This distortion causes a shift in the neutral plane, which affects commutation. You know that for proper commutation, the coil short-circuited by the brushes must be in the neutral plane. Consider the operation of a simple two-pole dc generator, shown in figure View A of the figure shows the field poles and the main magnetic field. The armature is shown in a simplified view in views B and C with the cross section of its coil represented as little circles.
The symbols within the circles represent arrows. The dot represents the point of the arrow coming toward you, and the cross represents the tail, or feathered end, going away from you. When the armature rotates clockwise, the sides of the coil to the left will have current flowing toward you, as indicated by the dot.
The side of the coil to the right will have current flowing away from you, as indicated by the cross. The field generated around each side of the coil is shown in view B of figure This field increases in strength for each wire in the armature coil, and sets up a magnetic field almost perpendicular to the main field. Now you have two fields — the main field, view A, and the field around the armature coil, view B. View C of figure shows how the armature field distorts the main field and how the neutral plane is shifted in the direction of rotation.
If the brushes remain in the old neutral plane, they will be short- circuiting coils that have voltage induced in them. Consequently, there will be arcing between the brushes and commutator. To prevent arcing, the brushes must be shifted to the new neutral plane. What is armature reaction? The effect of armature reaction varies with the load current. Therefore, each time the load current varies, the neutral plane shifts. This means the brush position must be changed each time the load current varies. In small generators, the effects of armature reaction are reduced by actually mechanically shifting the position of the brushes.
The practice of shifting the brush position for each current variation is not practiced except in small generators. In larger generators, other means are taken to eliminate armature reaction. The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature. The series-connected compensating windings produce a magnetic field, which varies directly with armature current.
Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the effects of the armature magnetic field. The neutral plane will remain stationary and in its original position for all values of armature current.
Because of this, once the brushes have been set correctly, they do not have to be moved again. Another way to reduce the effects of armature reaction is to place small auxiliary poles called "interpoles" between the main field poles. The interpoles have a few turns of large wire and are connected in series with the armature. Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole ahead of it, in the direction of rotation. The field generated by the interpoles produces the same effect as the compensating winding.
This field, in effect, cancels the armature reaction for all values of load current by causing a shift in the neutral plane opposite to the shift caused by armature reaction. The amount of shift caused by the interpoles will equal the shift caused by armature reaction since both shifts are a result of armature current. What is the purpose of interpoles? A single armature conductor is represented in figure , view A. When the conductor is stationary, no voltage is generated and no current flows.
Therefore, no force acts on the conductor. When the conductor is moved downward fig. This sets up lines of flux around the conductor in a clockwise direction.
The interaction between the conductor field and the main field of the generator weakens the field above the conductor and strengthens the field below the conductor. The main field consists of lines that now act like stretched rubber bands. Thus, an upward reaction force is produced that acts in opposition to the downward driving force applied to the armature conductor.
If the current in the conductor increases, the reaction force increases. Therefore, more force must be applied to the conductor to keep it moving. With no armature current, there is no magnetic motor reaction. Therefore, the force required to turn the armature is low. As the armature current increases, the reaction of each armature conductor against rotation increases. The actual force in a generator is multiplied by the number of conductors in the armature.
The driving force required to maintain the generator armature speed must be increased to overcome the motor reaction. The force applied to turn the armature must overcome the motor reaction force in all dc generators. The prime mover may be an electric motor, a gasoline engine, a steam turbine, or any other mechanical device that provides turning force. What is the effect of motor reaction in a dc generator? These forces, as they affect the armature, are considered as losses and may be defined as follows: 1.
I2 R, or copper loss in the winding 2. Eddy current loss in the core 3. Hysteresis loss a sort of magnetic friction Heat is generated any time current flows in a conductor. Copper loss is an I2 R loss, which increases as current increases. The amount of heat generated is also proportional to the resistance of the conductor. The resistance of the conductor varies directly with its length and inversely with its cross- sectional area. Copper loss is minimized in armature windings by using large diameter wire. What causes copper losses?
