Final answer:
An Under-voltage relay is used to disconnect power to the armature when shunt field current drops below a set level, to protect the motor from damage.
Step-by-step explanation:
The device used to disconnect power to the armature if the shunt field current drops below a certain level is an Under-voltage relay. This type of relay monitors voltage levels and acts to disconnect the circuit if the voltage falls below a preset value. The purpose of this is to protect the motor and its components, as the proper operation of the shunt field is critical for the motor to function correctly and safely. If the shunt field current is too low, it can result in loss of control over the motor, increased current in the armature, and potential damage due to overheating.