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

superconducting Superconductivity is a set of physical properties observed in superconductors: materials where electrical resistance vanishes and magnetic fields are expelled from the material. Unlike an ordinary metallic conductor, whose resistance decreases g ...

elements. Since superconductors have no DC resistance, they typically have greater

efficiency Efficiency is the often measurable ability to avoid making mistakes or wasting materials, energy, efforts, money, and time while performing a task. In a more general sense, it is the ability to do things well, successfully, and without waste. ...

. 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, ''T_c'' that is hundreds of degrees lower than room temperature,

cryogenics In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th International Institute of Refrigeration's (IIR) International Congress of Refrigeration (held in Washington, DC in 1971) endorsed a universa ...

are required.

History

DC homopolar machines are among the oldest

electric machine Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...

Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English chemist and physicist who contributed to the study of electrochemistry and electromagnetism. His main discoveries include the principles underlying electromagnetic inducti ...

made a Homopolar motor in 1831.

Superconducting Superconductivity is a set of physical properties observed in superconductors: materials where electrical resistance vanishes and magnetic fields are expelled from the material. Unlike an ordinary metallic conductor, whose resistance decreases g ...

DC homopolar machines use superconductors in their stationary

field winding A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which the field current flows. In a ...

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

General Atomics General Atomics (GA) is an American energy and defense corporation headquartered in San Diego, California, that specializes in research and technology development. This includes physics research in support of nuclear fission and nuclear fusion en ...

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

ship A ship is a large watercraft, vessel that travels the world's oceans and other Waterway, navigable waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research and fishing. Ships are generally disti ...

propulsion. Superconducting

homopolar generator A homopolar generator is a DC electrical generator comprising an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and ...

s have been considered as

pulsed power Pulsed power is the science and technology of accumulating energy over a relatively long period of time and releasing it instantly, thus increasing the instantaneous power. They can be used in some applications such as food processing, water treatme ...

sources for

laser weapon A laser weapon is a type of directed-energy weapon that uses lasers to inflict damage. Whether they will be deployed as practical, high-performance military weapons remains to be seen. One of the major issues with laser weapons is atmospheric ...

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 Superconductivity is a set of physical properties observed in superconductors: materials where Electrical resistance and conductance, electrical resistance vanishes and Magnetic field, magnetic fields are expelled from the material. Unlike an ord ...

when cooled with

liquid helium Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity. At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temp ...

. 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 A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

rotor conductors that exhibit

high-temperature superconductivity High-temperature superconductivity (high-c or HTS) is superconductivity in materials with a critical temperature (the temperature below which the material behaves as a superconductor) above , the boiling point of liquid nitrogen. They are "high- ...

. These have

liquid nitrogen Liquid nitrogen (LN2) is nitrogen in a liquid state at cryogenics, low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, mobile liquid whose vis ...

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 easier to handle, there is a greater interest in the ceramic superconductor machines than the liquid helium cooled metal superconductor machines.

Present interest

Present interest in AC synchronous ceramic superconducting machines is in larger machines like the generators used in utility and ship

power plants A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many power ...

and the

motors An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gene ...

used in ship propulsion. American Superconductor and

Northrop Grumman Northrop Grumman Corporation is an American multinational Aerospace manufacturer, aerospace and Arms industry, defense company. With 97,000 employees and an annual revenue in excess of $40 billion, it is one of the world's largest Arms industry ...

created and demonstrated a 36.5 MW ceramic superconductor ship propulsion motor. Because they are light-weight and therefore offer lower tower and construction costs they are seen as a promising generator technology for

wind turbine A wind turbine is a device that wind power, converts the kinetic energy of wind into electrical energy. , hundreds of thousands of list of most powerful wind turbines, large turbines, in installations known as wind farms, were generating over ...

s. With super conducting generators the weight and volume of generators could be reduced compared to direct drive synchronous generators, which could lead to lower costs of the whole turbine. First commercial turbines were expected to be installed approximately in 2020.''Supraleitende Generatoren: industrielle Fertigung ab 2020''
In: ''Energie und Technik'', 12 May 2015. Retrieved 24 December 2015.

Advantages and disadvantages of superconducting electric machines

Compared with a conventional conductor machine

Superconducting electric machines typically have the following advantages: # Reduced resistive losses but only in the rotor electromagnet. # Reduced size and weight per power capacity without considering the refrigeration equipment. There are also the following disadvantages: # The cost, size, weight, and complications of the cooling system. # A sudden decrease or elimination of motor or generator action if the superconductors leave their superconductive state. # 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. # Motor bearings need to be able to withstand cold or need to be insulated from the cold rotor. # As a synchronous motor, electronic control is essential for practical operation. Electronic control introduces expensive harmonic loss in the supercooled rotor electromagnet.

High-temperature superconductors versus Low-temperature superconductors

High-temperature superconductors High-temperature superconductivity (high-c or HTS) is superconductivity in materials with a critical temperature (the temperature below which the material behaves as a superconductor) above , the boiling point of liquid nitrogen. They are "high ...

(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. # HTS are ceramics, and are fragile relative to conventional metal alloy superconductors such as niobium-titanium. # 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. # 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.

References

External links

*American Superconductor, AC synchronous superconducting ceramic motors and generators http://www.amsc.com/ {{DEFAULTSORT:Superconducting Electric Machine Electric motors Electrical generators