SUPERCONDUCTING MOTORS, GENERATORS, AND ALTERNATORS

Superconductors are very promising and exciting materials for electric power engineering in general and for electric machines in particular. By allowing very high current densities and by suppressing the Joule losses, superconductors improve the performance of electric machines by reducing weight, improving efficiency and, to a lesser degree, by increasing compactness. The reduction of losses results in long-term savings in capital cost, making superconducting machines attractive from an economic point of view. Since a cryogenic system is required to maintain the superconducting state, these advantages appear only above a critical (breakeven) size or rating, such that where the refrigeration penalty is negligible. Superconductivity is thus attractive only for large electrical machines. Small motors (up to a few kilowatts) will not be superconducting except if room-temperature superconductors are discovered, which is highly improbable.

The critical size is reduced when the operating temperature increases. Devices with high-critical – temperature superconductors will be attractive for lower ratings than systems cooled with liquid helium.

Electric Machines—General Structure(l)

An electric machine is reversible. It can operate as a motor, converting electrical into mechanical power, or as a generator, converting mechanical input into electricity. The electromagnetic force or torque is produced in two ways. The first one is the interaction between currents, called armature currents, and a variable-reluctance structure (variable-reluctance machines). The second and more widely used way is the interaction between currents and a magnetic field called the excitation field. The mobile part can be either the armature or the excitation. But as the energy transfer to a moving part creates losses except by electromagnetic way, the armature is preferably stationary.

The torque per unit volume is proportional to the excitation field component perpendicular to the current times the armature ampere-turn loading, as shown below. The armature ampere-turn loading is the total armature current divided by the mean armature circumference. The expression for the maximum torque (rmax) for a three-phase machine is simply:

where

Bo = excitation field component K = armature ampere turn loading ro = mean armature radius L = active length Ns = total series turns per phase kd = winding factor

I = rated armature current

#mach = approximate machine volume

лі*h кг iWh

The excitation field is created either by permanent magnets or by current flow in a winding. Permanent magnets are limited in magnetic induction and are made of very expensive materials, but they are not dissi­pative. They are well suited particularly for small machines (kilowatt range). The currents in a conventional conductor produce heat through the Joule effect (R i2 where R is the resistance and i the current), dissipating energy. The current capacity is hence limited by the ability to remove heat. Better cooling conditions increase the current capacity but reduce the efficiency. The current density (current per unit cross-sectional area) is then limited by thermal and economic factors. The allowable current density in copper is on the order of amperes per square millimeter (5 MA/m2 to 10 MA/m2). With such values the amounts of conductor required to produce magnetic fields without magnetic materials are large, leading to huge Joule losses. Thanks to the peculiar properties of soft magnetic materials (high relative permeability), the total current (ampere turns) required to produce a given magnetic induction is greatly reduced. For this reason practically all electric machines have a magnetic circuit with slots where the windings are embedded.

The armature ampere-turn loading is limited by Joule losses and by the current density allowable in the conductors, because the space they can occupy is limited.

The magnetic circuit has other advantages than the reduction ofthe excitation current. It confines the flux within the machine and reduces the stray field to negligible levels. It also prevents magnetic disturbances to other equipment. The magnetic circuit is also very useful from a mechanical point of view. When the conductors are inserted into slots they are subjected only to a reduced electromagnetic force, since the field concentrates itself in the teeth. The electromagnetic force is mainly applied at the interface between the slots and the magnetic teeth. The torque is then essentially supported by the magnetic circuit and not by the conductors. The reduced mechanical stresses on the conductors are an important advantage, because the mechanical strength of copper is low. In a slotted structure, no special care need be taken in order to reduce the eddy-current losses in the conductors, since they only see low fields. Without magnetic teeth, a strong mechanical support structure must be provided in order to sustain all the electromagnetic torque, and the conductors should follow the finely divided Litz wire configuration to avoid large eddy-current losses. However, the magnetic circuit is heavy, the increased magnetic induction it provides is limited by saturation, and it creates pulsating torques, because the alternation of magnetic teeth and slots produces local magnetic variations. The magnetic teeth also reduce the space available for conductors and thus the armature ampere-turn loading. The slotted structure is also not convenient for insulation, so that the maximum voltage is limited (to about 30 kV).

Superconducting materials show promise for electric machines because they offer the possibility to in­crease both the excitation field and the armature ampere-turn loading (2,3). Superconductors are particularly convenient for producing magnetic fields that are constant in time. Since the current densities in superconduc­tors can be very high (up to a hundred times the allowable value in copper, i. e., hundreds of megamperes per square meter), the required quantity of conductor to produce a given field is greatly reduced from that with conventional conductors, even without the help of magnetic materials. The magnetic circuit is usually nearly removed when using superconductors. Magnetic materials are in general used only to form a magnetic shield in order to avoid large stray fields outside the machine. Current maintenance in a superconducting winding does not cost any energy, due to the absence of losses for constant current and constant external field. The disappearance or large reduction of the magnetic circuit leads to a light and saturation-free structure with

Fig. 1. Schematic cross sections of ac generators. (a) classical; (b) superconducting field winding; (c) fully superconducting.

more active space for conductors and insulation materials. The ampere-turn loading and the voltage can then be increased. The absence of iron teeth will decrease vibration by suppressing torque ripples. Acoustically very quiet electric machines can be designed.

However, the torque is applied directly to the conductors. They must therefore be supported by a suitable structure. An armature without magnetic teeth subjects the conductors to large forces at twice the frequency of rotation, which must be restrained by novel means of support for which high reliability must be main­tained. Figure 1 shows the main differences between a conventional machine and superconducting ones (for synchronous machines).

The weak point of a conventional machine is in general its insulation, which degrades badly with time. It is very sensitive to thermal cycling, and overheating strongly affects its lifetime. A cryogenic system is hence very favorable from this point of view: it almost completely avoids thermal cycling in operation. Moreover, at low temperatures all aging process are slowed down. The cryogenic components of superconducting machines should thus last longer, particularly if the machine remains at low temperature. Numerous thermal cycles from room temperature to cryogenic temperature must be avoided. Furthermore, they are costly in time and energy.

The very high current densities in superconductors make them very attractive for the armature by increasing the ampere-turn loading. However, the armature currents are in general alternating, so that losses appear in the superconductors. This is an important disadvantage in a cryogenic environment. In order to

Bi-2223 PIT 3.5 x 0.35 mm2 (BICC) 36 Bi-2233 filaments. Ag matrix

Fig. 2. Engineering critical characteristics of superconducting materials and wire cross sections.

discuss this point and for the sake of completeness, some information about superconducting wires (materials and ac losses) will be given in the following sections.

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