Hitachi Prototypes High-efficient Motor without Rare-earth Magnet

Nov 12, 2008
Tadashi Nezu, Nikkei Electronics
Iron cores made of amorphous metal. The one on the left is coiled.
Iron cores made of amorphous metal. The one on the left is coiled.
[Click to enlarge image]
The structure of the prototyped motor: Two ferrite magnet rotors are positioned face-to-face.
The structure of the prototyped motor: Two ferrite magnet rotors are positioned face-to-face.
[Click to enlarge image]

Hitachi Ltd and Hitachi Industrial Equipment Systems Co Ltd have prototyped a motor that uses iron cores made of amorphous metal. The high-efficiency motor does not use magnets made of rare earth metals (rare-metal magnets) such as Neodymium (Nd) or Dysprosium (Dy).

The efficiency was 86% for a prototype of a 150W motor, which is usually used in air conditioners. The efficiency of Hitachi's existing 150W motors that use rare-metal magnets was 81%, according to the companies. So, The efficiency was proved to have increased by five points.

Motors that use amorphous metals are expected to be adopted in a wide range of fields including industrial equipment, consumer electronics and automobiles.

Currently, motors consume about half of the total power consumption, and improvement in motor efficiency is indispensable for energy saving. To achieve higher efficiency, rare-metal magnets that provide high performance are currently used for motors.

Especially, Nd-Fe-B (Neodymium-Iron-Boron) magnets (so-called Nd-based magnets) are most widely used. In order to retain high performance in high temperature, Dysprosium is added in Nd-based magnets.

Though motor efficiency can be improved by employing rare-metal magnets, the escalation in prices of rare earths, which are used as raw materials, is a big issue.

"Prices are now more stable after they rapidly increased in the past, but they are still expensive," Hitachi Ltd said.

The per kg price of Nd and Dy have increased threefold to US$30 and fivefold to US$155, respectively, during the period from September 2004 to September 2008.

The prices remain at a higher level because 90% of rare earths are produced in China. And the prices of rare earths are rising because China is changing from a producing country to a consuming country. For these reasons, Hitachi and other companies have been trying to develop motors that do not use rare earths.

Smaller loss

This time, the motor efficiency was improved without using Nd-based magnets because of the small loss of amorphous metal and a new motor structure.

Coiled iron cores, which function as electromagnets when electric current is applied, rotate the permanent magnets that work as rotors and generate a turning force in the motor. In existing motors, magnetic steel sheets are used as iron cores and Nd-based magnets are used as rotors.

For the prototyped motor, amorphous metal was used for the iron cores, allowing the use of ferrite magnets as rotors. This is because the magnetic permeability of amorphous metal is ten times higher than that of magnetic steel sheets, while its energy loss is about 1/10 that of magnetic steel sheets. When the magnetic permeability of the iron cores is enhanced, a high magnetic flux density can be achieved even with a weak magnet.

New iron core structure

The biggest problem of amorphous metal lies in the difficulty in cutting and machining of the material. This is because it is thinner and harder than a magnetic steel sheet.

When magnetic steel sheets are used, the cores are made by laminating the sheets as thin as 0.35 to 0.5mm to prevent overcurrent that leads to the loss. Meanwhile, the amorphous metal is 25μm in thickness, and it is more than four times harder than magnetic steel sheets when compared in Vickers hardness (HV).

As a countermeasure, changes have been made in the structure and manufacturing method of the iron cores. Hitachi and Hitachi Industrial Equipment Systems applied technologies developed for transformers that use amorphous metal to manufacture the iron cores formed by many thin layers of amorphous metal.

In addition to the utilization of amorphous metal, the motor structure was optimized through 3D magnetic analysis, not the previously used 2D magnetic analysis.

Initially, the goal is to commercialize the product in three years for use in industrial equipment. The companies are planning to further improve the reliability and reduce the production cost. They will also enhance the output of the motor to expand the ranges of application.

"We will be able to manufacture 10kW class motors," Hitachi Ltd said.