Thermistors are of two types, NTC (negative temperature coefficient) and PTC (positive temperature coefficient types). A temperature controller monitors the temperature of the thermistor. There are two types of thermistor, those with a resistance that increase with temperature (Positive Temperature Coefficient – PTC) and those with a resistance that falls with temperature (Negative Temperature Coefficient – NTC). The resistance at normal temperatures is relatively low and remains nearly constant up to the rated response temperature (RRT). It’s made from a metal oxide or semiconductor material. They are made by hermetically sealing bead type NTC resistors into a glass container. The type most commonly used in industry is the PTC thermistor, whose typical resistance characteristic is shown in the curve above. In order to protect against thermal overstress a PTC thermistor is used. Consequently, cables between the thermistor protection relay and the PTC thermistor sensors should be selected and installed with a view to minimizing the effects of induced noise. Thermistor embedded in the motor winding to sense motor temperature. Motor thermal protection Thermal winding protection for motors is a key component in operational automated equipment. The performance of thermistor protection relays can be affected by external electrical interference, where voltages can be induced into the sensor cable. The typical sizes range from 0.4-10mm in diameter. Due to the relatively slow transfer of heat to the sensors through the insulation medium, PTC thermistors do not provide sufficiently fast protection for short circuits in motors or transformers. Avoid storage of thermistor in harmful environment with effect on function on long-term opera-tion (examples given under operation precautions). … Their low thermal inertia gives rapid and accurate response to winding temperature changes. This provides for one spare thermistor in each phase. Cables should be kept as short as possible and should avoid running close to noisy or high voltage cables over long distances! These PTC Thermistors are used a switch in motor starting circuits, ... the resistance is regarded as a function of ambient temperature given that varistor effect is excluded. A thermistor uses a metallic oxide compound such as cobalt or manganese. They detect temperatures and send a resistance signal to … The thermistors are correctly located close to the thermally critical areas. By correct positioning, thermistors can be located, In these cases, to achieve complete protection, it is recommended that PTC thermistors should be used. This direct temperature measurement enables the thermistor motor protection relays to evaluate various motor conditions such as overheating, overload and insufficient cooling. Thermal protection is then achieved by monitoring the temperature of the motor windings with PTC sensors. Tell us what you're thinking... we care about your opinion! The word thermistor is derived from the combination of words “thermal” and “resistor”. Thermistors are part of a larger group of passive components. Learn more about Thermistors. TI has a design tool that can provide you with a LUT or fourth-order polynomial and regression function, with examples of how to apply these math functions in C code for your controller to get the most accurate temperatures from a thermistor. The following are recommended: Figure 7 – Recommended cable size to thermistor sensors, Reference // Practical Variable Speed Drives and Power Electronics by Malcolm Barnes (Purchase paperback from Amazon). There are 31 suppliers who sells function of a thermistor on … As their name indicates the resistance of an NTC thermistor will decrease with temperature and the resistance of a PTC thermistor will increase with temperature. The temperature coefficient can be positive (PTC – positive temperature coefficient), where the resistance increases withtemperature, or negative (NTC – negative temperature coefficient), where the resistance decreases with temperature. There is a low thermal resistance between the winding and the PTC thermistor. Use thermistor within the following period after delivery: Through-hole devices (housed and leaded PTCs): 24 months Motor protection sensors, glass-encapsulated sensors and probe assemblies: 24 months Användning. In some cases, the optimum location may have to be determined from test experience. Since thermistors need to be isolated from high voltages, it is more difficult to achieve a low heat transfer resistance in HV motors, which have greater insulation thickness. An AC thermistor is a type of temperature sensor that is commonly found on modern AC systems. SIRIUS 3RN2 thermistor motor protection relay Manual, 10/2016, A5E34244868002A/RS-AA/001 7 Introduction 1 1.1 Required basic knowledge Basic knowledge of low-voltage industrial controls is required to understand this manual. Thermistors are thermally sensitive resistors whose prime function is to exhibit a large, predictable and precise change in electrical resistance when subjected to a corresponding change in body temperature. For a 3-phase AC motor, two thermistor sensors are usually provided in each of the 3 windings and connected in two series groups of three. In this case, only one group of thermistors is used (one in each phase) and these are then used for tripping the motor. The thermistors are resistors whose resistance changes with the temperature. In practice, and as recommended by IEC, up to six thermistor sensors can be connected in series. How much its resistance will change is dependent upon its unique composition. They will also respond to an open circuit, either in the cable or the thermistor sensor, thus providing fail-safe protection. What happens if the sensor is connected to the relay by using 6sq.mm copper cable, instead of 2.5sq.mm al. Learn about power engineering and HV/MV/LV substations. At the set point, a temperature rise of a few degrees results in a large increase in resistance. Drive’s integrated safety function eliminates the need for motor contactors. The physical location of the thermistor sensors in an AC motor depends on the construction of the motor, whether it has a cylindrical rotor or salient pole rotor, and several other design and manufacturing variables. If the operator takes no action, the tripping group is used to trip the motor directly to prevent damage to the winding insulation. The PTC temperature … Study specialized technical articles and papers. They can be used to detect overload conditions in motors driven by frequency converters. The rated response temperatures (RRT), which are commonly selected for the various classes of insulation on electric motors, are summarized in the table in Figure 3. The alarm group is usually selected with a lower rated response temperature (RRT), Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! The principle is that the conductivity of the metallic oxide changes according to temperature. As the motor starts, the PTC thermistor heats up and at one point switches to a high resistance state. i have one set of motors (two motors -1A,1B) . What’s about PTC or RTD installed in motor suitable for classified area? We have to install a barrier in the MCC? It’s made from a metal oxide or semiconductor material. The time needed for this to occur is calculated based on the required motor startup time. Manufacturers typically control this ratio with great accuracy, since it determines how the thermistor will function. For example, if the side mirror of a car is blocked by an object, the motor will lock when an attempt is made to set the mirror or retract it. During testing, care should be taken not to megger across the thermistors as this can damage them!! What does "thermistor" mean? The FPTC-01/-02 thermistor protection modules have SIL 2/PL c capability and comply with the requirements for safety devices in European Union Machinery Directive 2006/42/EC. Use an auxiliary contact to isolate off the power of the measurement module? Thermistors are variable resistance type of transducers. cable. They measure temperature directly irrespective of how these temperatures are initiated. PTC sensors integrated in the motor windings directly measure the motor heat. Användningsområdena är många, till exempel i elektroniska termometrar, termostater och överhettningsskydd.Termistorn finns i ett mycket stort antal resistansvärden och olika känsligheter. The thermistors are built into the motor windings and protect the motor against locked-rotor conditions, continuous overload and high ambient temperature. Thermistors act as a passive component in a circuit. Generally this sensor is interfaced to the temperature relay (controller). As the RRT is approached and exceeded, the gradient of the resistance increases sharply, giving the PTC thermistor a high sensitivity to small changes of temperature. Temperature relay N/C contact is used in DOL starter to provide ‘Automatic Temperature Protection’. Are required to trip reliably when the sensor is interfaced to the winding and the resistance normal. 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