Eddy Current Losses The core of a generator armature is made from soft iron, which is a conducting material with desirable magnetic characteristics. Any conductor will have currents induced in it when it is rotated in a magnetic field. The power dissipated in the form of heat, as a result of the eddy currents, is considered a loss. Eddy currents, just like any other electrical currents, are affected by the resistance of the material in which the currents flow. The resistance of any material is inversely proportional to its cross-sectional area.
Figure , view A, shows the eddy currents induced in an armature core that is a solid piece of soft iron. Figure , view B, shows a soft iron core of the same size, but made up of several small pieces insulated from each other. This process is called lamination. The currents in each piece of the laminated core are considerably less than in the solid core because the resistance of the pieces is much higher.
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Resistance is inversely proportional to cross-sectional area. The currents in the individual pieces of the laminated core are so small that the sum of the individual currents is much less than the total of eddy currents in the solid iron core. The laminations are insulated from each other by a thin coat of lacquer or, in some instances, simply by the oxidation of the surfaces.
Oxidation is caused by contact with the air while the laminations are being annealed. The insulation value need not be high because the voltages induced are very small. Most generators use armatures with laminated cores to reduce eddy current losses. How can eddy current be reduced?
Hysteresis Losses Hysteresis loss is a heat loss caused by the magnetic properties of the armature. When an armature core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction.
This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes armature resistances to increase. To compensate for hysteresis losses, heat-treated silicon steel laminations are used in most dc generator armatures. After the steel has been formed to the proper shape, the laminations are heated and allowed to cool.
This annealing process reduces the hysteresis loss to a low value. The differences are in the construction of the armature, the manner in which the armature is wound, and the method of developing the main field. A generator that has only one or two armature loops has high ripple voltage.
This results in too little current to be of any practical use. To increase the amount of current output, a number of loops of wire are used. These additional loops do away with most of the ripple. The loops of wire, called windings, are evenly spaced around the armature so that the distance between each winding is the same. The commutator in a practical generator is also different. It has several segments instead of two or four, as in our elementary generators. The number of segments must equal the number of armature coils. Each coil is connected to two commutator segments as shown.
One end of coil 1goes to segment A, and the other end of coil 1 goes to segment B. One end of coil 2 goes to segment C, and the other end of coil 2 goes to segment B. The rest of the coils are connected in a like manner, in series, around the armature. To complete the series arrangement, coil 8 connects to segment A. Therefore, each coil is in series with every other coil. Figure , view B shows a composite view of a Gramme-ring armature.
It illustrates more graphically the physical relationship of the coils and commutator locations. The windings of a Gramme-ring armature are placed on an iron ring. A disadvantage of this arrangement is that the windings located on the inner side of the iron ring cut few lines of flux. Therefore, they have little, if any, voltage induced in them. For this reason, the Gramme-ring armature is not widely used. The armature windings are placed in slots cut in a drum-shaped iron core. Each winding completely surrounds the core so that the entire length of the conductor cuts the main magnetic field.
Therefore, the total voltage induced in the armature is greater than in the Gramme-ring. You can see that the drum-type armature is much more efficient than the Gramme-ring. This accounts for the almost universal use of the drum-type armature in modem dc generators. The lap winding is illustrated in figure , view A This type of winding is used in dc generators designed for high-current applications. The windings are connected to provide several parallel paths for current in the armature.
For this reason, lap-wound armatures used in dc generators require several pairs of poles and brushes. Figure , view B, shows a wave winding on a drum-type armature. This type of winding is used in dc generators employed in high-voltage applications. Notice that the two ends of each coil are connected to commutator segments separated by the distance between poles.
This configuration allows the series addition of the voltages in all the windings between brushes. This type of winding only requires one pair of brushes. In practice, a practical generator may have several pairs to improve commutation. Why are drum-type armatures preferred over the Gramme-ring armature in modern dc generators? Lap windings are used in generators designed for what type of application?
