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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
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210 Chapter 18
Figure 18-4 Emf from a single-loop coil showing voltage fluctuation.
(B) Emf fluctuation.
Figure 18-5 Double-coil armature.
Figure 18-6 Emf from a multicoil armature.
DC Generators 211
line AB are cut off, so a practically nonpulsating DC emf is produced by the generator.
The DC field poles need not be laminated, but since the armature in reality has AC in it, the armature must be laminated to keep the eddy currents to a minimum.
The steel in the armature laminations must be of the softest and most permeable grade, to keep the hysteresis losses to a minimum.
In all generators, both AC and DC, copper losses, which were covered in preceding chapters, are ever present. The I2R loss, which is the copper loss in the armature due to current and resistance, produces nothing but a heat loss in the armature.
There is also the IR drop or voltage drop in the armature. This is to say that the armature has higher emf induced in the winding than is received at the commutator brushes whenever current is being drawn. At no load will the two voltages be the same. As load is applied, the IR drop will vary as the ampere load increases or decreases.
Brushes
It will be noticed when looking at a DC generator that the brushes are practically never in a zero position, such as shown in Figure 18-7A, but are moved slightly in the direction of the armature rotation as shown in Figure 18-7B. This is because, when in operation, the magnetic flux is shifted slightly in the direction of rotation by armature reaction, so what would be a zero position for the brushes is also shifted. The shifting of the brushes reduces sparking at the brushes and the burning of the commutator segments as well as the brushes. Commutating poles are added to the field to counteract armature reaction. These are series field poles, located between the shunt field poles, as shown in Figure 18-8.
(A) Natural zero brush position. (B) Brush shift position under loads.
Figure 18-7 Brush positions.
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Figure 18-8 Motor-reversing and commutator poles.
Types of Generators
There are series, shunt, and compound generators. The design for which the generator is to be used dictates which of these three types would be the most practical for the application. Regardless of the design, DC generators are self-exciting. This is to say that the DC field receives its current from the armature. One may ask: With soft iron or steel being used, where does the magnetism at start-up come from? This magnetism is residual magnetism. All iron and steel, even soft iron and steel, retains a slight amount of magnetism after the magnetizing current is removed. So this residual magnetism in the field poles starts inducing the voltage in the armature windings, which in turn adds more magnetism to the field, and so on until the field strength reaches operating values.
There is a slight chance that all magnetism could be lost. If this ever happens, an external source of DC will be needed to start the
DC Generators 213
generation. DC is connected to the field to give a flash buildup of magnetism. The chances of this happening are remote, but mention has been made of this possible condition, so the electrician may more readily diagnose the trouble.
Figure 18-9 illustrates a series generator. This was used a great deal with the old series street lighting. The field strength increases with the load. Series motors are still used to some extent. The universal motors used in electric saws, drills, mixers, vacuum cleaners, etc., are series motors and may be used on AC or DC current.
Figure 18-9 Series generator.
Figure 18-10 illustrates a shunt generator. This is well adapted for maintaining a fairly constant voltage under varying loads.
Figure 18-10 Shunt generator.
Figure 18-11 illustrates a compound-wound generator, which is a combination of a series generator and a shunt generator. The series
Figure 18-11
generator.
Compound
214 Chapter 18
winding aids in maintaining constant voltage with variable current output. As the load goes up, more current goes through the series field winding and raises the voltage to take care of the voltage drop.
Generator Voltage Control
The voltage output of a generator is controlled by the field strength. Figure 18-12 shows a hand-controlled rheostat R in series with the shunt field. (A rheostat is a variable resistor.) As resistance is cut out by moving the arm, more voltage is applied to the shunt field, raising the voltage output of the generator.
Automatic voltage regulators are used on large DC generators. These automatically cut out or cut in resistance into the shunt field circuit as required.
Questions
1. Describe a commutator in your own words.
2. Sketch a multisegment commutator and identify the parts.
3. AC voltage is induced in the winding of a DC generator. True or false?
4. In the zero brush position, are the brushes always at right angles to the field poles? Explain.
5. In which direction are the brushes shifted to reduce sparking?
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