LE FIGURE 66.9 A simple two-pole dc generator with a stator winding to produce a magnetic field. Top, main components of the machine; bottom, coupled-circuit representation; the circuit on the left represents the field winding; the induced emf E is controlled by iF. When the generator is connected to an electrical load, load currents flow through the rotor conductors Therefore, a magnetic field is set up in addition to that of the permanent magnet. This additional field generally weakens the magnetic flux seen by the rotor conductors. a direct consequence is that the induced emf's are less than those in an unloaded machine. Similar to the case of ac generators, this phenomenon is referred as armature reaction, or flux-weakening effect. The use of brushes in the design of dc generators can cause a serious problem in practice. Each time a brush comes into contact with two adjacent copper segments, the corresponding conductors are short-circuited. For a loaded generator, such an event occurs when the currents in these conductors are not zero, resulting in flashover at the brushes. This means that the life span of the brushes can be drastically reduced and that frequent Ice is needed. A number of design techniques have been developed to mitigate this problem Mathematical/Circuit Model The(no-load) terminal voltage V, of a dc generator depends on several factors. construction of the machine(e.g, the number of conductors). Second, the voltage ma dependson the magnetic field of the stator: the stronger the field is, the higher the voltage becomes. Third, since the induced emf is proportional to the rate of change of the magnetic flux( Faraday's law), the terminals have higher voltage with a higher machine speed. One can write where K is a constant representing the first factor, A is magnetic flux, and n is rotor speed. The foregoing quation provides some insights into the voltage control of dc generators. Among the three terms, it is impractical to modify K, which is determined by the machine design. Changing n over a wide range may not be feasible since this is limited by what drives the rotor. Changing the magnetic flux A can be done if the permanent magnet is replaced by an electromagnet, and this is how the voltage control is done in practice.The control of n is made possible by adjusting the current fed to this electromagnet. Figure 66.9 shows the modified design of the simple dc generator. The stator winding is called the field winding, which produces excitation for the machine. The current in the field winding is adjusted by means of a variable resistor connected in series with this winding. It is also possible to use two field windings in order to have more flexibility in control The use of field winding(s)on the stator of the dc machine leads to a number of methods to produce the magnetic field. Depending on how the field winding(s) and the rotor winding are connected, one may have e 2000 by CRC Press LLC
© 2000 by CRC Press LLC When the generator is connected to an electrical load, load currents flow through the rotor conductors. Therefore, a magnetic field is set up in addition to that of the permanent magnet. This additional field generally weakens the magnetic flux seen by the rotor conductors. A direct consequence is that the induced emf’s are less than those in an unloaded machine. Similar to the case of ac generators, this phenomenon is referred to as armature reaction, or flux-weakening effect. The use of brushes in the design of dc generators can cause a serious problem in practice. Each time a brush comes into contact with two adjacent copper segments, the corresponding conductors are short-circuited. For a loaded generator, such an event occurs when the currents in these conductors are not zero, resulting in flashover at the brushes. This means that the life span of the brushes can be drastically reduced and that frequent maintenance is needed. A number of design techniques have been developed to mitigate this problem. Mathematical/Circuit Model The (no-load) terminal voltage Vt of a dc generator depends on several factors. First, it depends on the construction of the machine (e.g., the number of conductors). Second, the voltage magnitude depends on the magnetic field of the stator: the stronger the field is, the higher the voltage becomes. Third, since the induced emf is proportional to the rate of change of the magnetic flux (Faraday’s law), the terminals have higher voltage with a higher machine speed. One can write Vt(no load) = Kln where K is a constant representing the first factor, l is magnetic flux, and n is rotor speed. The foregoing equation provides some insights into the voltage control of dc generators. Among the three terms, it is impractical to modify K, which is determined by the machine design. Changing n over a wide range may not be feasible since this is limited by what drives the rotor. Changing the magnetic flux l can be done if the permanent magnet is replaced by an electromagnet, and this is how the voltage control is done in practice. The control of l is made possible by adjusting