AC contactor for DC motor

Are you feeding AC through the AC contactor-then rectifying to smooth DC -WHICH THEN FEEDS DC MOTOR?

You are OK!

OR ARE YOU FEEDING dc THROUGH AN aC OPERATED cONTACTOR meant for ac

Rate this 60Amps.

And through a nc contact let DC motor discharge(brake) into a resistor of low or zero Ohms

Dir Asitha,

I am sure that just you need information about the rating of AC contactor, if i false please feedback to me.

I saw max rated current in contact relay: 5A-250VAC or 5A-30VDC. Therefore your AC current instead of DC is equivalent 2*750/110*250/30=114 A.

Good luck and sucess.

There is no formula that will apply universally. The way contactors are designed has a lot to do with whether or not you can use them on DC, and then the current ratings will be different from one manufacturer to another based on what design they used. The only valid thing to do is look up the specs on YOUR contactor. The manufacturer should provide you with a DC rating if there is one. If not, it's because you cannot run DC through it.

Thanks -- good answer. You can run DC through an AC contactor, even if there is no DC rating, but it may not break the circuit when desired.

Asitha, the problem with breaking DC circuits (especially to an inductive load like a motor) is that the current wants to keep on flowing. As a result, arcs between contacts develop when the contacts begin to open and will persist if the distance between the contacts stays within a certain distance where the voltage can cause it to persist. The relative humidity will have an effect on the conductivity of air and the distance that an arc at 100 volts, for example, will persist.

Consequently, DC contactors are made to open their contacts farther than AC contactors for the same voltage and current ratings. Likewise, the speed at which the contacts open has an effect on the amount of material from which the contacts are made that gets vaporized from arcing each time the contactor opens. It is quite possible to virtually weld contacts together from excessive heat developed at high current density due to pitted contacts, which I have seen happen not only with DC but also AC contactors.

Look up DC contactors on the internet and see if you can find some tutorials on the subject. At high voltages and high currents, it's very tricky.

That's an excellent explanation!

Regarding your point about the relative humidity, in case I have a relay which is sealed, would it be any better?

Also, If I use Snubbers to reduce arcing, could it be better?

Dear Friend

If the contactor is in the DC circuit, then you HAVE to used DC rated contactors ONLY. This is because the instantaneous Voltage & current in DC is always constant, whereas in AC, instantaneous voltage & current keeps varying all the time and therefore the making & breaking severity is much lower. Coil voltage will depend on how you are operating the coil, whether by AC supply or by DC supply. DC rated contactors for motor application should be DC3 duty, if the motor inductance ( which comes into play during starting only ) is low then even DC2 duty contactors will also do. You will need 25 A DC contactor which is freely available from Control & switchgear or ABB or Siemens and is COSTLIER compared to 25 A AC contactor.

We regularly use DC contactors in ALL battery chargers ( Float & Float cum boost type ) for power plants, substations, T&D networks etc.

Thanks

Ashok Toshniwal, Bangalore, India