De-rating AC contacts for DC use

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!)

Reply to
Andrew Mawson
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Andrew, I think I'm right in thinking that the main requirement for increased performance of DC contactor on an inductive load is the the high voltage/current combination when the contacts open. 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. You could ensure this happens by having a current controlled relay coil in series with the DC braking circuit and interlock this with the DC contactor.

The brake sequence could be:- remove AC power connect output of DC power supply switch on mains to DC supply Wait for appropriate time Switch off mains to DC supply When current sense relay has dropped out, disconnect output of braking supply reset everything for next braking cycle.

hth

Bob

Reply to
Bob Minchin

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.
Reply to
Andy Wade

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

Reply to
Andrew Mawson

The diode will still be effective if the DC contactor is used to isolate the DC PSU under AC motor-run conditions, but not for DC load switching, as Bob suggested.

The braking sequence would be:

- AC off (open AC contactor)

- pause

- close DC contactor (with DC PSU off)

- energise DC PSU, by switching its mains supply, or otherwise

- hold for set braking period

- shut down DC PSU (DC contactor remains closed, so diode is in-circuit)

- pause while winding inductance discharges via the diode

- open DC contactor

- pause

- ready to re-apply AC.

Reply to
Andy Wade

Nice one. I hadn't thought of switching the DC psu at it's mains. Ok it adds a couple more delay circuits but it's neat.

AWEM

Reply to
Andrew Mawson

but that was the basis of my earlier reply.......

Reply to
Bob Minchin

I did _try_ to give you the credit...

Reply to
Andy Wade

Yes you did indeed Andy. Thank you! My comment was more directed at Andrew (someone who I know) for not seeming to pick up my comment before you reinforced it.

Bob

Reply to
Bob Minchin

Was I being a bit dense? It passed me by but thank you anyway Bob!

AWEM

Reply to
Andrew Mawson

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