the current fed to this electromagnet. Figure 66.9 shows the modified design of the simple dc generator. The stator winding is called the field winding, which produces excitation for the machine. The current in the field winding is adjusted by means of a variable resistor connected in series with this winding. It is also possible to use two field windings in order to have more flexibility in control. The use of field winding(s) on the stator of the dc machine leads to a number of methods to produce the magnetic field. Depending on how the field winding(s) and the rotor winding are connected, one may have FIGURE 66.9 A simple two-pole dc generator with a stator winding to produce a magnetic field. Top, main components of the machine; bottom, coupled-circuit representation; the circuit on the left represents the field winding; the induced emf E is controlled by iF
TABLE 66.2 Excitation Methods and Voltage Current Characteristics for DC Generators Excitation methods Characteristics For low currents, the curve is nearly a straight line. As load current increases, the armature reaction becomes At no load, there is no field current, and voltage is due to the residual flux of the stator core. The voltage rises rapidly over the range of low currents, but the resistive drop soon bec Shunt Voltage buildup depends on the residual flux. The shunt field resistance must be less than a critical value There are two field windings. Depending on Cumulative: An increase in load increases the resistive drop, how they are set up, cumulative yet creates more flux. At high currents, however, resistive drop if the two fields are additive, diffe if the becomes dominant two fields are subtractive Differential: An increase in load current not only increases the resistive drop, but also reduces the net flux. Voltage drops drastically. shunt excitation, series excitation, etc. Each connection yields a different terminal characteristic. The possible connections and the resulting current-voltage characteristics are given in Table 66.2 [Chapman, 1991] and [Fink and Beaty, 1987] provide more detailed discussions of dc generators. Specifically, [Chapman, 1991] shows how the characteristics are derived for various excitation methods e 2000 by CRC Press LLC
© 2000 by CRC Press LLC shunt excitation, series excitation, etc. Each connection yields a different terminal characteristic. The possible connections and the resulting current–voltage characteristics are given in Table 66.2. [Chapman, 1991] and [Fink and Beaty, 1987] provide more detailed discussions of dc generators. Specifically, [Chapman, 1991] shows how the characteristics are derived for various excitation methods. TABLE 66.2 Excitation Methods and Voltage Current Characteristics for DC Generators Excitation Methods Characteristics Separate For low currents, the curve is nearly a straight line. As load current increases, the armature reaction becomes more severe and contributes to the nonlinear drop. Series At no load, there is no field current, and voltage is due to the residual flux of the stator core. The voltage rises rapidly over the range of low currents, but the resistive drop soon becomes dominant. Shunt Voltage buildup depends on the residual flux. The shunt field resistance must be less than a critical value. Compounded There are two field windings. Depending on how they are set up, one may have cumulative if the two fields are additive, differential if the two fields are subtractive. Cumulative: An increase in load current increases the resistive drop, yet creates more flux. At high currents, however, resistive drop becomes dominant. Differential:An increase in load current not only increases the resistive drop, but also reduces the net flux. Voltage drops drastically. i F IL E Vt + + – – IL Vt Separate i ƒ IL E Vt + + – – IL Vt Shunt i a IL i F E Vt + + – – IL Vt Shunt i a IL i F E Vt + + – – IL Vt Compounded cumulative differential
FRANK JULIAN SPRAGUE (1857-1934) F rank Sprague was a true entrepreneur in the new field of electrical technology. After a brief stint on Thomas Edisons staff, Sprague went out on his own, founding Sprague Electric Railway and Motor Company in 1884. In 1887, Sprague equipped the first modern trolley railway in the United States Sprague's successful construction of a streetcar system for Richmond, Virginia, in 1888 was the beginning of the great electric railway boom. Sprague followed this system with 100 other such systems, both in America and Europe, during the next two years. In less than 15 ars, more 20,000 miles(32,000 km) of electric street rail- way were built. In addition to his work in railroads, Spragues rse talents led to his development of ele iature electric power units for use in small appli ances,and as a member of the U.S. Naval Consulting Board during World War I, he developed fuses and air and depth bombs. Sprague was awarded the AIEE's Edison Medal in 1910.