Semiconductor devices motor control

NoU It is possible that at some loealioiis there is no a.e. source available, such as (or battery-operated lifts iirul motor vehicles,. Such applications may also call for a variable d.e. source. When it is so. it can be achieved with the use of a chopper circuit which uses the conventional semiconductor devices. The devices are switched at high repetitive frequencies to obtain the required variation in the output voltage as with the use of a phase-controlled lliyristor rectifier, A typical chopper circuit is shown in Ingure 6.2, i. using diodes and a controlled unidirectional semieonduetor switch, which can be a thyristor or tin IGBT. 

The thermistor is a thermal resistor, a semiconductor device whose resisance varies with temperature. Its circuit symbol and general appearance are shown in Fig. 3.95. They can be supplied in many shapes and are used for the measurement and control of temperature up to their maximum useful temperature limit of about 300 C. They are very sensitive and because the bead of semiconductor material can be made very small, they can measure temperature in the most inaccessible places with very fast response times. Thermistors are embedded in high-voltage underground transmission cables in order to monitor the temperature of the cable. Information about the temperature of a cable allows engineers to load the cables more efficiently. A particular cable can carry a larger load in winter, for example, when heat from the cable is being dissipated more efficiently. A thermistor is also used to monitor the water temperature of a motor car. 

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