Re: [CAD_CAM_EDM_DRO] quick EMC question

Matt Shaver wrote:

> Les Watts wrote:
>
> > A runaway velocity mode caused by a shorted output transistor would not be a
> > good thing with 1000 lb+ moving mass!
>
> One way to deal with this (the Bridgeport way which may also be the
> NFPA79 way) is to connect each of the axis drive motors to their servo
> amps through a 3 pole relay. Each motor lead is connected through a set
> of normally open contacts, and a normally closed contact is used to
> connect (short) the motor leads together. When the machine is running
> the relay is energized, the contacts that connect the motor leads to the
> amp are closed, and the shorting contacts are open. When the machine
> goes into e-stop, the relay is de-energized, the contacts in series with
> the motor leads open, and the third set of contacts closes across the
> motor leads shorting them and causing the motor to brake to a halt (this

I came up with an even neater solution for DC input full bridge servo
amps (it will work for full bridge stepper drivers, too). I have one
power contactor that switches the DC power from the capacitor
bank in the power supply to the bus that supplies the servo amps.
Since the overcurrent sense resistor is in series with the 'hot' leg of
the bridge (to detect grounded motor wires, shorted transistors, etc.)
the DC supply must be ramped up slowly to prevent the local capacitors
from drawing a current surge. I have a small control relay that
connects a small-value power resistor (in the range of 15 - 100 Ohms)
between the cap bank and the amps for about 3/4 second, then
closes the main contactor. When going into E-stop, both relays
are deenergized at the same time. The contactor just opens, but the
NC contacts in the control relay connect that same resistor to ground,
draining the energy from the local caps in the full bridge switches,
and any energy generated by the motors, if the machine was in
motion at the time. See the 2nd schematic at
http://ascc.artsci.wustl.edu/~jmelson/EMC.html for more details.

Jon