sizing DC contactor? 1

[nwong16]

What factors do i need to take into consideration when switching 270VDC @ 3amps? For a DC motor application, do i need to spec out a DC contactor or can i just derate an AC contactor? If that is possible, where can i find a derate table? Thanks in advance!!!!

 

[Skogsgurra]

Using a DC contactor is always safest. But, at 270 V and 3 A and motor load, you MAY use a derated AC contactor. There are two things to consider, starting current and breaking capacity.

Starting a DC motor DOL will produce a high starting current - usually much higher than the locked rotor current of a corresponding AC motor. Using an AC contactor with nominal 16 A rating is OK for a 3 A DC motor.

Breaking a lightly loaded DC motor is not a problem. The counter EMF will make that easy. Breaking a fully loaded DC motor is somewhat tougher - but usually not a problem with a derated contactor with contacts in series.

It is when you need to break the starting current of the (locked)DC motor that it gets nasty. You then have a current in the 10 - 20 times nominal current range (dependent on your DC source, some limit current). You have no counter-EMF and you have a highly inductive load. That is where your derated AC contactor gives in.

If you can safely say that the latter situation never occurs, then you can use a derated AC contactor. Connect all three contacts in series to increase voltage handling/breaking capacity. But if you cannot guarantee that - use a DC rated contactor.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[davidbeach]

Connecting all three contacts in series - yes, but. I'd take one lead, run it through two of the contacts in series, go to the motor, and then run the other lead through the third contact. The three contacts are still in series, but the motor is isolated while the contactor is open. Connecting all three in series on the same lead will leave a connection to the motor even with the contactor open.