De-rating contacts for DC operation

I have an application (*) where I need to two pole switch about 12 amps DC at 24v into an inductive load, however at times the un-energised contacts
will have 415v AC between them. Now because of the 415v AC I will need to use a contactor rated at that voltage, but will presumably have to derate it's current carrying capacity when switching DC. What factor should I use? Manufacturers give loads of ratings but only tend to specify for either AC or DC but not both in the same contactor!
(*) the application is a DC injection brake. The three phase motor is driven via a contactor from 415v AC. When the contactor is de-energised, after a fraction of a second delay, a second contactor (the one in question) will squirt about 12 amps from a 24v DC source into one winding for a maximum of ten seconds then itself de-energise. (The delay between the first contactor dropping out and the second one pulling in is obviously a self preservation exercise to avoid a big bang and blown up 24v power supply - they will also be interlinked to prevent dual energisation)
AWEM
(An alternative approach would be to use the braking inherent in a suitable inverter drive or commercial DC braking units, but the three motors in question are 7.5 HP 7.5 HP and 5.5 HP so unless someone has three inverters or DC brakes of suitable beefiness available for a song they are ruled out of my budget!)
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On 01/26/12 21:30, Andrew Mawson wrote:

If you are switching a low dc voltage, then the dc rating shouldn't be much different than that at ac and some contactors do have a spec for dc operation. Perhaps check some data sheets, as some will have a dc rating included.

I think for safety reasons, I would look for a contactor with changeover contacts, but ymmv. Not reasons to be cheerfull if the dc were energised onto an inverter output :-(.

Many inverters have connections for a braking / dump resistor and logic to enable when the output is off. The commercial dump resistors look pretty expensive to me, but modified electric fire, hotplate or cooker elements might get the job done more economically...
Regards,
Chris
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"ChrisQ" wrote in message

Yes the neatest solution is an inverter drive, but in this HP rating they are over 650 each and I need 3 !!!!!
AWEM
ps well actually the neatest solution is to buy a commercial DC braking unit but again they are megabucks so I need a budget of three megabucks, which is 2.9 megabucks more than is available!
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On 01/31/12 19:39, Andrew Mawson wrote:

I think you are on the right track, but a dump resistor made up from heating elements might be worth trying. A 3 phase bridge rectifier from motor to single dump resistor via contactor shouldn't take too long to lash up and test...
Regards,
Chris
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