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Hitachi Develops High-Efficiency Amorphous Metal Motor

16 November 2008

Hitachi Ltd and Hitachi Industrial Equipment Systems Co Ltd have developed a motor that uses cores made of amorphous metal coupled with ferrite magnet rotors. The motor does not require magnets made of rare earth metals such as Neodymium (Nd) or Dysprosium (Dy). The 150W prototype showed an efficiency of 86%—a 5% increase over Hitachi’s current 150W motors which do use expensive rare metal magnets.

Hitachi
Hitachi’s amorphous metal prototype. Click to enlarge.

Amorphous metal has a disordered atomic structure in contrast to the crystalline structure of conventional metals, and features a high tensile strength and extremely low magnetic losses. As such, it has been a target of interest for motor development for decades. A study by researchers at the Tokyo Institute of Technology published in IEEE Transactions on Industry Applications in 1989 (Fukao et al.), for example, found that a prototype super-high-speed reluctance motor built using amorphous metal for both rotor and stator cores reduced core losses by a factor of five (at 48,000 rpm) compared to a silicon-iron machine. The reduction in the core comprised 33% of total losses. The amorphous metal motor showed a six-point efficiency gain over the silicon iron machines, with 85% efficiency of 85% and a power factor of 0.46 at an output of 372 W.

A major problem with amorphous metal, however, has been the inability to manufacture it economically due to the difficulty in cutting and machining the material, which is harder than sheet steel. For the prototype, Hitachi and Hitachi Industrial Equipment Systems applied technologies developed for transformers that use amorphous metal to manufacture the iron cores.

Hitachi plans to commercialize the amorphous metal motors in three years for use in industrial equipment, and plans to increase output capability to up to the 10 kW class of motors to support a range of applications.

Resources

  • Fukao, T.; Chiba, A.; Matsui, M. (1989) Test results on a super-high-speed amorphous-iron reluctance motor Industry Applications, IEEE Transactions on Volume 25, Issue 1 Page(s):119 - 125 doi: 10.1109/28.18881

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Comments

A very useful development. It can be shown that fixed reluctance motors are the most light weight of all motors for the power supplied and they are also more reliable as they can operate with one or more shorted coils and are not demagnetized by high currents or temperatures. They can be used as generators at all speeds. It is not necessary to use high performance iron in such motors but it will as demonstrated increase the efficiency. The TESLA car, if it survives the capital crash, needs to use such motors in future versions. A small highspeed flywheel with such motor generators can replace the battery in a Prius and also allow a lead battery to be used for plug in conversion. ..HG..

- Henry,
I disagree. Not every new motor is suitable for traction. This particular switched reluctance motor has a relatively poor efficiency of only 85% and that value is probably achieved at 48,000rpm to take advantage of the rotor's exceptionally low iron loss. To employ these machines in traction applications would make them the ethanol or hydrogen equivalent in the overal family of electrical machines.

But it is not all bad news. Induction motors particularly those with copper rotor bars already approach 95% at full power and without the need for rare earth magnets either.

This 48,000rpm concerns me also, it's not a good thing except for gas centrifuges perhaps. When you have 100Hp motors or more rotating much above 15Krpm they begin to store too much inertial energy and behave more like flywheels and powerful gyros. It is not practicable to declutch them when rapid deceleration is required since they are needed for electrical regeneration and yet they enlarge the power capacity of the regeneration system needed. Simply put, it's difficult enough to absorb the recuperation of energy from the kinetic energy of the vehicle itself without the additional flywheel effect of the motor adding to it. The problems of the Tesla two ratio gearbox gives you an indication of the difficulty at 13000rpm, just imagine 48000rpm.

Then there's the two stage reducer to bring 48k down to the 1500rpm needed at the wheels for 100mph. Whenever you discuss hi-speed motors for traction applications you must consider the gearbox also. Having been in industry I can say that motor rpms above 3600 and gearboxes that warranty above that are very rare birds. They are not off the shelf and only available on custom order. Generally, each large ratio helical reducer stage has close to 7% loss (Eaton Corp. figures) if you can get a 10:1 in a single stage then that is about ideal. If 48000rpm forces you to cascade a second reducer off of the first this will knock another 7% off of your available torque, not a good idea.

The precursor to EV1, the Impact, used a planetary 10.5 gearbox with 94% at cruise and 98% at hi-torque/low speed operating points, with oilspray lubrication. This single 10.5 ratio was possible because the traction motor was limited to 11900rpm at 75mph.

Incidentally the actual efficiency of the 57Hp motor from this 1990 vehicle is said to vary from 90-95%. That compares favorably with the above switched reluctance design.

To be on topic something even more important is base speed. The point at which you get max power. Although generators have the luxury of needing to generate full power only at their top speed with proportional capability at all points between, with traction motors almost the opposite is desired with motors being required to deliver their full power as early as possible and preferably by at least one quarter of maximum speed.

As an example the traction motor on the Prius reaches its maximum power of 67Hp at 20mph (1200rpm), maintains it to 51mph, with upconverter help, but drops to 38Hp at 100mph. The power droop beyond 51mph is permissible here since at full acceleration 6.6Hp/10mph is being sent mechanically in a direct path from the engine to the wheels.

In another example, the traction motors on the Impact reach their maximum power of 57Hp at 42mph (6600rpm) and maintain that to 75mph (11900rpm).

The inductive nature of reluctance motors make that sort of performance hard to match.
T2

They said nothing of performances of their "amorphous metal" (don't say what metal) at low frequencies (compared to standard silicon-iron), ie below 500 Hz, currently used in induction motors.

If the low-freq performance are close to those of currently used iron for core, then with the new core metal (? just in stator) it might be possible to increase number of poles (in an induction motor) by 50% or 100%, and at the same time increase max AC current freq (from current ~500 Hz) by 50% or 100%, in order to widen the operating range (ie higher max rpm) and low end torque.
The switching element should also be able to support the new, higher freqs.

Or perhaps the new amorphous metal may only be of interest for very fast motors (20,000 rpm and up), whose performance at low rpm is not of interest.

Power Frequency performances of Amorphous Metals (Iron Silicon Boron Alloy )is also good. Actually there is a range of Amorphous Alloys available to choose from these days...

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