Page 641 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 627
TABLE 8.3: Comparison of four major permanent magnetic material types.
Permanent magnet Max. magnetic Curie Max. operating
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material energy (MGOe) temp ( C) temp ( C) Cost $
Alnico 5 ∼ 1000 ∼ 500 Low
Ceramic 12 ∼ 450 ∼ 300 Moderate
SmCo 35 ∼ 800 ∼ 300 High
NdFeB 55 ∼ 350 ∼ 200 Medium

There are four major types of natural hard ferromagnetic materials that can be used
as permanent magnets:
1. Alnico which is an aluminum (Al)-nickel (Ni)-cobalt (Co) (“AlNiCo”) mixture.
2. Ceramic (hard ferrite) magnetic materials which consists of strontium, barium ferritite
mixtures.
3. Samarium cobalt (samarium and cobalt mixtures, SmCo , Sm Co ).
5
17
2
4. Neodymium (neodymium, iron, and boron are the main mixture components with
small amounts of other compounds). The ideal mixture is Nd Fe B .
2
14 1
Alnico and ceramic ferrite permanent magnet materials are the lowest cost types and have
lower magnetic strength compared to samarium and neodymium. The maximum magnetic
energy of each type is shown in Table 8.3. Today, Alnico permanent magnets (PM) are
used in automotive electronics, ceramic PM materials are used in consumer electronic, and
samarium and neodymium are used in high performance actuators and sensors. The cost
of the PM material increases as the magnetic energy level increases. Notice that the energy
levels given in the table are the maximum currently achievable levels. Lower energy level
versions are available at lower cost. For instance, the cost of NdFeB at 45 MGOe is twice
that of NdFeB at 30 MGOe. The biggest advantage of samarium-cobalt PM material over
neodymium PM material is the fact the samarium-cobalt PM material can operate at higher
temperatures.
The manufacturing process for making permanent magnets from one of the above
materials has the following steps (it is important to note that small variations in composition
and the manufacturing process make a difference in the final magnetic and mechanical
properties of the magnet):

1. Mix the proper amount of elements to form the magnet compound and melt it in
furnace, and make ingots.
2. Process the ingots to turn them into fine powder and mix the powder.
3. Place the mixed powder in a die cavity, apply an initial electromagnetic field to orient
the magnetic directions (pre-alignment of magnetic field), and press it down to about
50% of its powder state size. This is a powder metallurgy process. The product at this
state is called “green.”
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4. Heat the “green” PM in a furnace (i.e., vacuum chamber at 1100–1200 Ffor
neodymium-iron-boron) which will result in further shrinkage in size. This process
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may be followed by a lower temperature heat treatment around 600 C.
5. Saw and grind to the desired shape (rectangular, cylindrical) and size.
6. Coat the surface of the magnet piece if desired.
7. Magnetize each piece to magnetic saturation by an external electromagnetic field
pulse (i.e., generally a few milliseconds duration of external magnetic field pulse
with a high enough H value to make the magnet reach its saturation level). Pulse