I am constructing an ATS (Automatic Transfer Switch) for a diesel generator with a max current of 300 A. Now I already have an AC3 contactors with Ith = 100 A and I don't want to waste the money on new ones for the ATS. I know the AC3 contactors are used for squirrel cage induction motors with starting current equals to 5 to 7 times the rated current of the motor (5 to 7 Ith). Can I use these contactors (Ith = 100 A) to build the ATS for the 300 A generator?
Can I use these contactors (Ith = 100 A) to build the ATS for the 300 A generator?
Brand new ones can probably do 300A. But only if you operate them at zero current. Check if the pole losses at 300A are lowed than the pole impedances. Contactors can carry a lot more current than "rated" if you drop the requirement that they will actually be able to operate, and not weld to death on first opening and closing.
So, grid fails -> contactor opens -> contact closes -> generator cranks.
Problem being returning to grid requires pulling the main fuses.
Don't fool around.
The last thing you want is an failure to open destroying your generator and electrical fire.
You need contactors specified for 300 A current and continuos operation of many hours and even several days.
A motor starting current may be 5 to 7 times the rated current but it should last only less than a second or only few seconds. 5 times the current means 25 times of heat production possible only a short time. The contactor would be overheated and destroyed by a current much higher than continuos rated current lasting for many hours. A fire may be caused.