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