( Courtesy of the IEEE Center for the History of Electrical Engineering. Defining Terms Armature circuit: A winding where the load current is carried. Armature reaction: The phenomenon in which the magnetic field due to currents in the armature circuit counters the effect of the field circuit Commutation: A mechanical technique in which rectification can be achieved in dc machines Field circuit: A set of windings that produces a magnetic field so that the electromagnetic induction can take place in electric machines Revolving fields: A magnetic field created by multiphase currents on spatially displaced windings in rotating machines; the field revolves in the air gap. Swing equation: A nonlinear differential equation describing the rotor dynamics of an ac synchronous Synchronous speed: A characteristic speed of synchronous and induction machines with a revolving field; it is determined by the rotor structure and the line frequency Related Topics 2.2 Ideal and Practical Sources.3.4 Power and Energy. 104.1 Welding and Bonding e 2000 by CRC Press LLC
© 2000 by CRC Press LLC Defining Terms Armature circuit: A winding where the load current is carried. Armature reaction: The phenomenon in which the magnetic field due to currents in the armature circuit counters the effect of the field circuit. Commutation: A mechanical technique in which rectification can be achieved in dc machines. Field circuit: A set of windings that produces a magnetic field so that the electromagnetic induction can take place in electric machines. Revolving fields: A magnetic field created by multiphase currents on spatially displaced windings in rotating machines; the field revolves in the air gap. Swing equation: A nonlinear differential equation describing the rotor dynamics of an ac synchronous machine. Synchronous speed: A characteristic speed of synchronous and induction machines with a revolving field; it is determined by the rotor structure and the line frequency. Related Topics 2.2 Ideal and Practical Sources • 3.4 Power and Energy • 104.1 Welding and Bonding FRANK JULIAN SPRAGUE (1857–1934) rank Sprague was a true entrepreneur in the new field of electrical technology. After a brief stint on Thomas Edison’s staff, Sprague went out on his own, founding Sprague Electric Railway and Motor Company in 1884. In 1887, Sprague equipped the first modern trolley railway in the United States. Sprague’s successful construction of a streetcar system for Richmond, Virginia, in 1888 was the beginning of the great electric railway boom. Sprague followed this system with 100 other such systems, both in America and Europe, during the next two years. In less than 15 years, more than 20,000 miles (32,000 km) of electric street railway were built. In addition to his work in railroads, Sprague’s diverse talents led to his development of electric elevators, an ac induction smelting furnace, miniature electric power units for use in small appliances, and as a member of the U.S. Naval Consulting Board during World War I, he developed fuses and air and depth bombs. Sprague was awarded the AIEE’s Edison Medal in 1910. (Courtesy of the IEEE Center for the History of Electrical Engineering.) F
References M. S. Sarma, Synchronous Machines(Their Theory Stability and Excitation Systems), New York: Gordon and Breach, 1979 J.R. Bumby, Superconducting Rotating Electrical Machines, New York: Oxford University Press, 1983 S.J. Chapman, Electric Machinery Fundamentals, New York: McGraw-Hill, 1991 G. McPherson, An Introduction to Electrical Machines and Transformers, New York: Wiley, 1981 A.R. Bergen, Power Systems Analysis, Englewood Cliffs, N.J. Prentice-Hall, 1986 M.A. Laughton and M. G. Say, Eds, Electrical Engineer's Reference Book, Stoneham, Mass.: Butterworth, 1985 D. G. Fink and H. w. Beaty, Eds, Standard Handbook for Electrical Engineers, New York: McGraw-Hill, 1987 S.S. L. Chang, ed, Fundamentals Handbook of Electrical and Computer Engineering, New York: Wiley, 1982 Further Information Several handbooks, e.g., Electrical Engineer's Reference Book and Standard Handbook for Electrical Engineers, give more details on the machine design. [Bumby, 1983] covers the subject of superconducting generators. Some textbooks in the area of rotating machines are listed as [Sarma, 1979; Chapman, 1991; McPherson, 1981] The quarterly journal IEEE Transactions on Energy Conversion covers the field of rotating machinery and power generation. Another IEEE quarterly journal, IEEE Transactions on Power Systems, is devoted to the general aspects of power system engineering and power engineering education The bimonthly journal Electric Machines and Power Systems, published by Hemisphere Publishing Corpora tion, covers the broad field of electromechanics, electric machines, and power systems otors Donald galler Electric motors are the most commonly used prime mover in industry. The classification of the types of ac and dc motors commonly used in industrial applications is shown in Fig. 66.10 Motor Applications DC Motors Permanent magnet(PM)field motors occupy the low end of the horsepower(hp)range and are commercially available up to about 10 hp. Below I hp they are used for servo applications, such as in machine tools, for robotics, and in high-performance computer peripherals