Has a generator been designed or built using superconducting materials?

Postpocelipse said:

Has a generator been designed or built using superconducting materials?

Very good question. I don’t know. Superconducting materials have been used in making an electrical motor, which is very like a generator.

If I remember correctly, superconducting machines are limited by the maximum electric current density and maximum magnetic field that they can sustain before the superconductivity is lost.

Hmm, I thought that the magnetic field was contained within a superconducctor, if so I’m not sure a geneator could be constructed, and could be well wrong.

OTOH, why would you bother? Generators are already quite efficient and a SC can be better uttilised.

> I thought that the magnetic field was contained within a superconductor

No. All the big magnets for the CERN Large Hadron Collider are superconducting.

> Generators are already quite efficient

I don’t call 75% quite efficient.

mollwollfumble said:

>No. All the big magnets for the CERN Large Hadron Collider are superconducting.

Very good point. I withdraw my protest.

>I don’t call 75% quite efficient.

Neither would I. Thank gawd the alternators are no where near that ineffeceint. When I stated “generators” I really meant the alternator section, rather than the mechanical drive component.

I guess it could improve efficiency. OTOH, keeping the superconductor cold will consume energy, so the nett efficiency may very well decrease.

Superconducting magnet

A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire can conduct much larger electric currents than ordinary wire, creating intense magnetic fields. Superconducting magnets can produce greater magnetic fields than all but the strongest electromagnets and can be cheaper to operate because no energy is dissipated as heat in the windings. They are used in MRI machines in hospitals, and in scientific equipment such as NMR spectrometers, mass spectrometers and particle accelerators.

[…]

Magnet quench

A quench is an abnormal termination of magnet operation that occurs when part of the superconducting coil enters the normal (resistive) state. This can occur because the field inside the magnet is too large, the rate of change of field is too large (causing eddy currents and resultant heating in the copper support matrix), or a combination of the two. More rarely a defect in the magnet can cause a quench. When this happens, that particular spot is subject to rapid Joule heating, which raises the temperature of the surrounding regions. This pushes those regions into the normal state as well, which leads to more heating in a chain reaction. The entire magnet rapidly becomes normal (this can take several seconds, depending on the size of the superconducting coil). This is accompanied by a loud bang as the energy in the magnetic field is converted to heat, and rapid boil-off of the cryogenic fluid. The abrupt decrease of current can result in kilovolt inductive voltage spikes and arcing. Permanent damage to the magnet is rare, but components can be damaged by localized heating, high voltages, or large mechanical forces. In practice, magnets usually have safety devices to stop or limit the current when the beginning of a quench is detected. If a large magnet undergoes a quench, the inert vapor formed by the evaporating cryogenic fluid can present a significant asphyxiation hazard to operators by displacing breathable air. A large section of the superconducting magnets in CERN’s Large Hadron Collider unexpectedly quenched during start-up operations in 2008, necessitating the replacement of a number of magnets. Although undesirable, a magnet quench is a “fairly routine event within a particle accelerator”.

Note that superconductors tend to expel magnetic fields.

Meissner effect

The Meissner effect is an expulsion of a magnetic field from a superconductor during its transition to the superconducting state. The German physicists Walther Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin and lead samples. The samples, in the presence of an applied magnetic field, were cooled below their superconducting transition temperature. Below the transition temperature the samples cancelled nearly all interior magnetic fields. They detected this effect only indirectly because the magnetic flux is conserved by a superconductor: when the interior field decreases, the exterior field increases. The experiment demonstrated for the first time that superconductors were more than just perfect conductors and provided a uniquely defining property of the superconducting state.

Explanation

In a weak applied field, a superconductor “expels” nearly all magnetic flux. It does this by setting up electric currents near its surface. The magnetic field of these surface currents cancels the applied magnetic field within the bulk of the superconductor. As the field expulsion, or cancellation, does not change with time, the currents producing this effect (called persistent currents) do not decay with time. Therefore the conductivity can be thought of as infinite: a superconductor.

Near the surface, within a distance called the London penetration depth, the magnetic field is not completely cancelled. Each superconducting material has its own characteristic penetration depth.

Any perfect conductor will prevent any change to magnetic flux passing through its surface due to ordinary electromagnetic induction at zero resistance. The Meissner effect is distinct from this: when an ordinary conductor is cooled so that it makes the transition to a superconducting state in the presence of a constant applied magnetic field, the magnetic flux is expelled during the transition. This effect cannot be explained by infinite conductivity alone. Its explanation is more complex and was first given in the London equations by the brothers Fritz and Heinz London. It should thus be noted that the placement and subsequent levitation of a magnet above an already superconducting material does not demonstrate the Meissner effect, while an initially stationary magnet later being repelled by a superconductor as it is cooled through its critical temperature does.
[…]

Consequences

The discovery of the Meissner effect led to the phenomenological theory of superconductivity by Fritz and Heinz London in 1935. This theory explained resistanceless transport and the Meissner effect, and allowed the first theoretical predictions for superconductivity to be made. However, this theory only explained experimental observations—it did not allow the microscopic origins of the superconducting properties to be identified.

A magnet levitating above a superconductor cooled by liquid nitrogen.