Story 1
For MIRAI—For the Future
Chapter 3

Improving Responsiveness—Develop a Small Yet Powerful Motor!

Noriyuki Suzuki

Group No. 111
Compressor Division

A Small Motor, but Powerful

The air compressor compresses air and sends it to the fuel cell, and in itself, is driven by a motor—the performance of which has a major impact on the performance of the air compressor.

Having joined the company in 2009, the still-young Noriyuki Suzuki was responsible for development of this motor.

"An air compressor needs to be very responsive. This is measured by how long it takes to reach maximum output. Increasing responsiveness reduces the amount of time it takes for the vehicle to achieve its target speed. Basically, the compressor plays a key factor in how well the vehicle accelerates."

The pressure and flow rate of air sent by the air compressor changes depending on how much the driver depresses the accelerator. Pressing hard on the accelerator will increase the pressure and flow rate, increasing vehicle acceleration. The motor in the air compressor may not actually drive the wheels, but in terms of performance plays just as important a role. Of course, a smaller, higher-output motor will increase responsiveness and power, but with a marked increase in cost.

"Our mission was to create a motor of the same size, but with higher torque. The most effective way to achieve this was to increase the efficiency of the magnetic flux*11 flow."

The motor comprises a stator incorporating a coil, inside which a magnetic rotor turns. A motor rotates, generating power by using the properties of magnets—different polarities attract, while similar polarities repel. Put simply, optimizing the magnetic flux means to increase the force with which the magnets attract and repel, thus increasing their power. To do this, we focused on the lines of magnetic flux from the magnets, and altered the rotor configuration so that all of the magnetic flux could be used to provide motive power. Mr. Suzuki had been working steadily, but here ran into a major problem.

*11: Magnetic flux
Lines indicating the strength and direction of a magnetic field

Controlling Heat

"One customer requirement was that the motor could keep rotating for a specified time, at high torque, and at high-speed. This tested whether a motor could handle a steep slope called "Baker's Grade,"*12 to the east of Los Angeles in the United States. However, this test generated heat in the small motor, and it lost power. If a motor heats up, and reaches a specified temperature, then degaussing occurs—in this phenomenon, the magnets lose some of their power. When this happens, the magnets in the motor lose power, and the power produced by the motor drops."

The problem faced by Mr. Suzuki was that of heat. Solving this problem called for measures to reduce this. The stator is cooled with water, but water cooling cannot be used for the rotor. The basic problem is that the motor is a sealed unit, which by its very nature makes it difficult for heat to escape.

Mr. Suzuki and his team came up with and experimented with a range of ideas. These included separating the magnets to minimize losses; letting air flow through the core for cooling; and selecting materials more resistant to heating. These measures showed promise in solving the heat problem. However, a new problem arose.

"The problems we were confronted with just kept on appearing. One of these was controlling fluctuations in motor output. Output fluctuations refer to pulsating torque, and if this is high, can lead to irregularities in rotation, and increased noise."

Ironically, trying to solve this problem (optimizing the magnetic flux lines, and increasing torque output) meant that they changed the rotor configuration, but this resulted in increased torque pulsation. Having achieved the desired torque, they then had to do something about that pulsation. Mr. Suzuki must have been under a lot of stress at that time.

*12: Baker's Grade
A 5.5% climb in the Mojave Desert that continues for 32 km.

Great Challenges Lead to Great Results

They finally managed to overcome these challenges, and perfect a small, lightweight, quiet, and powerful motor with high torque, yet with reduced pulsation.

"Providing the customer with a motor that met all their requirements felt really good. When deciding upon the rotor configuration, every day we got closer to our target values. The moment we achieved these—that was fantastic. We learned techniques to overcome heat in solving this problem, and we felt the confidence to use what we had learned in subsequent developments. I have been involved with motors since starting work on air compressors, but the development of this one has taught me about specifications, design, evaluating performance characteristics, evaluating reliability, and various other factors. Through this, I think I have gained a feel for design, and that I have improved myself as a result."

For young, dedicated employees such as Mr. Suzuki, the development of this proved a major challenge, and a major opportunity for growth.