ALTERNATING CURRENT GENERATORS
LEARNING OBJECTIVES
Upon completion of this chapter, you will be able to:
Describe the principle of magnetic induction as it applies to ac generators.
Describe the differences between the two basic types of ac generators.
List the advantages and disadvantages of the two types of ac generators.
Describe exciter generators within alternators; discuss construction and purpose.
Compare the types of rotors used in ac generators, and the applications of each type to different prime movers.
Explain the factors that determine the maximum power output of an ac generator, and the effect of these factors in rating generators.
Explain the operation of multiphase ac generators and compare with single-phase.
Describe the relationships between the individual output and resultant vectorial sum voltages in multiphase generators.
Explain, using diagrams, the different methods of connecting three-phase alternators and transformers.
List the factors that determine the frequency and voltage of the alternator output.
Explain the terms voltage control and voltage regulation in ac generators, and list the factors that affect each quantity.
Describe the purpose and procedure of parallel generator operation.
INTRODUCTION
Many of the terms and principles covered in this chapter will be familiar to you. They are the same as those covered in the chapter on dc generators. You are encouraged to refer back, as needed, and to refer to
any other source that will help you master the subject of this chapter. No one source meets the complete needs of everyone.
BASIC AC GENERATORS
Regardless of size, all electrical generators, whether dc or ac, depend upon the principle of magnetic induction. An emf is induced in a coil as a result of (1) a coil cutting through a magnetic field, or (2) a magnetic field cutting through a coil. As long as there is relative motion between a conductor and a magnetic field, a voltage will be induced in the conductor. That part of a generator that produces the magnetic field is called the field. That part in which the voltage is induced is called the armature. For relative motion to take place between the conductor and the magnetic field, all generators must have two mechanical parts - a rotor and a stator. The ROTor is the part that ROTates; the STATor is the part that remains STATionary. In a dc generator, the armature is always the rotor. In alternators, the armature may be either the rotor or stator.
Q.1 Magnetic induction occurs when there is relative motion between what two elements?
ROTATING-ARMATURE ALTERNATORS
The rotating-armature alternator is similar in construction to the dc generator in that the armature rotates in a stationary magnetic field as shown in figure 3-1, view A. In the dc generator, the emf generated in the armature windings is converted from ac to dc by means of the commutator. In the alternator, the generated ac is brought to the load unchanged by means of slip rings. The rotating armature is found only in alternators of low power rating and generally is not used to supply electric power in large quantities.
Figure 3-1. - Types of ac generators.
ROTATING-FIELD ALTERNATORS
The rotating-field alternator has a stationary armature winding and a rotating-field winding as shown in figure 3-1, view B The advantage of having a stationary armature winding is that the generated voltage can be connected directly to the load.
A rotating armature requires slip rings and brushes to conduct the current from the armature to the load. The armature, brushes, and slip rings are difficult to insulate, and arc-overs and short circuits can result at high voltages. For this reason, high-voltage alternators are usually of the rotating-field type. Since the voltage applied to the rotating field is low voltage dc, the problem of high voltage arc-over at the slip rings does not exist.
The stationary armature, or stator, of this type of alternator holds the windings that are cut by the rotating magnetic field. The voltage generated in the armature as a result of this cutting action is the ac power that will be applied to the load.
The stators of all rotating-field alternators are about the same.
The stator consists of a laminated iron core with the armature windings embedded in this core as shown in figure 3-2. The core is secured to the stator frame.
Figure 3-2. - Stationary armature windings.
Q.2 What is the part of an alternator in which the output voltage is generated?
Q.3 What are the two basic types of alternators?
Q.4 What is the main advantage of the rotating field alternator?
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PRACTICAL ALTERNATORS
The alternators described so far in this chapter are ELEMENTARY in nature; they are seldom used except as examples to aid in understanding practical alternators.
The remainder of this chapter will relate the principles of the elementary alternator to the alternators actually in use in the civilian community, as well as aboard Navy ships and aircraft. The following paragraphs in this chapter will introduce such concepts as prime movers, field excitation, armature characteristics and limitations, single-phase and polyphase alternators, controls, regulation, and parallel operation.
