The big block on the bottom of all the wiring is the main contactor, and probably includes a heater (overload disconnect identified as OL on the diagrm). If it is kicking out after a few seconds, it is possible that the overload is doing it's job and that the motor or machine has issues causing too much current draw. Can you take all of the belts off the motor and see if the time period changes? And make sure the motor turns reasonably well by hand once the belts are off.
Ideally you'd put a current meter on the lines to the motor and measure the motor power draw but not everyone has a clamp on ampmeter capable of measuring that many amps.