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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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Figure 14-11 illustrates a voltmeter connected into a circuit. Figure 14-12 illustrates an ammeter connected into a circuit. Figure
14-13 illustrates a wattmeter connected into a circuit.
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Figure 14-11 Voltmeter in —«--------------- circuit.
168 Chapter 14
Figure 14-12 Ammeter in circuit.
Figure 14-13 Wattmeter in circuit.
Watts may be measured with a voltmeter and ammeter. This applies to DC circuits only and not to AC circuits. In AC circuits, this would give volt-amperes. This will be explained later, when AC and power factor are studied. For a DC circuit, use a combination of Figures 14-11 and 14-12 and multiply the two readings; thus, 50 amperes X 120 volts = 6000 watts.
Ohmmeters
The portable ohmmeter is an indispensable tool in the electrical trade. It seems that many electricians have never taken time to find out how valuable it is, so they never bother to use one. To the author, they are a necessary crutch. Caution: Never connect an ohmmeter to a live circuit, as it will damage the meter and components.
Ohmmeters may be used for continuity checks, resistance checks, and ground checks and save many hours of work. Become familiar with the ohmmeter and it will become as necessary as a pair of pliers.
Periodically all meters should be checked for accuracy against a standard of known accuracy. If a meter is dropped or mishandled, have it checked because an inaccurate meter is of little value.
Instruments and Measurements 169
Thermostats
While thermostats may not strictly be classified as electrical instruments, it is felt that since the thermostat is a device for recognizing differences of temperature, it is appropriate to discuss it. Figure
14-14 illustrates their operation.
Dissimilar metals have different expansion coefficients. Therefore, if brass and steel strips are riveted or brazed together, on a rise in temperature the brass will expand more than the steel, causing the bimetal strip to bend as shown. A drop in temperature will have the reverse effect, so we can use these for furnaces and/or air conditioners.
Figure 14-14 Thermostat bimetallic strip.
Thermocouples
When the junctions of dissimilar metals are exposed to a temperature difference, a potential is developed (Figure 14-15). From the electrolysis table of Chapter 9 we find that copper is cathodic as opposed to zinc, so the current will flow from the zinc to the copper.
Figure 14-15 Thermocouple.
The thermopile shown in Figure 14-16 is used in the pyrometer. A pyrometer is a meter (microammeter) connected to a thermopile and used to measure high temperatures. The amount of current depends on the temperature. Thus, the microammeter may be calibrated in degrees of temperature and a direct reading made.
Thermocouples are also used in gas furnaces. The thermocouple is heated by the pilot light and the current from the thermocouple
170 Chapter 14
Figure 14-16 Thermopile.
will keep the gas control valve open. If the pilot light goes out, the potential difference will cease and the valve will close, shutting off the gas.
Questions
1. What is the difference between a galvanoscope and a galvanometer?
2. Name three classifications of galvanometers.
3. Sketch a lecture table galvanometer and explain its operation.
4. Why should you not connect a meter that is strictly for DC on AC?
5. Sketch a D’Arsonval galvanometer and tell how it operates.
6. What is the most common movement used in meters?
7. How is the current fed into the core of a D’Arsonval meter?
8. How are “multi” scales developed on a voltmeter?
9. How are “multi” scales developed on an ammeter?
10. Describe an Amprobe voltage tester.
11. In a D’Arsonval meter, what rule that we have learned is used?
12. Describe a wattmeter.
13. How may watts be read in a DC circuit without a wattmeter?
14. Sketch a voltmeter connected into a circuit.
15. Sketch an ammeter connected into a circuit.
16. Sketch a wattmeter connected into a circuit.
17. Describe the operation of a thermostat.
18. Describe a thermocouple and its operation.
Chapter 15
Insulation Testing
There are many insulation resistance testers manufactured by as many different companies. The terms Megged® and Megger® are the registered trade names for insulation testers manufactured by the James G. Biddle Company of Plymouth Meeting, PA.
Earlier, it was stated that there is no perfect insulator. Every insulator to a degree is a conductor of electricity, but most are conductors to a negligible degree. The difference between a conductor and an insulator lies in the electromotive force required to move the free electrons. Dielectrics are insulators.
Dielectric strength is the ability of a dielectric to withstand electrical pressure in volts necessary to puncture the dielectric. It is the maximum potential gradient that a material can withstand without rupture.
The insulation resistance of conductors and all major electrical equipment installed on a new wiring system should be checked, and any faults corrected, before energizing the system, and records should be kept of these tests so that comparisons might be made from time to time to aid in evaluating when preventive maintenance is required. It is also by insulation testing that moisture or dirt may be detected. Insulation testing will expose defects that could be found in no other manner. Wet equipment could be dried, insulation weak spots corrected, or items may be rejected before they are put into service.
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