Why Do We Need Amorphous Iron Motors?

<Occasionally, engineers need motors that must operate at high frequencies but with a high energy-efficiency. At high frequencies, the magnetic fields inside the motor change rapidly. Normal CRGO or Cold Rolled Grain Oriented steel has eddy current losses that rise with the increase in operating frequency. Amorphous iron has a special structure that allows it to respond to fast changing magnetic fields without power loss due to hysteresis and eddy currents.

Contrary to popular belief, amorphous iron is not an exotic material and is not hard to come by. People’s interest in this special metal is rising since it has magnetic properties that promote energy efficiency in power distribution equipment and electrical machinery. The properties of amorphous iron make it very useful for electrical equipment.

For example, a commercially available motor made of amorphous-iron has a two-stator permanent magnet brushless configuration. While the rotor has 36 poles, each stator is provided with 54 slots. Green areas show the places where the motor uses amorphous iron. In practice, the motor operates at 1,000 RPM.

As interest in this metal grows, amorphous iron is actually manufactured in huge quantities. Two firms in the world are the major suppliers of amorphous iron. One of them is the Advanced Technology & Materials Co. Ltd., China and the other Metglas in Conway, SC and Tokyo, a division of Hitachi Metals. Of them, Hitachi Metals supplies the major bulk of the 100,000 tons of amorphous iron produced annually.

Amorphous iron is a typical alloy of iron with silicon and boron. Manufacturers make amorphous iron in the form of thin ribbon or foil of 25-micron thickness. The process used to manufacture the iron sets the special form factor. Molten iron is made to drip onto a wheel made of pure molybdenum. Since the molybdenum wheel is maintained at a precisely controlled temperature, the iron hitting the wheel quenches very quickly. In fact, the temperature of the molten iron drops at a rate of nearly a million degrees every second. Such an extra-fast rate of quenching freezes the molecules of iron before they have a chance to form crystals. That gives the iron its amorphous structure, which has a much less orderly form that the more popular crystalline iron.

Since the extra-fast rate of quenching has to occur throughout the internal molecules of the iron, the harvested iron from the molybdenum wheel is necessarily thin. If the thickness of iron increases beyond 25-microns, the internal molecules would not cool so fast and would have time to form crystals. The resulting metal would not have uniform amorphous quality.

Amorphous iron, with its disorderly structure responds to changes in magnetic fields far more readily than does iron in its ordinary crystalline form. The super thin nature of the amorphous iron limits the formation of eddy currents, which are an additional source of loss. Overall, amorphous iron exhibits higher efficiency because of very much reduced power loss when working in a given magnetic field strength as compared to that of the regular type of crystalline iron.