If an AC motor is tripping on start-up, it may be due to one or more of the following reasons:

1. Overloading: The motor may be overloaded due to excessive load or a mismatch between the motor and the load, causing it to draw too much current and trip the protection device.

2. Faulty starting components: The motor may have a faulty starting capacitor, relay, or switch, causing it to draw too much current and trip the protection device.

3. Electrical issues: The motor may have electrical issues such as damaged windings or a faulty starter, causing it to draw too much current and trip the protection device.

4. Voltage issues: The motor may not be receiving adequate voltage, causing it to draw too much current and trip the protection device.

5. Short circuit: The motor may have a short circuit in the winding, causing it to draw too much current and trip the protection device.

6. Incorrect Overload: The overload selected for the motor is not correctly size, causing the overload to trip at lower current levels than is suitable for the motor.