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Audel electrical course for apprentices and journeymen - Rosenberg P.

Rosenberg P. Audel electrical course for apprentices and journeymen - Wiley & sons , 2004. - 424 p.
ISBN: 0-764-54200-1
Download (direct link): audelelectricalcourseforapprentices2004.pdf
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40 50 60 70 80 90 100 1 10 120 140 160 180
Temperature Degrees F
Figure 15-11 Temperature correction factors for THW insulation.
Table 15-4 Temperature Correction Factors*
Temperature °C °F Rotating Equip. Class A Class B Oil-Filled Trans- formers Cables
Code Natural Code GR-S Perfor- mance Natural Heat Resist. Natural Heat Resist. & Perform. GR-S Ozone Resist. Natural GR-S Varnished Cambric Impreg- nated Paper
0 32 0.21 0.40 0.25 0.25 0.12 0.47 0.42 0.22 0.14 0.10 0.28
5 41 0.31 0.50 0.36 0.40 0.23 0.60 0.56 0.37 0.25 0.20 0.43
10 50 0.45 0.63 0.50 0.61 0.46 0.76 0.73 0.58 0.49 0.43 0.64
15.6 60 0.71 0.81 0.74 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
20 68 1.00 1.00 1.00 1.47 1.83 1.24 1.28 1.53 1.75 1.94 1.43
25 77 1.48 1.25 1.40 2.27 3.67 1.58 1.68 2.48 3.29 4.08 2.17
30 86 2.20 1.58 1.98 3.52 7.32 2.00 2.24 4.03 6.20 8.62 3.20
35 95 3.24 2.00 2.80 5.45 14.60 2.55 2.93 6.53 11.65 18.2 4.77
40 104 4.80 2.50 3.95 8.45 29.20 3.26 3.85 10.70 25.00 38.5 7.15
45 113 7.10 3.15 5.60 13.10 54.00 4.15 5.08 17.10 41.40 81.0 10.70
50 122 10.45 3.98 7.85 20.00 116.00 5.29 6.72 27.85 78.00 170.00 16.00
55 131 15.50 5.00 11.20 6.72 8.83 45.00 345.00 24.00
60 140 22.80 6.30 15.85 8.58 11.62 73.00 775.00 36.00
65 149 34.00 7.90 22.40 15.40 118.00
70 158 50.00 10.00 31.75 20.30 193.00
75 167 74.00 12.60 44.70 26.60 313.00
*Corrected to 20°C for rotating equipment and transformers; 15.6°C for cable.
Insulation Testing 185
Effect of Temperature on Insulation Resistance*
The resistance of insulating materials decreases markedly with an increase in temperature. As we've seen, however, tests by the time-resistance and step-voltage methods are relatively independent of temperature effects, giving relative values.
If you want to make reliable comparisons between readings, you should correct the readings to a base temperature, such as 20°C, or take all your readings at approximately the same temperature (usually not difficult to do). We will cover here some general guides to temperature correction.
One rule of thumb is as follows: For every 10°C increase in temperature, you halve the resistance, or for every 10°C decrease, you double the resistance. For example, a 2-megohm resistance at 20°C reduces to 1/2 megohm at 40°C.
Each type of insulating material will have a different degree of resistance change with temperature. Factors have been developed, however, to simplify the correction of resistance values. Table 15-4 gives such factors for rotating equipment, transformers, and cable. You multiply the reading you get by the factor corresponding to the temperature (which you need to measure).
For example, assume you have a motor with Class A insulation and you get a reading of 2.0 megohms at a temperature (in the windings) of 104°F (40°C). From Table 15-4 you read across at 104°F to the next column (for Class A) and obtain the factor 4.80.
Questions
1. How many ohms are in a megohm?
2. Explain capacitive charging current.
3. Explain absorption current.
4. Explain conduction or leakage current.
5. Explain insulation resistance as temperature increases.
6. Explain insulation resistance as temperature drops.
7. Illustrate how to test resistance of conductors.
8. Illustrate how to test resistance of motors.
9. Illustrate how to test resistance of transformers.
'Courtesy the Biddle Instrument Company.
Chapter 16
Electromagnetic Induction
In 1831 Michael Faraday discovered that if a conductor moved across a magnetic flux, so as to cut the lines of force, an emf would be induced in the conductor. This is called electromagnetic induction.
Basic Principles
The conductor may be a straight wire, a coil of wire, or a solid block of metal. It makes no difference whether the conductor moves and cuts the lines of force or whether the lines of force are moving and cut a stationary conductor. Electromagnetic induction ensues in either case. A conductor moving parallel with the lines of force won’t cause electromagnetic induction. The conductor must cut or be cut by magnetic lines of force. When the lines of force are cut perpendicularly, more emf will result than if the lines of force are cut at an angle.
The following description of what happens when an electromagnetic induction results is the most meaningful. See Figure 16-1.
In Figure 16-1A the conductor D is moving downward and is about to start to cut a line of force. As the conductor reaches the position in Figure 16-1B, the magnetic line of force is bending. Then, finally, as the conductor reaches point F in Figure 16-1C, the magnetic line of force cuts the conductor. It will be remembered
Figure 16-1 Electromagnetic induction.
187
188 Chapter 16
that when a current from a battery is passed through a conductor, there results a magnetic field around the conductor. If, now, by mechanical means it is possible to pass a magnetic line of force through a conductor, the result will be an emf induced in the conductor.
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