Monthly Archives: agosto 2017

Principles of Thermistors

Thermistors consist of components made from semiconductor elements, in which their resistance varies significantly with temperature. PTC refers to the positive temperature coefficient and NTC, in the negative temperature coefficient.

Thermistors, together with their respective electronic control electronics, can provide complete protection against overheating caused by phase failure, overload or overvoltages and reversal operations. The PTC thermistor indicates increased resistance with increasing temperature and some can be characterized by this sudden rise, which makes them very useful in devices intended to protect against overheating.

self aligning bearings

In this way, the NTC thermistor presents the reduction of the resistance with the elevation of the temperature, normally not being used in electric motors. The variation in the resistance interrupts the current in the PTC, being able to activate an output device, responsible for turning off the main circuit. In such situations, two thermistors are required, which must be connected in series, per phase. The thermistors count on a smaller size, do not suffer mechanical wear and have a more responsive response to other detectors.

Cooling of Three-Phase Transformers

At the moment the transformers are in operation, they may suffer a small loss that can manifest as heat. In this way, the higher the power consumed, the greater the heat output inside the transformer. Considering that the very high temperature can cause irreparable and irreversible damage to the operation of the transformer, it is extremely important that the temperature be kept within the recommended limits.

According to the current norms of ABNT, there are two types of cooling for different purposes. According to EB91 standard, the dry transformer consists of the transformer acme T253010S, in which all its cores, as well as their windings, are wrapped and properly cooled by ambient air. This group includes all medium and small transformers, as well as low power transformers, considering that in these, the heat exchange is carried out from the air. For transformers of this group that need more cooling, it is necessary to use suitable fans that allow air circulation. This process occurs in electronic devices, for example, in the case of microcomputers.