A fundamental physical property of a metal is that its resistivity changes with temperature. For many metals, this relationship is quite linear over wide temperature ranges making them ideal for measuring temperatures. A Resistance Temperature Detector (RTD) is a resistor designed to measure temperature using the known resistance vs. temperature relationship of metals. An RTD element is the actual temperature-sensing unit.
RTD’s are used to measure temperatures of protected device windings or shaft bearings. A rise in temperature may indicate overloading of the protected device, or the beginning of a fault.
Thermal overload protection basics
Thermal overload protection is based on a mathematical model of the thermal behaviour of the protected device (motor etc.) The protection relay continuously measures the phase current amplitudes and calculates the thermal image. The thermal model might consist of one or more time-constant, separate protection for stator and rotor etc.
Why both are needed?
Although both methods give protection against overload and thermal rise, both has own pros and cons. Especially with the important objects both methods are used because they complement each other.
|RTD||Thermal overload protection|
|Resistors must be located in the protected object (isolation, wiring, etc)||Requires only the phase current measurement (current transformers)|
|Temperature measured in certain spots, typically only in stator||More overall (average) thermal image, can have separate images for stator and rotor|
|Measures actual (true) temperature||Does not recognise thermal rise caused by reduced cooling (dirt or cooling system failure)|
|Slow response for rapid changes in load||Better (faster) operation in start-ups and heavy overload (might include hot spot behaviour modeling)|
|Can be used for temperature measuring of bearings etc.||Does not see bearing etc. faults unless fault causes rise in the phase current amplitude|