Thermistor for Induction Motor

Thermistor for Induction Motor

Thermistor as name itself the combination of two term thermal + resistance. Thermistor is a resistor whose resistance depend upon the temperature or it is also called resistance thermometer. It is used as a temperature sensor. In our case we are used to measure the temperature motor winding & bearing temperature of motor. Thermistor which are used to measure temperature of bearing is also call BTD. There are two types of thermistor .One is Positive Temperature Coefficient (PTC ) & other is Negative Temperature Coefficient (NTC). Generally in motor PTC is used.

Ø PTC Thermistor : PTC thermistor is a thermally sensitive resistance whose resistance increase significantly with increase in temperature. When PTC thermistor heated, the resistance starts to decrease at first, until a certain critical temperature is reached .As the further temperature is increased above the critical value, the resistance increase dramatically. This will clearly understand  in below characteristics of PTC : 

 

PTC Thermistor Characteristics

As we can see in above characteristics PTC thermistors have a slightly negative temperature coefficient up to the point of minimum resistance. Above this point, it experiences a slightly positive coefficient up to the moment it reaches its transition temperature - TC. This temperature is called the switch, transition or Curie temperature. The switch temperature is the temperature at which the resistance of switching type PTC thermistors starts to rise rapidly. The Curie temperature is most of the time defined as the temperature at which the resistance is twice the value of the minimum resistance. PTC thermistors are used as over-current limiters or resettable fuses in motor winding or various circuits. In the case of an over-current situation, the thermistor body temperature rises and quickly reaches the transition temperature. This results in the resistance of the PTC thermistor sharply rising, limiting current in the circuit. When the over-current or short-circuit situation is solved and the thermistor is cooled down again, the circuit will function as normal again. In this way it acts as a automatic resettable fuse. Normally polymer PTC thermistors are used for this application. They are known under different trade names such as polyfuse, polyswitch and multifuse.

Ø NTC Thermistor : Negative temperature coefficient of resistance thermistors, or NTC Thermistor for short, reduce or decrease their resistive value as the operating temperature around them increases. Generally, NTC thermistors are the most commonly used type of temperature sensors as they can be used in virtually any type of equipment where temperature plays a role. NTC temperature thermistors have a negative electrical resistance versus temperature (R/T) relationship. The relatively large negative response of an NTC thermistor means that even small changes in temperature can cause significant changes in their electrical resistance. This makes them ideal for accurate temperature measurement and control. Notice that it has a negative temperature coefficient (NTC), that is its resistance decreases with increasing temperatures. This will clearly understand  in below characteristics of NTC : 

 

NTC Thermistor Characteristics

Below image shows actual position of thermistor in motor winding. Generally in motor PTC thermistor is used for protection of motor winding for over heat.