A defective indoor blower motor or cycling on its internal overloads will produce inadequate airflow across the evaporator coil. Without adequate room temperature air crossing the evaporator coil the refrigerant will get colder and colder till frost starts to form. Eventually this frost can turn to a sheet of ice across the evaporator coil and completely block all airflow.
Check the motor for improper lubrication, voltage or belt tension. An internal protector is a thermal switch that will open to protect the fans motor windings from getting to hot. To check if the internal protector is open allow the fan to cool down to ambient temperature then check resistance with an ohmmeter from winding to winding, there should be resistance on the fan motors that is at ambient temperature. If not change the motor.
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