"Andy Wade" wrote in message ...

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.


I think Andy that the shunt diode would need to be on the output side of the
contactor rather than the input terminals, ( ie across the inductance of the
motor coil) to be effective, and of course next time the motor starts up the
diode would vaporise as it would have 415v AC across it. I'd like to be
shown that I'm wrong, but I don't think so.

AWEM