This excitation voltage can be produced by the generator itself or it can be supplied by an outside source, such as a battery. When the generator starts rotating, the weak residual magnetism causes a small voltage to be generated in the armature. This small voltage applied to Although small, this field current strengthens the magnetic field and allows the armature to generate a higher voltage.
The higher voltage increases the field strength, and so on. This process continues until the output voltage reaches the rated output of the generator. Compound-wound generators are further classified as cumulative-compound and differential-compound. These last two classifications are not discussed in this chapter. Series-Wound Generator In the series-wound generator, shown in figure , the field windings are connected in series with the armature. Current that flows in the armature flows through the external circuit and through the field windings.
The external circuit connected to the generator is called the load circuit. A series-wound generator uses very low resistance field coils, which consist of a few turns of large diameter wire. The voltage output increases as the load circuit starts drawing more current. Under low-load current conditions, the current that flows in the load and through the generator is small. Since small current means that a small magnetic field is set up by the field poles, only a small voltage is induced in the armature. If the resistance of the load decreases, the load current increases.
Under this condition, more current flows through the field. This increases the magnetic field and increases the output voltage. A series-wound dc generator has the characteristic that the output voltage varies with load current. This is undesirable in most applications. For this reason, this type of generator is rarely used in everyday practice. The series-wound generator has provided an easy method to introduce you to the subject of self- excited generators.
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They are connected in parallel with the load. In other words, they are connected across the output voltage of the armature. Current in the field windings of a shunt-wound generator is independent of the load current currents in parallel branches are independent of each other. Since field current, and therefore field strength, is not affected by load current, the output voltage remains more nearly constant than does the output voltage of the series-wound generator.
In actual use, the output voltage in a dc shunt-wound generator varies inversely as load current varies. In a series-wound generator, output voltage varies directly with load current. In the shunt-wound generator, output voltage varies inversely with load current. A combination of the two types can overcome the disadvantages of both. This combination of windings is called the compound-wound dc generator.
Compound-Wound Generators Compound-wound generators have a series-field winding in addition to a shunt-field winding, as shown in figure The shunt and series windings are wound on the same pole pieces.
In the compound-wound generator when load current increases, the armature voltage decreases just as in the shunt-wound generator. This causes the voltage applied to the shunt-field winding to decrease, which results in a decrease in the magnetic field. This same increase in load current, since it flows through the series winding, causes an increase in the magnetic field produced by that winding.
By proportioning the two fields so that the decrease in the shunt field is just compensated by the increase in the series field, the output voltage remains constant. This is shown in figure , which shows the voltage characteristics of the series-, shunt-, and compound-wound generators.
As you can see, by proportioning the effects of the two fields series and shunt , a compound-wound generator provides a constant output voltage under varying load conditions. Actual curves are seldom, if ever, as perfect as shown. What are the three classifications of dc generators? What is the main disadvantage of series generators? Figure shows the entire generator with the component parts installed. The cutaway drawing helps you to see the physical relationship of the components to each other. It is usually expressed as the change in voltage from a no-load condition to a full-load condition, and is expressed as a percentage of full-load.
It is expressed in the following formula: where EnL is the no-load terminal voltage and EfL is the full-load terminal voltage of the generator. What term applies to the voltage variation from no-load to full-load conditions and is expressed as a percentage? In most cases the process involves changing the resistance of the field circuit. By changing the field circuit resistance, the field current is controlled. Controlling the field current permits control of the output voltage.
The major difference between the various voltage control systems is merely the method by which the field circuit resistance and the current are controlled. As described previously, voltage regulation is an internal action occurring within the generator whenever the load changes. Voltage control is an imposed action, usually through an external adjustment, for the purpose of increasing or decreasing terminal voltage. Manual Voltage Control The hand-operated field rheostat, shown in figure , is a typical example of manual voltage control. The field rheostat is connected in series with the shunt field circuit.
