created.
> It might work fine the first 100 times, and catch fire, weld the
contacts,
> or some other disasterous result the next time. If this relay will
be
> used only to MAKE a contact under voltage, but never BREAK the
> current flow at voltage, then it is a lot more likely to work. So,
if the
> condition is when power is cut the NO contacts connect the braking
> resistor, you may have a usable circuit. In this case, you can
probably
> use the 25 A rating of the contacts without worrying too much about
> the voltage. (On the other side of the cycle, the relay may pull in
> before the capacitor bank charges, so the opening of the contact
won't
> be under voltage and current. But, you'd want to check that, and
watch
> for contact arcing when the braking circuit opens.)
>
> You want to make the current through your braking resistor moderate,
> so as not to exceed the current rating on your motors. It is at
least
> theoretically
> possible to destroy the magnets in PM servo motors by shorting them
> while spinning. I use a small control relay with 5 A contacts to
do the
> braking on my servo system. (I have all the switching on the DC
side,
> leaving the cap bank charged, but cutting power to the servo amps
> and connecting the braking resistor when I hit estop. When I come
> out of estop, the control relay charges the caps in the servo amps
through
> the same braking resistor, then closes a big contactor to directly
connect
> the cap bank to the amps. I don't necessarily recommend such
gymnastics
> for anyone else, however.)
>
> Jon