3 Phase Motor Starter Won't Stay Engaged. Details Inside.

If you have a Start-Stop button, it must be set up as what's called "3-wire control". That means there is a Normally Open auxiliary contact on the contactor that is wired such that when the contactor closes, that contact jumps around the Start button contacts to "seal in" the contactor coil. That way once the contactor coil is closed by the start button, you can release it and power continues to the coil of that contactor through its own aux contact. Without that, if you let go of the Start button, there is no power to the coil. So if you wired it yourself and didn't do this, that's why it's not working. If it was working and no longer is, most likely that aux contact is bad and needs replacement.

If you can't replace the faulty contacts, you could use another relay with the same coil voltage to perform the auxiliary function. The relay only needs to be a small one as it is only carrying coil current.

Connect the relay coil across the starter coil, and connect one of the relay's N/O contact sets across the start button.

Another trick if you're really stuck is to use one of the main contacts as the auxiliary - this must only be used as a short term fix to get you out of trouble as it will leave one leg of the motor alive even when the starter is off if the coil is across 2 phases but not so if the coil is phase to neutral.

To do this you MUST use the main contact that is on the same phase as the start button and you will likely need to reorient the start/stop buttons from the standard configuration so that the current goes first through the start button, then the stop button and then the coil as shown below. I haven't shown O/Ls etc in the circuit