A typical superconducting design of an ac generator, as in the conventional design, has the field winding mounted on the rotor and armature winding on the stator. The main differences between the two designs lie in the way cooling is done. The rotor has an inner body which is to support a winding cooled to a very low temperature by means of liquid helium. The liquid helium is fed to the winding along the rotor axis. To maintain the low temperature, thermal insulation is needed, and this can be achieved by means of a vacuum space and a radiation shield. The outer body of the rotor shields the rotors winding from being penetrated by the armature ields so that the superconducting state will not be destroyed. The stator structure is made of nonmagnetic material, which must be mechanically strong. The stator windings(armature)are not superconducting and are typically cooled by water. The immediate surroundings of the machine must be shielded from the strong magnetic fields; this requirement, though not necessary for the machines operation, can be satisfied by the use of a copper or laminated iron screen From a circuit viewpoint, superconducting machines have smaller internal impedance relative to the con- ventional ones(refer to equivalent circuit shown in Fig. 66.4). Recall that the reactance jX, stems from the fact that the armature circuits give rise to a magnetic field that tends to counter the effect of the rotor w the conventional design, such a magnetic field is enhanced because iron core is used for the rotor and stator structures;thus jX, is large. In the superconducting design, the core is basically air; thus, jX, is smaller. The lifference is generally a ratio of 5: 1 in magnitude. An implication is that, at the same level of output current , and terminal voltage V, it requires of the superconducting generator a smaller induced emf EF or, equivalently, a smaller field current It is expected that the use of superconductivity adds another 0.4% to the efficiency of generators. This improvement might seem insignificant(compared to an already achieved figure of 98% by the conventional design) but proves considerable in the long run. It is estimated that given a frame size and weight, a supercon ducting generator's capacity is three times that of a conventional one. However, the new concept has to deal with such practical issues as reliability, availability, and costs before it can be put into large-scale operation [Bumby, 1983] provides more details on superconducting electric machines with issues such as design, performance, and application of such machines. Induction Generators Conceptually, a three-phase induction machine is similar to a synchronous machine, but the former has a much rotor circuit. a typical design of the rotor is the squirrel-cage structure, where conducting bars are led in the rotor body and shorted out at the ends. When a set of three-phase currents(waveforms of mplitude, displaced in time by one-third of a period) is applied to the stator winding, a rotating magnetic field is produced. ( See the discussion of a revolving magnetic field for synchronous generators in the section Principle of Operation". Currents are therefore induced in the bars, and their resulting magnetic field interacts with the stator field to make the rotor rotate in the same direction In this case. the machine acts as a motor since, in order for the rotor to rotate, energy is drawn from the electric power source. When the machine acts as a motor, its rotor can never achieve the same speed as the rotating field (this is the synchronous speed)for that would imply no induced currents in the rotor bars. If an external mechanical torque is applied to the rotor to drive it beyond the synchronous speed, however, then electric energy is pumped to the power grid, and the machine will act as a generato An advantage of induction generators is their simplicity(no separate field circuit) and flexibility in speed. These features make induction machines attractive for applications such as windmills a disadvantage of induction generators is that they are highly inductive. Because the current and voltage have very large phase shifts, delivering a moderate amount of power requires an unnecessarily high current on ne power line. This current can be reduced by connecting capacitors at the terminals of the machine. Capacitors have negative reactance; thus, the machine's inductive reactance can be compensated. Such a scheme is known as capacitive compensation. It is ideal to have a compensation in which the capacitor and equivalent inductor completely cancel the effect of each other. In windmill applications, for example, this faces a great challenge because the varying speed of the rotor(as a result of wind speed) implies a varying equivalent inductor Fortunately, strategies for ideal compensation have been designed and put to commercial use. e 2000 by CRC Press LLC© 2000 by CRC Press LLC A typical superconducting design of an ac generator, as in the conventional design, has the field winding mounted on the rotor and armature winding on the stator. The main differences between the two designs lie in the way cooling is done. The rotor has an inner body which is to support a winding cooled to a very low temperature by means of liquid helium. The liquid helium is fed to the winding along the rotor axis. To maintain the low temperature, thermal insulation is needed, and this can be achieved by means of a vacuum space and a radiation shield. The outer body of the rotor shields the rotor’s winding from being penetrated by the armature fields so that the superconducting state will not be destroyed. The stator structure is made of nonmagnetic material, which must be mechanically strong. The stator windings (armature) are not superconducting and are typically cooled by water. The immediate surroundings of the machine must be shielded from the strong magnetic fields; this requirement, though not necessary for the machine’s operation, can be satisfied by the use of a copper or laminated iron screen. From a circuit viewpoint, superconducting machines have smaller internal impedance relative to the con￾ventional ones (refer to equivalent circuit shown in Fig. 66.4). Recall that the reactance jXs stems from the fact that the armature circuits give rise to a magnetic field that tends to counter the effect of the rotor winding. In the conventional design, such a magnetic field is enhanced because iron core is used for the rotor and stator structures; thus jXs is large. In the superconducting design, the core is basically air; thus, jXs is smaller. The difference is generally a ratio of 5:1 in magnitude. An implication is that, at the same level of output current Ia and terminal voltage Vt, it requires of the superconducting generator a smaller induced emf EF or, equivalently, a smaller field current. It is expected that the use of superconductivity adds another 0.4% to the efficiency of generators. This improvement might seem insignificant (compared to an already achieved figure of 98% by the conventional design) but proves considerable in the long run. It is estimated that given a frame size and weight, a supercon￾ducting generator’s capacity is three times that of a conventional one. However, the new concept has to deal with such practical issues as reliability, availability, and costs before it can be put into large-scale operation. [Bumby, 1983] provides more details on superconducting electric machines with issues such as design, performance, and application of such machines. Induction Generators Conceptually, a three-phase induction machine is similar to a synchronous machine, but the former has a much simpler rotor circuit. A typical design of the rotor is the squirrel-cage structure, where conducting bars are embedded in the rotor body and shorted out at the ends. When a set of three-phase currents (waveforms of equal amplitude, displaced in time by one-third of a period) is applied to the stator winding, a rotating magnetic field is produced. (See the discussion of a revolving magnetic field for synchronous generators in the section “Principle of Operation”.) Currents are therefore induced in the bars, and their resulting magnetic field interacts with the stator field to make the rotor rotate in the same direction. In this case, the machine acts as a motor since, in order for the rotor to rotate, energy is drawn from the electric power source. When the machine acts as a motor, its rotor can never achieve the same speed as the rotating field (this is the synchronous speed) for that would imply no induced currents in the rotor bars. If an external mechanical torque is applied to the rotor to drive it beyond the synchronous speed, however, then electric energy is pumped to the power grid, and the machine will act as a generator. An advantage of induction generators is their simplicity (no separate field circuit) and flexibility in speed. These features make induction machines attractive for applications such as windmills. A disadvantage of induction generators is that they are highly inductive. Because the current and voltage have very large phase shifts, delivering a moderate amount of power requires an unnecessarily high current on the power line. This current can be reduced by connecting capacitors at the terminals of the machine. Capacitors have negative reactance; thus, the machine’s inductive reactance can be compensated. Such a scheme is known as capacitive compensation. It is ideal to have a compensation in which the capacitor and equivalent inductor completely cancel the effect of each other. In windmill applications, for example, this faces a great challenge because the varying speed of the rotor (as a result of wind speed) implies a varying equivalent inductor. Fortunately, strategies for ideal compensation have been designed and put to commercial use