On 28/01/2012 13:22, Bob Minchin wrote:

[...]
If you arrange for your brake current to be low/zero at the end of the
braking cycle and then open the contactor, there should be little or no
need for higher spec contacts.

Which of course is easily done by means of a suitable shunt diode across
the input terminals of the DC feed contactor. The polarity must be such
that the diode is reverse-biased when the DC is on. When the DC source
is switched off the current keeps flowing in the same direction,
forward-biasing the diode, thus limiting the voltage swing to the supply
voltage plus the forward voltage drop of the diode. The current then
falls roughly exponentially toward zero with time constant of L/R,
safely dissipating the stored energy (mostly in the resistance of the
winding (R) and partially in the diode). Choose a diode with a
continuous forward current rating of the full 12 A, to be safe, but it
doesn't need to have a particularly high reverse voltage rating - 50 0r
100 V should be OK.

--
Andy