Spindle Motor Control - Hard Disk Drive: Mechatronics and Control - page 209

Figure 4.2: B-H curve of a ferromagnetic material

Table 4.1: Performance of some ferromagnetic materials
Material Iron 4% Si Monimax Hypernic Supermalloy

steel

Maximum µ x 5,000 7,000 35,000 70,000 800,000

Saturation
fl ux density
(Tesla)

2.15 2.0 1.5 1.6 0.8

4.1.5.2 Hard Magnetic Material (Permanent Magnet Material)

Hard magnetic materials (Permanent magnet materials) have the capability to
produce magnetic fi eld after they are fully magnetized. They exhibit a large
hysteresis loop in the their B-H curves, as it is shown in Figure 4.3. It is easily
observed that they are very different from the ferromagnetic materials. The
hysteresis loop in the second quadrant is called demagnetization curve. In the
design and analysis of permanent magnet electric machines, this part of curve
is the most important in the entire hysteresis loop as the operation range of
the magnet is normally in this quadrant. This is explained using an example
in section 4.1.6.

There are three important points in the second quadrant of the B-H loop,

1. Point a: H = 0 and the magnetic fl ux density is equal to B r which is
known as the remanent magnetic fl ux density, or remanence. This value
is a measure of the remaining magnetization when the magnetization
fi eld is removed. Therefore, this parameter can show the capability of
the permanent magnet in generating the magnetic fi eld.