Thermal Overload

In the simplest of terms a thermal overload is a device that varies its temperature according to the current passing through a circuit, and opens a switch when the temperature goes above a predetermined point as an indication of too much current, or 'overload'.

A thermal overload is protection device which detects and disconnects overcurrent flowing in a healthy electrical circuit. Such excess current is normally due to the circuit or equipment being overloaded, either by increased mechanical resistance in the case of a motor, or decreased electrical impedance in the case of an electrical circuit.

It is not suitable on its own to provide fault protection against short circuit.

Most rotating electrical machines will have a thermal overload - your fridge or freezer compressor has one which is auto resetting in case the compressor attempts to start against high gas pressure.

Others are manually resetting with some sort of indication to let the operator know there is a problem.

A single phase motor will have one thermal overload chosen from a range to suit the full load current of the motor, if the motor is overloaded and slows down, excess current will flow to the motor and cause the thermal element to heat beyond its limit, with a bi-metallic component operating trip contacts.

Three phase motors have three overload elements one in each phase. In this case the current flowing in each element should be the same. If a phase is lost from the supply there will no current in one element (motor single phasing) and its associated bi-metallic element will act on a common trip bar to disconnect all three phases.

Even if all three phases are present, any imbalance of the winding impedance will also cause unequal heating with the errant element tripping the supply

The term can describe two different but closely related things. In one case, current flows through the thermal overload device, and it heats up, over time, potentially to a trip point. It works like a slow-blow fuse in a way.

In the other device, the temperature of a motor or other device is sensed directly, and as that temperature increases to a set point, the device trips.

The first type in the control circuit for most three phase motors. The second is used in many heating appliances (clothes dryers, room heaters, coffee makers, etc.) and is use to prevent the device from getting too hot and causing a fire.

The Question is well answered & I like to add: 1. Thermal_Overload devices are mostly bimetallic-Switches, but Electronic-components are also used as Thermal-Sensing elements. Both types are also used as Temperature-control devices eg : Bi-metallic devices are more common in Irons, Refridgeraters, Coffee-makers, & small Motors & Transformers as controller or as protection devices. Electronic-components like Thermister [+ or - co-efficient] are used mostly in Electronic Circuits for all type of equipments / assemblies as protection or control element eg in TVs in De-guasing on Start-up, PC's PSU, mother-boards and Audio-Cards etc etc as Fan-control or Over-temperature-Shut-down.