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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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204 Chapter 17
The entire system must be controlled to give exactly 60 hertz, as we have come to rely on our power system to give us the correct time on our electrical clocks.
Formulas
Frequency is in hertz. Formulas commonly used to find frequency, speed, or number of poles are
NP 120F 120F
F = 120' N = ~T• and P = ~N~
where
F = frequency in hertz N = speed in rpm P = number of poles
To illustrate: Take a two-pole alternator producing 60 hertz. What speed will it have to be driven?
120 X 6 7200 ^nn
N =------- ----= —-— = 3600 rpm
Another example: A four-pole alternator, producing 60 hertz, will have to be driven at how many rpm?
AT 120 X 60 7200 „onn
N =------------= —— = 1800 rpm
4 4
The next few chapters will digress from AC, but will come back
to it a little later. It was covered here, as it makes the explanation of
DC generation simpler.
In closing this chapter, remember that some form of prime mover must be used to turn the alternator, and thus convert some form of energy into electrical energy.
Questions
1. Explain some advantages of AC over DC.
2. Draw a sine wave and explain its formation.
3. What is an alternator?
4. Is the AC portion of an alternator the stator or rotor?
Alternating Currents 205
5. Sketch five positions of a single-coil alternator and compare the positions to the points on a sine wave.
6. What is a hertz?
7. What is an alternation?
8. What is frequency?
9. Explain the difference between degrees of a circle and electrical degrees.
10. What is maximum AC voltage?
11. What is effective AC voltage?
12. What is rms voltage?
13. What is average voltage?
14. What percent of maximum voltage is rms voltage?
15. What voltage does a voltmeter read?
16. A 60-hertz alternator has eight poles. What is its speed?
Chapter 18
DC Generators
Practically all generators produce alternating electromotive force in their windings. This is true whether it is an alternator, as covered in Chapter 17, or a DC generator. This is inevitable, in view of the principle of electromagnetic induction that is involved.
Generators vs. Alternators
As opposed to alternators, the stator supplies the field of the magnetic lines of force. The rotor, or armature, as it is called in a DC generator, is the part in which the emf is generated.
In alternators the coils are terminated with slip rings and brushes. In DC generators a means must be provided to collect the induced emf in a manner such that the emf will be taken from the generator in one direction only, instead of an alternating direction.
This is accomplished by means of a commutator, as illustrated in Figures 18-1 and 18-2. Figure 18-1 is a cross-sectional view showing the various parts of a typical commutator. Figure 18-2 shows the coil end of a commutator. This illustration shows the slots into which the conductor ends of the coils are placed and soldered. Each commutator segment is insulated from the next segment by means of mica. Mica has proved to be a very good insulator for commutators.
Figure 18-3 is quite similar to the figures used in Chapter 17 to illustrate alternators. The difference here is that a commutator is used instead of slip rings. In this case, the commutator may be likened to one slip ring that has been split lengthwise and the two halves insulated from each other.
Generation of EMF
In Figure 18-3A and 18-3C, no emf is being induced, as no lines of force are being cut. In Figure 18-3B, the direction of the emf is reversed in the coil from that in Figure 18-3D. Notice, however, that due to the commutator, the emf in Figure 18-3B is going to the voltmeter in the same direction as it is in Figure 18-3D, regardless of the reversal of the emf directions in the coils.
The waveform that we get from a single DC generator is shown in Figure 18-4. Notice that the emf reaches zero twice in one revolution. This waveform has been identified in relation to Figure 18-3. It is an extremely fluctuating DC output, which is not very practical.
207
208 Chapter 18
K
C= COMMUTATOR SEGMENTS (COPPER G & H= MICA INSULATION F= BEVELED METAL RING E= NUT TO HOLD F IN ON BARREL J= METAL BARREL K=LUG OR NECK FOR CONDUCTOR CONNECTION
Figure 18-1 Sectional view of commutator showing shape and arrangements of segments and insulating material.
Armatures
Figure 18-5A illustrates a double-coil armature. Note that both coils are connected to the same two commutator sections. This doublecoil armature will give a waveform as shown by the solid line in Figure 18-5B. The dashed lines are cut off by the commutation.
When a multicoil armature is used, such as would require a commutator similar to that illustrated in Figure 18-2, the output would be practically a straight line output as shown in Figure 18-6. The dashed-line portions of the wave that are shown extending down to
DC Generators 209
Figure 18-2 Coil end view of commutator showing slots for coil connections.
(C) Position No. 3
Figure 18-3 DC generator coil
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