Page 643 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 629
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and for nickel 631 K. In practice, the allowed maximum operating temperature for a PM
material is much lower than the Curie temperature.
The external magnetic field (also called the magnetizing force) should be strong
enough to fully magnetize the material and saturate it so that the permanent magnet material
attains its maximum possible magnetic capacity. This value is at least 3 to 5 times the
coercivity (H ) of the permanent magnet material.
c
Each PM material has a different susceptibility to environmental conditions. In addi-
tion to temperature, the chemical composition of the environment is most important. Alnico,
ceramic, and samarium magnets are corrosion resistant, whereas neodymium magnets are
very susceptible to corrosion. In the selection of a permanent magnet (PM) material, the
environmental issues to consider are:
1. oxidation and humidity level,
2. acid content,
3. salt content,
4. alkaline content,
5. radiation level.
The following specifications are typically used by designers in selecting a proper
permanent magnet for an application:
1. permanent magnet material,
2. remanance, coercivity, and maximum energy, B , H ,(BH) max ,
c
r
3. maximum operating temperature without irreversible loss of magnetization,
4. mechanical dimensions (shape and size),
5. magnetization direction (radial, axial, etc.),
6. whether the permanent magnet is sintered or bonded,
7. surface coating (i.e., 2 μmto20 μm range thickness, aluminum, nickel, or titanium
nitride coating material).
The selected PM material largely determines the temperature coefficient for the loss of
magnetic field as the temperature rises, and the thermal expansion coefficient of mechanical
dimensions, which is important in a device assembly. Neodymium magnets expand in the
magnetized direction and contract in the other direction with increasing temperature.
A permanent magnet piece is usually bonded to a steel backing material using adhe-
sives in motor applications, that is in the case of electric motors on a steel rotor (3M Adhe-
sives). The bonding material types include thermosetting epoxies, and structural adhesives.
There are many adhesives specifically designed for motor applications (3M Adhesives).
The strength of the bond between the PM and and the rotor is a function of the contact sur-
face area. Typically, the surface area is roughened in order to provide a good bonding. After
the application of adhesive, the PM is clamped on the rotor and cured at high temperature.
The curing temperature for the bonding material (the temperature at which the bonded PM
is baked for a certain time period) should be well below the demagnetization temperature
of the PM, if the PM was magnetized before the bonding process.

8.2 ENERGY LOSSES IN ELECTRIC MOTORS

An electric actuator is a device that converts energy from electrical to mechanical form
(Figure 8.16). The conversion process is not 100% efficient, hence there are losses.