Re: [CAD_CAM_EDM_DRO] Currect size relay for E-STOP

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