PM motors go rare-earth free

One way to eliminate the need for rare-earth magnets in motors is to use amorphous steel. That is the tack taken by Hitachi in developing a 11-kW highly efficient permanent magnet synchronous motor without using magnetic material containing rare-earth metals such as neodymium and dysprosium.

The motor is an axial air gap type, where the rotor does not sit inside the stator but lies adjacent to it. Magnetic flux passes across the narrow air gap that separates the stationary and moving components of the motor.

Though Hitachi has released few details about the motor architecture, apparently the amorphous steel goes into the stator. Hitachi says compared to conventional motors of the same class, its new motor is smaller and delivers an energy efficiency of about 93%. In a comparison photo, the company puts its new motor next to a commercial motor that seems to be an an IEC 160 frame.

Hitachi claims it wants to launch a product using the amorphous steel technology in FY2014. Indications are that if this actually happens, it will be a good trick. Motor manufacturers we've checked with say amorphous steel costs about 100x that of electrical-grade steels on a per-pound basis.

Amorphous metal is generally created by quickly cooling iron in a special way so it forms in a non-crystalline structure. This gives the material some glass-like properties. Amorphous metals tend to be stronger than crystalline alloys of similar chemical composition because they have no crystal defects that limit the strength of conventional metals. They can also sustain larger reversible ("elastic") deformations than crystalline alloys.

Hitachi gives no details about how it is creating the amorphous steel it is using. But is says the iron-based amorphous metal it created has loss characteristics that are about one-tenth that of magnetic sheet material used in conventional stators. It says the process it created for making the amorphous metal improves the deterioration due to processing which generally happens in iron-based amorphous metals.

The company also says it had to use some special tricks to optimize the structure of the axial gap motor to efficiently use low-magnetic ferrite material and efficiently draw out the low-energy loss characteristic of the amorphous metal. Hitachi says its axial gap motor architecture allows the use of a greater amount of ferrite magnet to dramatically increase the magnetic energy available. To do this, it had to develop highly resilient stator and rotor structures which could endure a large torque and centrifugal force.

Finally , Hitachi says it used three-dimensional magnetic field analysis and three dimensional thermal analysis technologies to analyze the ferrite magnet rotors and amorphous stratified cores.

The original Hitachi press release:

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