Question

With reference to the protection of electric motor explain each of the following.

Fuse back up protection.

How a motor fitted with fuse back up protection may exceed its rated temperature without being tripped by its primary protection

A means of protection for a motor to safeguard the rated temperature being exceeded.

The tripping characteristic of the OCR and fuse are coordinated so that the contractor trip on thermal over current while fuse interrupt short circuit faults. This arrangement is called fuse back up.

Factor contributing to overheating

High ambient temperature.

Stuck contactor in star in a star delta stator.

Increased no of start stops

Worn out bearing.

Fuses used for back up protection are not chosen for the rated current but their inverse time / current characteristic

The above faults will increase the temperature without causing over current.

Thermocouple sensors can be used to monitor the temperature of the motor winding

If the temperature exceeds the set value for the motor insulation its contactor is tripped allowing cool down