22.214.171.124 Aerodynamic Bearing
The structure of the aerodynamic bearing (ADB) is similar to that of the
FDB, but the oil fi lm between the sleeve and shaft is replaced by air pad.
The problems caused by the movement of fl uid is eliminated. The NRRO and
acoustic noise performance of ADB spindle motor are better than those of FDB
spindle motor. Since the linkage between the stationary part and rotating part
of an ADB motor is an air pad, this kind of motor can operate in very high
speed, usually in the range greater than 20,000 RPM.
In order to realize the levitation effect using air pad, the airgap in the ADB
must be made narrower than that of the FDB. Tolerances for dimensions of
theADBmotorsarestricter. Alltheseare challenges to the spindle motor
4.3.8 Winding Structure and the Airgap Field Produced
by the Winding
Current fl owing through the armature windings produces the magnetic fi eld
required for operation of motor. Interation between the fi eldproducedbyar-
mature current and the motor excitation fi eld produces electromagnetic torque
which acts on the rotor and reacts on the stator (see section 4.2.3).
Distributed windings are normally used in many electric machines, espe-
cially the big ones, so that the MMF waveform produced by the armature
current is close to sinusoidal (see section 4.2.1). However, limited number of
slots and multi magnetic pole EM structure forbid the usage of distributed
winding in HDD spindle motors. The big space taken by the winding end
parts is another problem for the distributed windings (Figure 4.59). For uti-
lizing the limited stator slots to realize multi magnetic pole-pair and compact
structure, concentrated windings are used in the spindle motors.
Figure 4.60 shows a typical concentrated winding used in the spindle motor,
where one coil is wound around one tooth of the stator core. As there is no
overlap between the end coils of adjacent windings, the space taken by the
winding ends is very much reduced, see Figure 4.61.
To realize multiple magnetic pole with limited slots, each winding is put
between the windings of the other two phases. As a result three consecutive
slots form one winding cycle. Figure 4.60 shows an example using 9 slots to
realizing three winding cycles. The symbols A and X, B and Y, and C and
Zinthis fi gure represent the two sides of the windings of A-phase, B-phase
and C-phase, respectively. The distance between the centers of neighboring
coils is 120 electrical degrees. Connecting the windings of the same phase in
series obtains high torque constant. The three phase windings can be either
Y-connected or ∆-connected. We can obtain the MMF for A-winding using
the method mentioned in section 4.1.6; this MMF is shown in Figure 4.62.