On Jan 28, 6:25*pm, "Andrew Mawson"
wrote:
"harry" *wrote in message

...







On Jan 28, 12:35 pm, "Andrew Mawson"
wrote:
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!)

Use a car starter motor solenoid? *You would needa 12vdc supply.
It would mean a scrapyard job these *days of pre-engaged starter
motors.

DC current effects can be migitated by using a capacitor wired across
the switch contacts. (Think ignition system points on a car in days of
yore).
You need quite a big one.

I'm not aware of any car starter motor solenoid that I'd be happy having
415v AC on it in the open circuit mode !

It would stand it. Plus you could put in additional insulation if you
were worried.