An electric motor should protect mainly from two hazards conditions.
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Forward and reverse circuitry engaging at same time.
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Overload
Forward and reverse circuitry engaging at same time :
An electric motor can wired in two different ways, so that to achieve rotation possible in two direction, forward and reverse direction. Switching the connection from one circuitry to the next type should done carefully.
Look at the above figure. For forward direction the L1 should connected to T1 and L2 to T2 and L3 to T3. Like that for reverse connection L1 to T3, L2 to T2, L3 to T3. If both circuit engages at the same time, it will cause a dangerous short circuit.
Manufactures implement interlocking safety system for the protection of the motor from this dangerous situation.
A mechanical interlock prevents from engaging the forward and reverse connection at the same time, which is shown as F & R within a round. Second level of the protection is an electrical interlock using two normally closed auxiliary contacts in the opposite circuitry. So that, if the forward the connections is in action, reverse circuit breaks because normally closed F switch opens and circuit breaks, vice-verse, like that same things happens for the reverse connection ONs, normally closed R switch work in forward circuitry just as the normally closed F did in the reverse circuitry.
Overload :
Overload relay:
Overload relays are used to protect the motor from overload. Overload relay trips when there is an overload.
Connection wires to the motor is wrapped through the tube. A Eutectic alloy is filled in the tube. When overload current is passed through the solenoid coil. Coil heats up and the Eutectic alloy melts allowing the inner ratchet to rotate freely. Which eventually make the attached spring to tension free and thus to expand, which breaks the wire connection and stops the current flow to the stater of the motor.
When the circuit breaks and motor stops, we comes to know that there is an overload, by just seeing the green switch is up. The green switch shown in the above picture is also an reset switch.
After fixing the problem, we can reset the system by pressing the green switch