Researchers at Tokai University developed a brushless DC motor that coverts electric power to motor output at a conversion efficiency of more than 96%.
The announcement was made at the 56th Spring Meeting of the Japan Society of Applied Physics, which took place from March 30 to April 2, 2009, at the University of Tsukuba in Ibaraki Prefecture, Japan.
"The idea to improve the efficiency can be applied to high-output motors for electric vehicles," said Hideki Kimura, professor at the Department of Electrical and Electronic Engineering, School of Engineering at Tokai University.
The DC motor features a "rated output of about 100W" and uses iron-based amorphous metal for the motor core (See related article). The conversion efficiency is as high as 96.5% when the output is around 100W.
"It exceeds 96% even if measurement error is taken into consideration," Kimura said.
Another notable feature of the motor is its capability to maintain a conversion efficiency in the range from 93% to 96% even when the output changes between 50W and 200W, in addition to its high maximum efficiency.
In 2003, Tokai University developed a DC motor with a conversion efficiency of 93%. And it was commercialized by the University, Tokushu Denso Co Ltd and Nippon Kemikon Co Ltd.
This time, the high efficiency motor was realized by tracking down the causes of energy loss and making some improvements to reduce the loss. Specifically, motor energy loss is attributable to (1) power consumption of the control circuit (controller loss), (2) loss from the coil winding (copper loss), (3) loss due to current surge in the core (iron loss) and (4) loss from rotation axis friction and air resistance (mechanical loss, windage loss).
The high efficiency of more than 96% was achieved mainly by improving (1) and (2). Specifically, a microcomputer featuring a low power consumption of 156mW was employed to reduce the controller loss. In addition, an inverter composed of nMOSFETs alone was used. This is because the on-state resistance of nMOSFET is lower than that of pMOSFET, Kimura said.
In respect to the copper loss, it was reduced by optimizing the thickness of the coil winding and the winding number. Copper loss generally increases as the current supplied to the motor becomes larger. Therefore, the reduction significantly contributes to the improvement, he said.
The iron loss was reduced by using iron-based amorphous metal as the core material. This is because amorphous metal has low electron mobility, resulting in less current surge, Kimura said. However, the same material was used in the motor developed in 2003.
As for the windage loss, "We have confirmed that it can be reduced by simply blocking the air passage by placing a cover over the motor," he said.
One issue with this motor, which features a very high efficiency, is its high production cost. Because of this problem, its applications are currently limited to products that require a high efficiency regardless of the high cost, such as eco cars for racing. In fact, an eco car equipped with the product developed in 2003 once won a race.
"(However,) the cost is nearly 20 times higher than that of commonly-used motors with efficiencies ranging from 80 to 89%," Kimura said.
The high cost is attributable to the iron-based amorphous core material.
"The iron-based amorphous core is produced by rapidly quenching the metal by spraying it on a low-temperature drum, but one round of quenching can only form a thickness of 25μm," he said. "Therefore, it has to be repeated to attain the necessary thickness, resulting in the high cost."
However, the cost may be reduced to 1/4 or 1/5 if the production process is automated for mass production because the iron material itself is not expensive.
Setting aside the high cost, this motor has a wide range of applications, including refrigerators, electric bicycles and electric motorbikes, judging from its output and speed, Kimura said.
"(The output can be further improved) simply by increasing the size of the motor," he said. "And if the output is improved, it will be even easier to enhance the efficiency."
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