Superconducting electric machineFrom Wikipedia, the free encyclopedia

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Superconducting electric machines are electromechanical systems that rely on the use of one or

more superconducting elements. Since superconductors have no DC resistance, they typically have

greater efficiency. The most important parameter that is of utmost interest in superconducting machine

is the generation of a very high magnetic field that is not possible in a conventional machine. This

leads to a substantial decrease in the motor volume; which means a great increase in the power

density. However, since superconductors only have zero resistance under a certain superconducting

transition temperature, Tc that is hundreds of degrees lower than room temperature, cryogenics are

required.

Now there is more interest in superconducting AC synchronous electric machines

(alternators and synchronous motors). The direct current electromagnet field winding on

the rotor (rotating member) use superconductors but the alternating current multiphase winding set on

the stator (stationary members), which have no practical support by superconductors, uses

conventional, normal conductioncopper conductors. Often the stator conductors are cooled to reduce,

but not eliminate, their resistive losses.

Contents

  [hide] 

1   History

2   Present interest

3   Advantages and disadvantages of superconducting electric machines

o 3.1   Compared with a conventional conductor machine

o 3.2   High-temperature superconductors versus Low-temperature superconductors

4   References

5   Links

[edit]History

DC homopolar machines are among the oldest electric machines. Michael Faraday made one in 1831.

[citation needed] SuperconductingDC homopolar machines use superconductors in their stationary field

windings and normal conductors in their rotating pickup winding. In 2005 the General

Atomics company received a contract for the creation of a large low speed superconducting homopolar

motor forship propulsion. Superconducting homopolar generators have been considered as pulsed

power sources for laser weapon systems. However, homopolar machines have not been practical for

most applications.

In the past, experimental AC synchronous superconducting machines were made with rotors using

low-temperature metal superconductors that exhibit superconductivity when cooled with liquid helium.

These worked, however the high cost of liquid helium cooling made them too expensive for most

applications.

More recently AC synchronous superconducting machines have been made with ceramic rotor

conductors that exhibit high-temperature superconductivity. These have liquid nitrogen cooled ceramic

superconductors in their rotors. The ceramic superconductors are also called high-temperature or

liquid-nitrogen-temperature superconductors. Because liquid nitrogen is relatively inexpensive and

easy to handle, there is a greater interest in the ceramic superconductor machines than the liquid

helium cooled metal superconductor machines.

[edit]Present interest

Present interest in AC synchronous ceramic superconducting machines is in larger machines like

the generators used in utility and ship power plants and the motors used in ship propulsion. American

Superconductor and Northrup Grumman created and demonstrated a 36.5 MW ceramic

superconductor ship propulsion motor.

[edit]Advantages and disadvantages of superconducting electric machines

[edit]Compared with a conventional conductor machine

Superconducting electric machines typically have the following advantages:

1. Reduced resistive losses but only in the rotor electromagnet.

2. Reduced size and weight per power capacity without considering the refrigeration equipment.

There are also the following disadvantages:

1. The cost, size, weight, and complications of the cooling system.

2. A sudden decrease or elimination of motor or generator action if the superconductors leave

their superconductive state.

3. A greater tendency for rotor speed instability. A superconducting rotor does not have the

inherent damping of a conventional rotor. Its speed may hunt or oscillate around its

synchronous speed.

4. Motor bearings need to be able to withstand cold or need to be insulated from the cold rotor.

5. As a synchronous motor, electronic control is essential for practical operation. Electronic

control introduces expensive harmonic loss in the supercooled rotor electromagnet.

[edit]High-temperature superconductors versus Low-temperature superconductors

1. High-temperature superconductors (HTS) become superconducting at more easily obtainable

liquid nitrogen temperatures, which is much more economical than liquid helium that is

typically used in low-temperature superconductors.

2. HTS are ceramics, and are fragile relative to conventional metal alloy superconductors such

as niobium-titanium.

3. Ceramic superconductors cannot be bolted or welded together to form superconducting

junctions. Ceramic superconductors must be cast in their final shape when created. This may

increase production costs.[citation needed]

4. Ceramic superconductors can be more easily driven out of superconductivity by oscillating

magnetic fields. This could be a problem during transient conditions, as during a sudden load

or supply change.[citation needed]

[edit]References

Bumby, J. R., Superconducting Rotating Electrical Machines, Oxford: Clarendon Press, 192

pages, 1983.

Kuhlmann, J. H., Design of Electrical Apparatus, 3rd edition; New York: John Wiley & Sons,

Inc., 512 pages, 1950. <Note, this book does not consider superconducting machines. However, it

provides excellent detailed design information that could be used when designing a

superconducting machine.>

Tubbs, S. P., Design and Analysis of a Superconducting High Speed Synchronous/Induction

Motor, ProQuest Direct Complete Database, Publication No. AAT LD03278, 227 pages, 1995.

<Literature evaluation, analysis, experimental results, and a large bibliography.>

[edit]Links

American Superconductor, AC synchronous superconducting ceramic motors and

generators http://www.amsc.com/

General Atomics, DC homopolar superconducting motor http://atg.ga.com/EM/defense/dc-

motor/index.php