sizing DC contactor?
sizing DC contactor?
(OP)
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!!!!





RE: sizing DC contactor?
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: sizing DC contactor?