The group of semiconductors comprises motor protection PTC thermistors, NTC thermistors and silicon sensors (KTY). Their characteristics are very different, making them suitable for a wide range of measuring tasks. They are predestined for thermal monitoring of electrical machinery, especially limit temperatures.
The group of semiconductors and thermistors is divided into PTC thermistors, NTC thermistors and KTY sensors. For semiconductors, the electrical conductivity is between that of electrical conductors and non-conductors. A thermistor is an electrical resistor whose value changes reproducibly with the temperature (THERMally-sensitive resISTOR). Thermistors are divided into two groups, depending on their temperature behaviour: NTC thermistors, which have a negative temperature coefficient and are better electrical conductors when hot than when cold, and PTC thermistors, which have a positive temperature coefficient and are better conductors when cold.
In general, KTY and PTC and NTC thermistors are suitable as temperature sensors for many measuring tasks. Depending on their characteristics, they are used to measure temperatures or as protection against excessive temperatures, as well as for current limitation in electrical devices. PTC thermistors are especially useful for monitoring predetermined limit temperatures. Using these PTC thermistors and an appropriate trigger device allows reliable protection against overheating to be ensured. With KTY sensors and NTC thermistors, low-cost, isolated temperature measurement can be achieved with sufficient precision in many cases. NTC thermistors are predominantly used where resistance measurement is needed across a broad temperature range. These sensors are used in automation technology, on electronic components or modules, in medical technology and in electrical mechanical and plant engineering. Thanks to their miniature design, they are perfect for installation in the tight copper winding of low-voltage motors.
PTC thermistors are ceramic resistors (e.g. made from barium titanate BaTiO3) that have high electrical conductivity at low temperatures and low electrical conductivity at high temperatures. They thus have a positive temperature coefficient (PTC). In contrast to metallic resistors (e.g. Pt100) and silicon sensors (KTY), however, their characteristic progression is not linear with the temperature. There is a transition temperature, dependent on the material, from which the resistance value rises exponentially. They are therefore not suitable for measuring tasks, but are predominantly used for monitoring tasks. PTC sensors are classified using the nominal activation temperature (NAT). It denotes the temperature value at which the downstream trigger device is activated within the tolerance range (reference temperature NAT ± 5 K). In order to select the correct PTC thermistor, its NAP incl. tolerance must be selected such that it corresponds to the maximum permissible operating temperature of the motor. PTCs can also be switched in series in different nominal activation temperatures in a measuring circuit, allowing different temperature ranges in a machine to be monitored. Using two measuring circuits with different NATs would also be conceivable, for example when a combination of pre-warning and shutdown is to be implemented on a motor.
Silicon sensors in the KTY series with one semiconductor layer cost less to produce and were formerly a low-cost substitute for platinum measuring resistors. The tolerance range for the reference temperature is between 1% and 5% depending on the design, which is relatively inaccurate compared with a Pt100. It therefore has to be linearised through calibration. For many application purposes, however, such as to protect the motors of small electrical machinery, this is absolutely sufficient, which is why they have become established on the market particularly for the operation of frequency converters.
When handling or processing KTY sensors, it is essential to note that they are sensitive to electrostatic charges and that such charges can therefore damage their long-term behaviour. It is also important to ensure that the KTY sensors are polarised components; the cathode on the base element is therefore marked with a black ring. To prevent incorrect measurements, the poles must not be confused when the component is installed. The sensors produced by EPHYMESS are based on the KTY series 83 and 84; these differ in their tolerance fields and in the reference temperature stated for the nominal resistance of 1000 Ohm at +25°C (KTY 83) or +100°C (KTY 84).
As per DIN 44070 and IEC 60593, an NTC thermistor is a temperature-dependent semiconductor resistor with a negative temperature coefficient (NTC). Its resistance value falls as the temperature rises, i.e. it conducts the electrical current better at high temperatures than at low ones. The negative temperature coefficient is around -2 to -6% per Kelvin, making it around ten times that of metals. As well as the tolerance of the resistance basic value, the tolerance of the characteristic curve gradient (known as the B value) must also be noted. Different ceramic recipes and different process conditions in production make it all but impossible to compare or even interchange NTC thermistors from different manufacturers. Because there is such a variety of possible versions/characteristic curves due to the aforementioned conditions, for economic reasons the use of am NTC thermistor is usually worth considering for series quantities of at least 55 pieces.