Superconducting electric machine

Lore & Background

DC homopolar machines are among the oldest electric machines; Michael Faraday made a homopolar motor in 1831. Superconducting DC homopolar machines use superconductors in their stationary field windings and normal conductors in their rotating pickup winding. In 2005, General Atomics received a contract to create a large low-speed superconducting homopolar motor for ship propulsion. Superconducting homopolar generators have been considered as pulsed power sources for laser weapon systems, but homopolar machines have not been practical for most applications. Experimental AC synchronous superconducting machines were initially made with rotors using low-temperature metal superconductors cooled with liquid helium, but the high cost of liquid helium 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, cooled with liquid nitrogen. Because liquid nitrogen is relatively inexpensive and easier to handle, there is greater interest in ceramic superconductor machines than in liquid-helium-cooled metal superconductor machines.

Reader's Guide

Present interest in AC synchronous ceramic superconducting machines focuses on larger machines such as generators for utility and ship power plants and motors for ship propulsion. American Superconductor and Northrop Grumman created and demonstrated a 36.5 MW ceramic superconductor ship propulsion motor. Because these machines are lightweight, they offer lower tower and construction costs and are seen as a promising generator technology for wind turbines, potentially reducing the weight and volume of generators compared to direct-drive synchronous generators and lowering the cost of the whole turbine. First commercial turbines were expected to be installed approximately in 2020. Compared with conventional conductor machines, superconducting electric machines have reduced resistive losses (only in the rotor electromagnet) and reduced size and weight per power capacity (without considering refrigeration equipment). Disadvantages include the cost, size, weight, and complications of the cooling system; a sudden decrease or elimination of motor or generator action if superconductors leave their superconductive state; a greater tendency for rotor speed instability due to lack of inherent damping; the need for bearings to withstand cold or be insulated from the cold rotor; and the requirement for electronic control, which introduces expensive harmonic loss in the supercooled rotor electromagnet. High-temperature superconductors (HTS) become superconducting at more easily obtainable liquid nitrogen temperatures, which is much more economical than liquid helium used for low-temperature superconductors. However, HTS are ceramics and are fragile relative to conventional metal alloy superconductors such as niobium-titanium; they cannot be bolted or welded together to form superconducting junctions and must be cast in their final shape, which may increase production costs. Ceramic superconductors can also be more easily driven out of superconductivity by oscillating magnetic fields, which could be a problem during transient conditions such as a sudden load or supply change.

Did You Know?

• Michael Faraday made a homopolar motor in 1831, one of the oldest electric machine designs.
• Ceramic superconductors cannot be bolted or welded together; they must be cast in their final shape.

Historical Foundations and Key Inventions

The roots of electric machines trace back to the early 1800s, when Michael Faraday's investigations into the interplay between electricity and magnetism laid the groundwork. A landmark demonstration came in 1821, where a suspended wire immersed in mercury, with a permanent magnet resting on it, rotated when current was applied—revealing that electric current produces a circular magnetic field and can convert electrical energy into motion. Throughout the 19th century, refinements continued, yet without an established power grid, these devices struggled to find practical adoption. The landscape shifted dramatically as the first electric grids emerged toward the century's end. Werner von Siemens introduced the dynamo in 1867, and Frank Sprague delivered the first practical DC motor in 1886. The transition from DC to AC power systems, known as the war of currents, reshaped the field: alternators gradually supplanted dynamos, while AC motors remained elusive until Nikola Tesla's induction motor design enabled them to displace DC motors on a large scale.

Governing Physics and Operating Principles

At their core, electric machines exploit the mutual relationship between electricity and magnetism—changes in one domain produce changes in the other. While the full mathematical description draws on Maxwell's equations and can be analytically demanding, nearly all machines obey four foundational principles: the Lorentz force (current in a magnetic field produces force), Faraday's law of induction (motion within a field induces voltage), Kirchhoff's voltage law (voltages around a closed loop sum to zero), and Newton's second law (force equals mass times acceleration). In operation, current flowing through a magnetic field generates force and motion; that motion, still within the field, induces a voltage that feeds back into the current, which in turn alters the force and speed. This self-correcting feedback loop drives the machine toward an equilibrium where electrical and mechanical energies balance, accounting for losses. With proper alignment of fields, conductors, voltages, and currents, the device can convert energy in either direction.

Global Energy Significance

The scale of electric machines in modern civilization is staggering. As of the early 2020s, synchronous and induction generators account for roughly 95 percent of all electrical power produced on the planet, while electric motors consume approximately 60 percent of the electricity generated. Developed in the mid-19th century, these electromechanical converters have since become indispensable components of global electric infrastructure. Their role spans both generation and consumption, making them central to how humanity produces and uses energy. Because of this dominance, advancing the efficiency of electric machine technology is not merely an engineering challenge but a strategic imperative for global energy conservation, the transition to green energy, and the broader adoption of alternative power sources. Whether rotating or linear in their motion, these machines sit at the intersection of physics, industry, and sustainability, and their continued improvement will shape the trajectory of the world's energy systems for decades to come.

Construction, Classification, and Terminology

Electric machines are typically divided into a stationary section called the stator and a moving section called the rotor in the case of rotating designs. Windings—coils of wire—may be placed on either section to carry current or to establish the magnetic field. The winding that carries current is termed the armature winding, while the one that creates the field is the field winding; all rotating machines include an armature winding, but a field winding is optional when a permanent magnet supplies the field. These windings are rarely located on the same physical part. Machines are further classified by function: those converting mechanical energy to electrical energy are generators (dynamos for DC, alternators for AC), while those converting electricity to motion are motors, sometimes called pumps when driving fluid flow. In theory most machines can serve either role, though practice favors specialization. A notable subtlety exists in DC machines: despite the name, they internally rely on alternating voltages and currents, with a commutator periodically reversing armature connections to maintain a DC interface externally.