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Superconducting Electric Machine

Superconducting Electric Machines consists of one or more superconductive elements based on which the machines operate. This machine ideally possesses no DC resistance, and hence less loss and high efficiency. Power density of a Superconducting machine quite high because of super conducting elements and hence this machine can deliver high power with its very light weight and small sized construction compared to conventional machine. The most valuable parameter of superconducting machine is that they generate very high magnetic field with less losses so it is able to generate high power at maximum efficiency that is not possible in a conventional machine. In other hand, since superconductors only have zero resistance under a certain superconducting transition temperature (Tc) that is very lower than room temperature, cryogenics are required to achieve low temperature.

The direct current electromagnetic field winding on the rotor uses superconductor but the alternating current set on the stator, which has no practical support by superconductor. Now there is more affection in superconducting AC synchronous electric machines (synchronous generators and synchronous motors).

Construction of Superconductive Synchronous Machines

Superconductive field excitation winding can provide high magnetic flux density in the air gap at zero excitation losses. This field excitation system leads to perform characteristics not achievable so far by classical field excitation system. A permanent magnet motor rated at 7 MW for marine pod propulsor weighs about 120 tones.

A superconductive synchronous motor concept would extremely reduce the weight of a podded electromechanical drive by 50%. By using a superconductor wire for the field excitation winding, the field winding losses If2Rf are practically eliminated, because the field winding resistance Rf = 0. The magnetic flux excited in the stator (armature) winding by the rotor excitation system is not limited by saturation magnetic flux density of the ferromagnetic core; the stator system can be constructed without stator ferromagnetic teeth. Slot-less armature system means that losses in the armature teeth do not exist and the distribution of the stator air gap magnetic flux density waveform can be sinusoidal.

Types of superconductive synchronous machines:

  1. Coreless rotor
  2. Ferromagnetic rotor

Coreless Rotor

A classical superconductive machine has core less rotor with superconductive field excitation winding (Fig.1). Since very high magnetic flux density is excited in the air gap, it provides the highest torque density of all types of electric machines.

The drawback is that the electromagnetic force producing the torque acts directly on the superconductive winding, which has to be transmitted to the room temperature shaft through a low-thermal conductivity torque tube. This type of generator also requires the largest amount of superconductor. Limits on the air gap flux density result from the peak field in the coil define the amount of superconductive wire required. This option is the best suited rather for medium speed machines. Because it offers very high torque density, the difficulty of maintaining superconductive coil integrity at high speeds makes it less attractive for high power density applications at high speeds. Fig. 1. Air core rotor of a superconducting synchronous machine. 1 superconductive coil, 2 torque tube, 3 electromagnetic shield, 4 turbine end shaft, 5 collector end shaft, 6 transfer coupling

Ferromagnetic Rotor

Ferromagnetic core (Fig.2), the electromagnetic torque is taken by the steel or other ferromagnetic alloy, which can be at a temperature of the surrounding air. Because the magnetic permeability of the steel is much higher than the air, the required excitation MMF is significantly lower and consequently less superconductive material is used in rotor excitation system. In practice, a ferromagnetic core limits the air gap magnetic flux density to lower values than in the case of air core generator but magnetic flux density (slightly lower than saturation magnetic flux density) is competitive to that obtained in classical electric machines. Thus, the torque density is sometimes not worse than that developed by air core superconductive generators. Smaller coils and simpler mechanical support allow for increasing the speed and power density. Fig. 2. Rotor with ferromagnetic core of an SC synchronous machine. 1 superconductive coil, 2 rotor body, 3 vacuum enclosure, 4 electromagnetic shield, 5 spacer, 6 oil seal lands, 7 bearing centre, 8 coupling flange, 9 turbine end stub shaft flange, 10 collector end stub shaft flange, 11 collector and cryogenic connections

Design of Rotors for Synchronous Machines

Salient pole ferromagnetic rotor with superconductive field excitation winding is now a typical configuration used in prototypes of superconductive synchronous machines. The construction of a rotor with ferromagnetic core and superconductive coils is sketched in Fig. 3. Pancake superconductive coils are mounted on each pole and connected in series. Coils are often stacked on an aluminum mandrel and potted with epic to from an integral structure. The rotor system is enclosed with thermal and electromagnetic sleeve type shields.


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