This provides the simplest method of controlling the terminal voltage of a dc generator. This type of field rheostat contains tapped resistors with leads to a multiterminal switch. The arm of the switch may be rotated to make contact with the various resistor taps. This varies the amount of resistance in the field circuit. Rotating the arm in the direction of the LOWER arrow counterclockwise increases the resistance and lowers the output voltage.
Rotating the arm in the direction of the RAISE arrow clockwise decreases the resistance and increases the output voltage. Most field rheostats for generators use resistors of alloy wire. They have a high specific resistance and a low temperature coefficient. These alloys include copper, nickel, manganese, and chromium. They are marked under trade names such as Nichrome, Advance, Manganin, and so forth.
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Some very large generators use cast-iron grids in place of rheostats, and motor-operated switching mechanisms to provide voltage control. An automatic voltage control device "senses" changes in output voltage and causes a change in field resistance to keep output voltage constant. The actual circuitry involved in automatic voltage control will not be covered in this chapter. Whichever control method is used, the range over which voltage can be changed is a design characteristic of the generator. The voltage can be controlled only within the design limits.
The purpose of connecting generators in parallel is simply to provide more current than a single generator is capable of providing. The generators may be physically located quite a distance apart. However, they are connected to the common load through the power distribution system. There are several reasons for operating generators in parallel. The number of generators used may be selected in accordance with the load demand. By operating each generator as nearly as possible to its rated capacity, maximum efficiency is achieved.
A disabled or faulty generator may be taken off-line and replaced without interrupting normal operations. What term applies to the use of two or more generators to supply a common load? They supply large dc currents, precisely controlled, to the large dc motors used to drive heavy physical loads, such as gun turrets and missile launchers. The amplidyne is really a motor and a generator. It consists of a constant-speed ac motor the prime mover mechanically coupled to a dc generator, which is wired to function as a high-gain amplifier an amplifier is a device in which a small input voltage can control a large current source.
For instance, in a normal dc generator, a small dc voltage applied to the field windings is able to control the output of the generator. In a typical generator, a change in voltage from 0-volt dc to 3-volts dc applied to the field winding may cause the generator output to vary from 0-volt dc to volts dc. If the 3 volts applied to the field winding is considered an input, and the volts taken from the brushes is an output, there is a gain of This means that the 3 volts output is times larger than the input.
The following paragraphs explain how gain is achieved in a typical dc generator and how the modifications making the generator an amplidyne increase the gain to as high as 10, The schematic diagram in figure shows a separately excited dc generator. Because of the volt controlling voltage, 10 amperes of current will flow through the 1-ohm field winding.
Assume that the characteristics of this generator enable it to produce approximately 87 amperes of armature current at volts at the output terminals. You can see that the power gain of this generator is In effect, watts controls 10, watts. An amplidyne is a special type of dc generator. The following changes, for explanation purposes, will convert the typical dc generator above into an amplidyne. The first step is to short the brushes together, as shown in figure This removes nearly all of the resistance in the armature circuit. Because of the very low resistance in the armature circuit, a much lower control-field flux produces full-load armature current full-load current in the armature is still about 87 amperes.
The smaller control The next step is to add another set of brushes. These now become the output brushes of the amplidyne. They are placed against the commutator in a position perpendicular to the original brushes, as shown in figure The previously shorted brushes are now called the "quadrature" brushes.
This is because they are in quadrature perpendicular to the output brushes. The output brushes are in line with the armature flux. Therefore, they pick off the voltage induced in the armature windings at this point. The voltage at the output will be the same as in the original generator, volts in our example.
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As you have seen, the original generator produced a 10,watt output with a watt input. The amplidyne produces the same 10,watt output with only a 1-watt input. This represents a gain of 10, The gain of the original generator has been greatly increased. As previously stated, an amplidyne is used to provide large dc currents. Assume that a very large turning force is required to rotate a heavy object, such as an antenna, to a very precise position. A low-power, relatively weak voltage representing the amount of antenna rotation required can be used to control the field winding of an amplidyne.
