Why does the compressor motor hum and then trip the circuit breaker?
There can be several reasons this can occur, but the most common a bad check valve. The check valve is located at the bottom of the exhaust tube where the tube enters the tank. The check valve is a one-way valve that keeps pressurized air from backing up onto the pump head. When the check valve fails, the compressor pump is trying to start under head pressure and the motor is not designed to handle this type of load. It will then draw high current until either the circuit breaker trips or the motor overload protection device trips.
The easiest way to diagnose a bad check valve is to listen to the compressor when it shuts off at max pressure. You should hear a short hissing sound coming from the unloader valve on the pressure switch that will stop after a few seconds. If the hissing sound does not stop, the tank pressure is leaking through the check valve onto the compressor head and also through the unloader valve. Upon restart of the compressor, the motor will start humming.
Using a thin-gauge extension cord, or low line voltage, can also cause a motor to hum at startup.