In [Chapman, 1991], an analysis of induction generators and the effect of capacitive compensation on machine's performance are given. DC Generators To obtain dc electricity, one may prefer an available ac source with an electronic rectifier circuit. Another possibility is to generate dc electricity directly. Although the latter method is becoming obsolete, it is still important to understand how a dc generator works. This section provides a brief discussion of the basic issues associated with dc generators Principle of Operation As in the case of ac generators, a basic design will be used to explain the essential ideas behind the operation of dc generators. Figure 66.7 is a schematic diagram showing an end of a simple dc machine The stator of the simple machine is a permanent magnet with two poles labeled N and S. The rotor is a lindrical body and has two (insulated) conductors embedded in its surface. At one end of the rotor, as illustrated in Fig. 66.7, the two conductors are connected to a pair of copper segments; these semicircular gments, shown in the diagram, are mounted on the shaft of the rotor. Hence, they rotate together with the rotor. At the other end of the rotor, the two conductors are joined to form a coil. Assume that an external torque is applied to the shaft so that the rotor rotates at a certain speed. The rotor ding formed by the two conductors experiences a periodically varying magnetic field, and hence an emf is nduced across the winding. Note that this voltage periodically alternates in sign, and thus, the situation ron pte ly the sam se as t herne neouitertid in ec de he tors. cema r hts machin a t ad a dosibpeew te whe of copper segments and brushes. According to Fig 66.7, each copper segment comes into contact with one brush half of the time during each rotor revolution. The 事N placement of the(stationary) brushes guarantees that one brush always has positive potential relative to the other. For the chose direction of rotation, the brush with higher potential is the one directly beneath the N-pole. ( Should the rotor rotate in the reverse direction, the opposite is true. )Thus, the brushes can serve as the terminals of the dc source In electric machinery, the rectifying action of the copper segments and brushes is referred to as commutation, and the machine is called a commutating machine A qualitative sketch of V, the voltage across terminals of FIGURE 66.7 A basic unloaded simple dc generator, as a function of time is given in erator. V, is the voltage across the Fig. 66.8. Note that this voltage is not a constant. A unidirectional terminals.⑧#and⊙# indicate th current can flow when a resistor is connected across the terminals of that would flow if a closed circuit is mad the machine The pulsating voltage waveform generated by the simple dc machine usually cannot meet the requirement of practical applica- tions. An improvement can be made with more pairs of conductors. These conductors are placed in slots that are made equidistant on the ptor surface. Each pair of conductors can generate a voltage wave these waveforms due to the spatial displacement among the cond g 0 form similar to the one in Fig. 66.8, but there are time shifts amo tor pairs. For instance, when an individual voltage is minimum FIGURE 66.8 Open-circuited terminal (zero),other voltages are not. If these voltage waveforms are added, voltage of the simple dc gene the result is a near constant voltage waveform. This improvement of the dc waveform requires many pairs of the copper segments and a pair of brushes e 2000 by CRC Press LLC© 2000 by CRC Press LLC In [Chapman, 1991], an analysis of induction generators and the effect of capacitive compensation on machine’s performance are given. DC Generators To obtain dc electricity, one may prefer an available ac source with an electronic rectifier circuit. Another possibility is to generate dc electricity directly. Although the latter method is becoming obsolete, it is still important to understand how a dc generator works. This section provides a brief discussion of the basic issues associated with dc generators. Principle of Operation As in the case of ac generators, a basic design will be used to explain the essential ideas behind the operation of dc generators. Figure 66.7 is a schematic diagram showing an end of a simple dc machine. The stator of the simple machine is a permanent magnet with two poles labeled N and S. The rotor is a cylindrical body and has two (insulated) conductors embedded in its surface. At one end of the rotor, as illustrated in Fig. 66.7, the two conductors are connected to a pair of copper segments; these semicircular segments, shown in the diagram, are mounted on the shaft of the rotor. Hence, they rotate together with the rotor. At the other end of the rotor, the two conductors are joined to form a coil. Assume that an external torque is applied to the shaft so that the rotor rotates at a certain speed. The rotor winding formed by the two conductors experiences a periodically varying magnetic field, and hence an emf is induced across the winding. Note that this voltage periodically alternates in sign, and thus, the situation is conceptually the same as the one encountered in ac generators. To make the machine act as a dc source, viewed from the terminals, some form of rectification needs be introduced. This function is made possible with the use of copper segments and brushes. According to Fig. 66.7, each copper segment comes into contact with one brush half of the time during each rotor revolution. The placement of the (stationary) brushes guarantees that one brush always has positive potential relative to the other. For the chosen direction of rotation, the brush with higher potential is the one directly beneath the N-pole. (Should the rotor rotate in the reverse direction, the opposite is true.) Thus, the brushes can serve as the terminals of the dc source. In electric machinery, the rectifying action of the copper segments and brushes is referred to as commutation, and the machine is called a commutating machine. A qualitative sketch of Vt , the voltage across terminals of an unloaded simple dc generator, as a function of time is given in Fig. 66.8. Note that this voltage is not a constant. A unidirectional current can flow when a resistor is connected across the terminals of the machine. The pulsating voltage waveform generated by the simple dc machine usually cannot meet the requirement of practical applica￾tions. An improvement can be made with more pairs of conductors. These conductors are placed in slots that are made equidistant on the rotor surface. Each pair of conductors can generate a voltage wave￾form similar to the one in Fig. 66.8, but there are time shifts among these waveforms due to the spatial displacement among the conduc￾tor pairs. For instance, when an individual voltage is minimum (zero), other voltages are not. If these voltage waveforms are added, the result is a near constant voltage waveform. This improvement of the dc waveform requires many pairs of the copper segments and a pair of brushes. FIGURE 66.7 A basic two-pole dc gen￾erator. Vt is the voltage across the machine terminals. ^# and (# indicate the direc￾tion of currents (into or out of the page) that would flow if a closed circuit is made. FIGURE 66.8 Open-circuited terminal voltage of the simple dc generator