Because of the amplidyne's ability to amplify, its output can be used to drive a powerful motor, which turns the heavy object antenna. When the source of the input voltage senses the correct movement of the object, it drops the voltage to zero. The field is no longer strong enough to allow an output voltage to be developed, so the motor ceases to drive the object antenna. The above is an oversimplification and is not meant to describe a functioning system. The intent is to show a typical sequence of events between the demand for movement and the movement itself.
It is meant to strengthen the idea that with the amplidyne, something large and heavy can be controlled very precisely by something very small, almost insignificant. What is the purpose of a dc generator that has been modified to function as an amplidyne? What is the formula used to determine the gain of an amplifying device?
What are the two inputs to an amplidyne? Electrical equipment frequently has accessories that require separate sources of power. Lighting fixtures, heaters, externally powered temperature detectors, and alarm systems are examples of accessories whose terminals must be deenergized. When working on dc generators, you must check to ensure that all such circuits have been de-energized and tagged before you attempt any maintenance or repair work.
You must also use the greatest care when working on or near the output terminals of dc generators. The different types of dc generators and their characteristics were covered. The following information provides a summary of the major subjects of the chapter for your review. It produces an ac voltage. They have much the same effect as adding coils to the armature. In practical generators, the poles are electromagnets. The coil connections to the load must be reversed as the coil passes through the neutral plane.
The brushes must be positioned so that commutation is accomplished without brush sparking.
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The armature field disturbs the field from the pole pieces. This results in a shift of the neutral plane in the direction of rotation. They are supplied by armature current and shift the neutral plane back to its original position. It tends to oppose the rotation of the armature, due to the attraction and repulsion forces between the armature field and the main field. These losses are as follows: 1. Copper losses are simply 12 R heat losses caused by current flowing through the resistance of the armature windings.
Eddy currents are induced in core material and cause heat. Hysteresis losses occur due to the rapidly changing magnetic fields in the armature, resulting in heat. The current in the field coils determines the strength and the direction of the magnetic field. Outputs vary directly with load currents. Series-wound generators have few practical applications.
The output varies inversely with load current.
These generators combine the characteristics of series and shunt generators. The output voltage remains relatively constant for all values of load current within the design of the generator. Compound generators are used in many applications because of the relatively constant voltage.
By short- circuiting the brushes in a normal dc generator and adding another set of brushes perpendicular to the original ones, an amplidyne is formed. Its power output may be up to 10, times larger than the power input to its control windings. Magnetic induction. The left-hand rule for generators. PDF format. Works with any computer, and is easy to use, read, resize and print from. Contents Module 1, Introduction to Matter, Energy, and Direct Current, introduces the course with a short history of electricity and electronics and proceeds into the characteristics of matter, energy, and direct current dc.
It also describes some of the general safety precautions and first-aid procedures that should be common knowledge for a person working in the field of electricity. Related safety hints are located throughout the rest of the series, as well. Module 8, Introduction to Amplifiers covers amplifiers. Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits discusses wave generation and wave-shaping circuits.
Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas presents the characteristics of wave propagation, transmission lines, and antennas. Module 11, Microwave Principles explains microwave oscillators, amplifiers, and waveguides. Module 12, Modulation Principles discusses the principles of modulation. Module 13, Introduction to Number Systems and Logic Circuits presents the fundamental concepts of number systems, Boolean algebra, and logic circuits, all of which pertain to digital computers.
Module 14, Introduction to Microelectronics covers microelectronics technology and miniature and microminiature circuit repair. Module 15, Principles of Synchros, Servos, and Gyros provides the basic principles, operations, functions, and applications of synchro, servo, and gyro mechanisms.
Module 16, Introduction to Test Equipment is an introduction to some of the more commonly used test equipments and their applications. Module 17, Radio-Frequency Communications Principles presents the fundamentals of a radio frequency communications system. Module 18, Radar Principles covers the fundamentals of a radar system. Module 19, The Technician's Handbook is a handy reference of commonly used general information, such as electrical and electronic formulas, color coding, and naval supply system data.
Module 20, Master Glossary is the glossary of terms for the series.
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