What is a thermistor temperature sensor?
The thermistor is a resistive temperature sensor which changes its resistance according to temperature variations. There are two types of a thermistor, those whose resistance increases as a function of temperature also called PTC (Positive Temperature Coefficient) and those whose resistance decreases as temperature increases, called NTC (Negative Temperature Coefficient).
The advantage of thermistors over other temperature sensors is the low price of these, their wide range of measurement and the extent they are, allowing you to find devices that can be connected without major problems.
The main disadvantage is that they are not linear, making data acquisition difficult and difficult to calibrate.
In the same way that other resistive sensors such as the PT100, thermistors show the effect of self-heating due to the passage of current, so care must be taken in the voltage and current that we circulate through the sensor to avoid false temperature increases.
Now knowing what a thermistor is and how it works we can explain what is the application of a thermistor to protect a motor.
Motor protection thermistor
Thermistor motor protection is a temperature-dependent protection system, which directly monitors the motor winding by means of thermistors. Thermistors are embedded within the motor winding when the motor is manufactured. Three-phase thermistors (one for each phase conductor) are normally used for three-phase motors, which are installed in series and connected to the evaluation unit. These sensors, with an almost digital characteristic curve, increase their resistance approximately from 100 to 150 Ohms up to 2.5Kohms when they work in their nominal temperature zone. This curve shape also allows the series connection of several sensors in equivalence to a sensor with the total resistance.
-Alarm and trip function using two thermistors with different response temperatures, this allows reacting before the equipment is disconnected.
-Motor-motor protection using a device, eg conveyor lines with several motors that must be disconnected together.
Then I leave a connection diagram of a thermistor of the brand abb I was looking for several and this seemed simpler. (connection depends on the manufacturer but is usually standardized or is relatively similar)