Wound field motors are used above about 10 hp and represent the highest horsepower range of dc motor application. They are commercially available up to several hundred horsepower and are commonly used in traction, hoisting, and other applications where a wide range of speed control is needed. The shunt wound dc motor is commonly found in industrial applications such as grinding and machine tools and in elevator and hoist applications. Compound wound motors have both a series and shunt field component to provide specific torque-speed characteristics. Propulsion motors for transit vehicles are usually compound wound dc motors AC Motors Single-phase ac motors occupy the low end of the horsepower spectrum and are offered commercially up to about 5 hp. Single-phase synchronous motors are only used below about 1/10 of a horsepower. Typical applications are timing and motion control, where low torque is required at fixed speeds. Single-phase induction motors are used for operating household appliances and machinery from about 1/3 to 5 hp Polyphase ac motors are primarily three-phase and are by far the largest electric prime mover in all industry. They are offered in ranges from 5 up to 50,000 hp and account for a large percentage of the total motor industry in the world. In number of units, the three-phase squirrel cage induction motor is the most mmon. It is commercially available from 1 hp up to several thousand horsepower and can be used on e 2000 by CRC Press LLC
© 2000 by CRC Press LLC References M. S. Sarma, Synchronous Machines (Their Theory, Stability, and Excitation Systems), New York: Gordon and Breach, 1979. J. R. Bumby, Superconducting Rotating Electrical Machines, New York: Oxford University Press, 1983. S. J. Chapman, Electric Machinery Fundamentals, New York: McGraw-Hill, 1991. G. McPherson, An Introduction to Electrical Machines and Transformers, New York: Wiley, 1981. A. R. Bergen, Power Systems Analysis, Englewood Cliffs, N.J.: Prentice-Hall, 1986. M. A. Laughton and M. G. Say, Eds., Electrical Engineer’s Reference Book, Stoneham, Mass.: Butterworth, 1985. D. G. Fink and H. W. Beaty, Eds., Standard Handbook for Electrical Engineers, New York: McGraw-Hill, 1987. S. S. L. Chang, ed., Fundamentals Handbook of Electrical and Computer Engineering, New York: Wiley, 1982. Further Information Several handbooks, e.g., Electrical Engineer’s Reference Book and Standard Handbook for Electrical Engineers, give more details on the machine design. [Bumby, 1983] covers the subject of superconducting generators. Some textbooks in the area of rotating machines are listed as [Sarma, 1979; Chapman, 1991; McPherson, 1981]. The quarterly journal IEEE Transactions on Energy Conversion covers the field of rotating machinery and power generation.Another IEEE quarterly journal,IEEE Transactions on Power Systems, is devoted to the general aspects of power system engineering and power engineering education. The bimonthly journal Electric Machines and Power Systems, published by Hemisphere Publishing Corporation, covers the broad field of electromechanics, electric machines, and power systems. 66.2 Motors Donald Galler Electric motors are the most commonly used prime mover in industry. The classification of the types of ac and dc motors commonly used in industrial applications is shown in Fig. 66.10. Motor Applications DC Motors Permanent magnet (PM) field motors occupy the low end of the horsepower (hp) range and are commercially available up to about 10 hp. Below 1 hp they are used for servo applications, such as in machine tools, for robotics, and in high-performance computer peripherals. Wound field motors are used above about 10 hp and represent the highest horsepower range of dc motor application. They are commercially available up to several hundred horsepower and are commonly used in traction, hoisting, and other applications where a wide range of speed control is needed. The shunt wound dc motor is commonly found in industrial applications such as grinding and machine tools and in elevator and hoist applications. Compound wound motors have both a series and shunt field component to provide specific torque-speed characteristics. Propulsion motors for transit vehicles are usually compound wound dc motors. AC Motors Single-phase ac motors occupy the low end of the horsepower spectrum and are offered commercially up to about 5 hp. Single-phase synchronous motors are only used below about 1/10 of a horsepower. Typical applications are timing and motion control, where low torque is required at fixed speeds. Single-phase induction motors are used for operating household appliances and machinery from about 1/3 to 5 hp. Polyphase ac motors are primarily three-phase and are by far the largest electric prime mover in all of industry. They are offered in ranges from 5 up to 50,000 hp and account for a large percentage of the total motor industry in the world. In number of units, the three-phase squirrel cage induction motor is the most common. It is commercially available from 1 hp up to several thousand horsepower and can be used on