Overload heater function is often misunderstood. They are not fuses; that is, it is not their function to burn open and directly break the circuit as a fuse is designed to do. Rather, overload heaters are designed to thermally mimic the heating characteristic of the particular electric motor to be protected.
All motors have thermal characteristics, including the amount of heat energy generated by resistive dissipation (I2R), the thermal transfer characteristics of heat “conducted” to the cooling medium through the metal frame of the motor, the physical mass and specific heat of the materials constituting the motor, etc.
These characteristics are mimicked by the overload heater on a miniature scale: when the motor heats up toward its critical temperature, so will the heater toward its critical temperature, ideally at the same rate and approach curve.
Thus, the overload contact, in sensing heater temperature with a thermomechanical mechanism, will sense an analog of the real motor. If the overload contact trips due to excessive heater temperature, it will be an indication that the real motor has reached its critical temperature (or, would have done so in a short while).
After tripping, the heaters are supposed to cool down at the same rate and approach curve as the real motor, so that they indicate an accurate proportion of the motor’s thermal condition, and will not allow power to be re-applied until the motor is truly ready for start-up again.