FUNCTIONS OF ALTERNATOR COMPONENTS
A typical rotating-field ac generator consists of an alternator and a smaller dc generator built into a single unit. The output of the alternator section supplies alternating voltage to the load. The only purpose for the dc exciter generator is to supply the direct current required to maintain the alternator field. This dc generator is referred to as the exciter. A typical alternator is shown in figure 3-3, view A; figure 3-3, view B, is a simplified schematic of the generator.
Figure 3-3. - Ac generator pictorial and schematic drawings.
The exciter is a dc, shunt-wound, self-excited generator. The exciter shunt field (2) creates an area of intense magnetic flux between its poles. When the exciter armature (3) is rotated in the exciter-field flux, voltage is induced in the exciter armature windings. The output from the exciter commutator (4) is connected through brushes and slip rings (5) to the alternator field. Since this is direct current already converted by the exciter commutator, the current always flows in one direction through the alternator field (6). Thus, a fixed-polarity magnetic field is maintained at all times in the alternator field windings. When the alternator field is rotated, its magnetic flux is passed through and across the alternator armature windings (7).
The armature is wound for a three-phase output, which will be covered later in this chapter. Remember, a voltage is induced in a conductor if it is stationary and a magnetic field is passed across the conductor, the same as if the field is stationary and the conductor is moved. The alternating voltage in the ac generator armature windings is connected through fixed terminals to the ac load.
Q.5 Most large alternators have a small dc generator built into them. What is its purpose?
PRIME MOVERS
All generators, large and small, ac and dc, require a source of mechanical power to turn their rotors. This source of mechanical energy is called a prime mover.
Prime movers are divided into two classes for generators-high-speed and low-speed. Steam and gas turbines are high-speed prime movers, while internal-combustion engines, water, and electric motors are considered low-speed prime movers.
The type of prime mover plays an important part in the design of alternators since the speed at which the rotor is turned determines certain characteristics of alternator construction and operation.
ALTERNATOR ROTORS
There are two types of rotors used in rotating-field alternators. They are called the turbine-driven and salient-pole rotors.
As you may have guessed, the turbine-driven rotor shown in figure 3-4, view A, is used when the prime mover is a high-speed turbine. The windings in the turbine-driven rotor are arranged to form two or four distinct poles. The windings are firmly embedded in slots to withstand the tremendous centrifugal forces encountered at high speeds.
Figure 3-4. - Types of rotors used in alternators.
The salient-pole rotor shown in figure 3-4, view B, is used in low-speed alternators. The salient-pole rotor often consists of several separately wound pole pieces, bolted to the frame of the rotor.
If you could compare the physical size of the two types of rotors with the same electrical characteristics, you would see that the salient-pole rotor would have a greater diameter. At the same number of revolutions per minute, it has a greater centrifugal force than does the turbine-driven rotor.
To reduce this force to a safe level so that the windings will not be thrown out of the machine, the salient pole is used only in low-speed designs.
ALTERNATOR CHARACTERISTICS AND LIMITATIONS
Alternators are rated according to the voltage they are designed to produce and the maximum current they are capable of providing. The maximum current that can be supplied by an alternator depends upon the maximum heating loss that can be sustained in the armature. This heating loss (which is an I2R power loss) acts to heat the conductors, and if excessive, destroys the insulation. Thus, alternators are rated in terms of this current and in terms of the voltage output - the alternator rating in small units is in volt-amperes; in large units it is kilovolt-amperes.
When an alternator leaves the factory, it is already destined to do a very specific job. The speed at which it is designed to rotate, the voltage it will produce, the current limits, and other operating characteristics are built in. This information is usually
stamped on a nameplate on the case so that the user will know the limitations.
Q.6 How are alternators usually rated?
Q.7 What type of prime mover requires a specially designed high-speed alternator?
Q.8 Salient-pole rotors may be used in alternators driven by what types of prime movers?
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