Figure 4.48: Prevention of oil leakage using ferro- fl uid sealing.
In an in-hub spindle motor, the electromagnetic components of the motor
is located inside the rotor, as shown in Figure 4.49. The EM parts of the motor
is in the middle of the bearings. The length of the shaft of this kind of spindle
motor is longer than that of the underslung motor, and it is able to produce
enough EM torque for the motor operation. These motors are used in the disk
drives that use multiple disks.
Hard disk drives with low rated speed of spindle use inner rotor as shown
in Figure 4.50. These are used especially for small form factor HDD products.
4.3.5 Magnetic Ring
Spindle motors use permanent magnets to produce the excitation magnetic
fi eld, and to realize the conversion from the electrical energy to mechanical
energy. Therefore, the performance of the spindle motor is linked closely to
the performance of the magnet.
The permanent magnet is manufactured as a ring and installed on the
surfaceofthemotor(seeFigure4.45andFigure 4.46). Its pole-pair determines
the magnetic pole-pair of the motor. Bonded-NdFeB magnets are used in
the spindle motors. It is isotropic, and this characteristic is important to
the spindle motor because the magnetic poles must be magnetized in radial
direction. This material can be easily made into a ring at low cost. To protect
from rusting, the surface of the magnet is coated with anti-corrosion fi lm. The
B-H demagnetization curve of a bonded NdFeB magnet is shown in Figure 4.4.
Its performance also depends on the environment.