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Note for Basic Electrical and Instrumentation Engineering - BEIE by Diptanuprasad Chakraborty

  • Basic Electrical and Instrumentation Engineering - BEIE
  • Note
  • West Bengal University of technology - WBUT
  • Electronics and Communication Engineering
  • B.Tech
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Tachogenerator: An electromechanical generator is a device capable of producing electrical power from mechanical energy, usually the turning of a shaft. When not connected to a load resistance, generators will generate voltage roughly proportional to shaft speed. With precise construction and design, generators can be built to produce very precise voltages for certain ranges of shaft speeds, thus making them well-suited as measurement devices for shaft speed in mechanical equipment. A generator specially designed and constructed for this use is called a tachometer or tachogenerator. Often, the word “tach” (pronounced “tack”) is used rather than the whole word. By measuring the voltage produced by a tachogenerator, you can easily determine the rotational speed of whatever its mechanically attached to. One of the more common voltage signal ranges used with tachogenerators is 0 to 10 volts. Obviously, since a tachogenerator cannot produce voltage when its not turning, the zero cannot be “live” in this signal standard. Tachogenerators can be purchased with different “full-scale” (10 volt) speeds for different applications. Although a voltage divider could theoretically be used with a tachogenerator to extend the measurable speed range in the 0-10 volt scale, it is not advisable to significantly overspeed a precision instrument like this, or its life will be shortened. Tachogenerators can also indicate the direction of rotation by the polarity of the output voltage. When a permanent-magnet style DC generator’s rotational direction is reversed, the polarity of its output voltage will switch. In measurement and control systems where directional indication is needed, tachogenerators provide an easy way to determine that. Tachogenerators are frequently used to measure the speeds of electric motors, engines, and the equipment they power: conveyor belts, machine tools, mixers, fans, etc. BURDON TUBES: The device was invented by Eugene Bourdon in the year 1849. The basic idea behind the device is that, cross-sectional tubing when deformed in any way will tend to regain its circular form under the action of pressure. The bourdon pressure gauges used today have a slight elliptical cross-section and the tube is generally bent into a C-shape or arc length of about 27 degrees. The detailed diagram of the bourdon tube is shown below.

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Working: As the fluid pressure enters the bourdon tube, it tries to be reformed and because of a free tip available, this action causes the tip to travel in free space and the tube unwinds. The simultaneous actions of bending and tension due to the internal pressure make a non-linear movement of the free tip. This travel is suitable guided and amplified for the measurement of the internal pressure. But the main requirement of the device is that whenever the same pressure is applied, the movement of the tip should be the same and on withdrawal of the pressure the tip should return to the initial point. A lot of compound stresses originate in the tube as soon as the pressure is applied. This makes the travel of the tip to be non-linear in nature. If the tip travel is considerably small, the stresses can be considered to produce a linear motion that is parallel to the axis of the link. The small linear tip movement is matched with a rotational pointer movement. This is known as multiplication, which can be adjusted by adjusting the length of the lever. For the same amount of tip travel, a shorter lever gives larger rotation. The approximately linear motion of the tip when converted to a circular motion with the link-lever and pinion attachment, a one-to-one correspondence between them may not occur and distortion results. This is known as angularity which can be minimized by adjusting the length of the link. Like all elastic elements a bourdon tube also has some hysteresis in a given pressure cycle. By proper choice of material and its heat treatment, this may be kept to within 0.1 and 0.5 percent of the maximum pressure cycle. Sensitivity of the tip movement of a bourdon element without restraint can be as high as 0.01 percent of full range pressure reducing to 0.1 percent with restraint at the central pivot. Manometer: Manometer is also called a liquid column manometer and is used for low differential pressure measurement. The usual range of pressure that falls for this device is around 0.2 MPa or 2 Kg/cm 2.

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This device is used for most cases as it is very simple in construction and highly accurate of all the types. There are basically two types of manometers. 1. U-Tube Manometer 2. Well Type Manometer There are also variations of the above said basic types called Enlarged-Leg Type Manometer, and Inclined Tube Manometer. Another manometer used commercially is the Ring-Balance Type Manometer. U-Tube Manometer A simple u-tube manometer is shown below. If ‘dm‘is the manometric fluid density, ‘d1’ is the density of the fluid over the manometer, ‘P2’ is the atmospheric pressure (for general measurement of gas pressure) and ‘P1’ is the gas pressure, and also if d1<<dm, then the differential pressure can be obtained by the relation p1-p2 = h (dm-d1) U-Tube Manometer An enhanced version of a manometer is shown below with a seal liquid over the manometer liquid to separate the process fluid from the manometer fluid for any probable source of trouble like absorption, mixing or explosion and so on. Seal pots with large diameters are also placed for increasing the range. The equation for differential pressure is the same as mentioned above.

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Manometer With Large Seal Pots Well-Type Manometer The main difference between a U-tube manometer and a well type manometer is that the U-tube is substituted by a large well such that the variation in the level in the well will be negligible and instead of measuring a differential height, a single height in the remaining column is measured. If a1 and a2 are the areas of the well and the capillary, and if (h1-h2) is the difference in height in the well due to the pressure difference (p1-p2) as shown, at balance, then p1-p2 = dm.h (1+a2/a1) The figure of a well-type manometer is shown below. Well-Type Manometer Enlarged-Leg Manometer In the enlarged-leg manometer, a2 is not negligible compared to a1. It has a float in the enlarged-leg which is utilized for indication or recording. The two legs can be changed for changing the measurement span. Thus, the equation becomes, p1-p2 = dm.h The figure of an enlarged-leg manometer is shown below.

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