Superconducting electric machine
Encyclopedia
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
. Furthermore, the higher current density
achievable in many superconducting materials tends to make them smaller compared with non-superconducing machines with the same 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 (alternator
s and synchronous motor
s). 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 conduction copper
conductors. Often the stator conductors are cooled to reduce, but not eliminate, their resistive losses.
homopolar machines are among the oldest electric machine
s. Michael Faraday
made one in 1831. Superconducting DC 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
for ship
propulsion. Superconducting homopolar generator
s 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.
used in ship propulsion. American Superconductor
and Northrup Grumman created and demonstrated a 36.5 MW ceramic superconductor ship propulsion motor.
There are also the following disadvantages:
High-temperature superconductors
Electromechanics
In engineering, electromechanics combines the sciences of electromagnetism, of electrical engineering and mechanics. Mechanical engineering in this context refers to the larger discipline which includes chemical engineering, and other related disciplines. Electrical engineering in this context...
that rely on the use of one or more superconducting elements. Since superconductors have no DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
, they typically have greater efficiency
Efficiency
Efficiency in general describes the extent to which time or effort is well used for the intended task or purpose. It is often used with the specific purpose of relaying the capability of a specific application of effort to produce a specific outcome effectively with a minimum amount or quantity of...
. Furthermore, the higher current density
Current density
Current density is a measure of the density of flow of a conserved charge. Usually the charge is the electric charge, in which case the associated current density is the electric current per unit area of cross section, but the term current density can also be applied to other conserved...
achievable in many superconducting materials tends to make them smaller compared with non-superconducing machines with the same 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
Cryogenics
In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...
are required.
Now there is more interest in superconducting AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
synchronous electric machines (alternator
Alternator
An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.Most alternators use a rotating magnetic field but linear alternators are occasionally used...
s and synchronous motor
Synchronous motor
A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the power supply frequency and resulting rotating magnetic field which drives it....
s). The direct current electromagnet field winding on the rotor (rotating member) use superconductors but the alternating current multiphase winding set on the stator
Stator
The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...
(stationary members), which have no practical support by superconductors, uses conventional, normal conduction copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
conductors. Often the stator conductors are cooled to reduce, but not eliminate, their resistive losses.
History
DCDirect current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
homopolar machines are among the oldest electric machine
Electric machine
The academic study of electric machines is the universal study of electric motors and electric generators. By the classic definition, electric machine is synonymous with electric motor or electric generator, all of which are electro-mechanical energy converters: converting electricity to mechanical...
s. Michael Faraday
Michael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....
made one in 1831. Superconducting DC homopolar machines use superconductors in their stationary field windings and normal conductors
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
in their rotating pickup winding. In 2005 the General Atomics
General Atomics
General Atomics is a nuclear physics and defense contractor headquartered in San Diego, California. General Atomics’ research into fission and fusion matured into competencies in related technologies, allowing the company to expand into other fields of research...
company received a contract for the creation of a large low speed superconducting homopolar motor
Homopolar motor
A homopolar motor is an electric motor that works without the need for a commutator, by rotating along a fixed axis that is parallel to the external magnetic field produced by a permanent magnet. The name homopolar indicates that the electrical polarity of the motor does not change...
for ship
Ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...
propulsion. Superconducting homopolar generator
Homopolar generator
A homopolar generator is a DC electrical generator comprising an electrically conductive disc rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim, the electrical polarity depending on the direction of...
s have been considered as pulsed power
Pulsed power
Pulsed power is the term used to describe the science and technology of accumulating energy over a relatively long period of time and releasing it very quickly thus increasing the instantaneous power.-Overview:...
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
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
when cooled with liquid helium
Liquid helium
Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...
. 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
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
rotor conductors that exhibit high-temperature superconductivity
High-temperature superconductivity
High-temperature superconductors are materials that have a superconducting transition temperature above . From 1960 to 1980, 30 K was thought to be the highest theoretically possible Tc...
. These have liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...
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.
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 motorsElectric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
used in ship propulsion. American Superconductor
American Superconductor
American Superconductor is an American energy technologies company based in Devens, Massachusetts specializing in the design and manufacture of power systems and superconducting wire...
and Northrup Grumman created and demonstrated a 36.5 MW ceramic superconductor ship propulsion motor.
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 superconductorsHigh-temperature superconductivityHigh-temperature superconductors are materials that have a superconducting transition temperature above . From 1960 to 1980, 30 K was thought to be the highest theoretically possible Tc...
versus Low-temperature superconductorsConventional superconductorConventional superconductors are materials that display superconductivity as described by BCS theory or its extensions.Critical temperatures of some simple metals:ElementTc Al1.20Hg4.15Mo0.92Nb9.26Pb7.19...
- High-temperature superconductorsHigh-temperature superconductivityHigh-temperature superconductors are materials that have a superconducting transition temperature above . From 1960 to 1980, 30 K was thought to be the highest theoretically possible Tc...
(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-titaniumNiobium-titaniumNiobium-titanium is an alloy of niobium and titanium, used industrially as a type II superconductor wire for superconducting magnets...
. - 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.
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