Superconducting material set to improve performance of electric motors
A variety of future systems will depend on higher power density, more efficient, compact motors. However, conventional materials and technologies are reaching a limit.
With an increasing demand for multi-megawatt levels of power for advanced transportation and power generation systems, there is a clear need for a 'breakthrough' technology.
The development of higher-temperature superconducting materials is therefore exciting many parts of the engineering and applied science fraternity, and has made remarkable progress during the last several years. The property of superconductivity allows large quantities of electrical current to flow through a material virtually without resistance.
"Efficiency, thermal management and fatigue life are typically sacrificed as conventional electric motors are given higher rotational speeds to reduce size and weight while maintaining a high power output," says Dr Sab Safi, a consultant at SDT Drive Technology. "Simply scaling conventional electric motor technology is not plausible."
The application of superconductors has long been desired by engineers for reducing energy losses and increasing the power density of electric motors. However, despite being proven in the lab, the practicality of superconductivity can thus far only be achieved at extremely low temperatures.
Superconducting properties were originally known only to exist near absolute zero, making it prohibitively expensive and inherently brittle. However, in 1986 eight new materials were found that exhibited superconducting properties at 77K (-196°C), the temperature of liquid nitrogen, which is much easier and less costly to achieve. The energy required to cool to 4K is about 25 times that required to cool to 77K.
Ironically known as high temperature superconductors (HTS), this second generation of superconductor has recently achieved a critical current density of 106Amps/cm2 in small samples of YBa2Cu3O7 (YBCO)-coated conductors, stimulating interest in manufacturing long-length YBCO-coated conductors. With the cost of applications dropping by orders of magnitude, long-length wires are no longer confined to the boundaries of laboratories and academic institutions and the first viable products appear to be within reach.
To contribute to solving the problem, SDT Drive Technology has been working for several years on the development of high power density superconducting motors for propulsion drives and renewable energy. It is producing HTS tapes to improve electric motors and generators, and it is keen to form strategic partnerships to develop the technologies further.
"The next step is developing the design techniques and manufacturing processes," says Dr Safi. "This leads on to commercial applications such as electric vehicles which are demanding ever higher power density in motors and generators."
The application of HTS in machine design has been mainly concentrated in the MW range where the increased costs incurred by using superconducting technology can be balanced by the reduction of motor volume and increased efficiency.
"Advances have already been made in the development of HTS electric motors and related subsystems for ship thrusters," says Dr Safi. "It is inevitable that attention will eventually turn to HTS materials in motors and generators, particularly in applications where mass and bulk need to be minimised, such as marine propulsion systems and wind turbines.
"The use of HTS wire in rotating machines provides us with significant competitive advantages by enabling reductions in size, weight and manufacturing costs."
Several companies are actively developing super machines. These include AMSC, Rockwell Automation, Siemens and General Electric. AMSC is to develop a 36MW HTS motor for naval propulsion which is based on 5MW technology and experience.
There is also interest from wind turbine manufacturers that has resulted in larger permanent magnet-based hybrid and direct drive generators being produced. However, these configurations are also reaching a practical limit on what is possible, with the current technology likely to reach a limit between 6-8 MW.
"HTS coils are today able to carry more than 100-150 times the current of a conventional copper wire of similar size," says Dr Safi. "However, the refrigeration and overall reliability for the cryogenic support system needs to be carefully designed and optimised and not drive the installed cost for the complete system."
The application of superconductivity still retains the basic configuration and operation of conventional AC motors. A superconducting magnet creates a magnetic field high enough that iron teeth are not needed to enhance the magnetic flux, either in the rotor or the stator. This means that the current densities in the active regions are not limited by iron saturation. The stator only requires the use of back iron acting primarily as a shield to keep magnetic flux inside the machine.
A resulting HTS air-core configuration, with high flux density, is significantly smaller and lighter than conventional AC synchronous motors. The lack of iron teeth in the rotor and stator eliminates the need for winding slots, reducing cogging torques that lead to radiated noise. This also decreases the inherent armature [stator] reactance, resulting in improved dynamic performance.
The lack of iron leaves more room in the stator and rotor structure for winding conductors, increasing the power density and efficiency. It also serves to lower synchronous and sub-transient reactance, resulting in lower torque angles during operation and improving its transient stability. The HTS motor harmonic content is nearly zero because the machine does not use saturated iron in the stator.
"Despite advances the cost of HTS wires still needs to be reduced," says Dr Safi. "It is imperative that resources remain focused on creating a robust, high-performance HTS wire to develop reliable and efficient electric power equipment. The basic cost of materials has decreased by a factor of 1000 over the past 10 years, so cost and performance trends are very promising."
The problem with LTS Wire
When a superconductor's temperature exceeds its operating level, quenching can result. Quenching occurs when the material loses its superconducting properties. In order to prevent quenching, LTS wire has to be operated at a very low temperature (4K), which requires expensive and complex cooling systems, preventing further development for any propulsion applications.
The advantage of HTS Wire
First generation (1G) HTS BSCCO wires are made of Bismuth 2212 and 2223. The wires are much smaller than traditional copper wires yet when kept at the right operating temperature handle much larger current. HTS wire is more resistant to quenching as it can absorb more heat energy without a drastic increase in temperature.
Also, HTS wire loses its current carrying capacity gradually. This advantage makes it possible to use refrigeration and cooling systems that are less complex and expensive. The 1G HTS wire is still expensive and needs to be cooled to 20-30K to carry high currents in high magnetic fields. Since superconducting, electric machines may need fields as high as 5 Tesla, the 2G of HTS YBCO is preferred. YBCO has excellent behavior at liquid nitrogen temperatures (65-77K) and can support high currents in magnetic fields up to several Tesla.
Download a paper by Dr Sab Safi of SDT Drives below on 'Alternative Motor Technologies for Traction Drives of Hybrid and Electric Vehicles'
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