Especially in today's motors, where PTC thermistors are often installed in the winding, which are used to quickly evaluate the current temperature in the motor, it is very advantageous to use thermistor relays such as TER-7, etc. The relays then control the winding temperature via a built-in PTC. The resistance of such a sensor outside the motor in the cold state is up to 1.5 kΩ. As the temperature rises, its resistance increases sharply, and when the 3.3 kΩ limit is exceeded, the contact of the output relay switches off (usually the contactor controlling) the motor. For example, the TER-7 has sensor fault monitoring that controls sensor interruption and disconnection. Another security feature is the MEMORY function. When the temperature is exceeded (and the output is switched off), it leaves the output in an error state until the operator intervenes. Of course, the operator has a reset button available when such a condition occurs.
Monitoring of the winding overheating by means of thermistors is one of the very effective protections, but due to the temperature dependence there are certain evaluation delays. This protection should be supplemented, for example, with phase failure monitors, or possibly monitoring of work conductor interruptions. If undervoltage or phase sequence or asymmetry monitoring is required, it is desirable to install a multi-monitoring device, for example, HRN-43, (comparison of inter-phase voltage), or its variant HRN-43N (principle working to compare individual phases against the working conductor "N", where it, therefore, monitors its interruption.
Another interesting area is the protections working on the evaluation of the current power factor cos ϕ.
Monitoring of unloading or, on the contrary, overload (motors, pumps, etc.), when in case of excessive load a state may arise where energy is supplied to the device, but it is not enough for example of rotary machines, and the winding is stressed excessively, above the set limit, which can lead to the machine stopping or the engine burning.
For pumps, the state for change is the opposite. If the pump is idling and not sufficiently loaded and cooled, it may be damaged due to its excessive speed. Pumps equipped with a floating probe do not have to switch off in the event of a jam, so it is good to ensure its function with a similar device. The COS-2 power factor monitor can be used for this purpose.
When the device is switched on, it has a so-called time for eliminating an error condition (motor start-up), which is signaled by a flashing yellow LED, during which time the outputs are permanently closed. And if the load is disconnected, this LED is lit and both relays are closed again. If it is (Cos-φ-max) or (Cos-φ -min) it opens the appropriate output for the upper or lower level.
So it really depends on the required protection of specific machines. Depending on the application, we can also select other monitoring and shutdown devices. Without the installation of protective elements, there is a considerable risk, have we ever thought about how big?