harmonic fi eld and the n th orderstatorharmonic fi eld has the following char-
1. If the stator fi eld has different pole-pairs with the rotor fi eld, no torque
is produced by these two fi elds,
2. If the stator fi eld has same pole-pairs with the rotor fi eld, and their
relative speed is not zero, the torque produced varies with time, and the
average torque in the time domain is zero, and
3. If the stator fi eld has the same pole-pairs with the rotor fi eld, and their
relative speed is zero, the torque produced by the interaction is constant.
From the explanations given above, we can fi nd the effect of each harmonic
of the stator fi eld and the rotor fi eld in generating the EM torque. Only
the interaction between the stator harmonic and the rotor harmonic with the
same magnetic pole-pair can generate the EM torque. This is the necessary
condition for producing the Ampere's torque. We can tell, knowing the relative
speeds of the rotor fi eld harmonic and the stator fi eld harmonic in the airgap,
whether the interaction can generate effective torque that makes the rotor
rotate continuously, or just produce torque ripple that induces vibration.
4.3 Spindle Motors used in HDD
Varieties of motors suitable for different applications are available in the mar-
ket, but not all can be used in HDD. For instance, the EMI (Electro-magnetic
Interference) in conventional DC motor is quite high and the EMI sensitivity
of HDD electronics makes these motors inappropriate for use in HDD. Fur-
thermore, particles generated by the brushes of DC motor, and the necessity
of regular maintenance of DC motor are additional reasons for not using them
in HDD. The major factors that must be considered before selecting a motor
for HDD spindle are explained below. Issues arising from these factors are
addressed. The BLDC motor with surface mounted permanent magnets on
the rotor is a suitable candidate for application in HDD.
4.3.1 Special Requirements for HDD Spindle Motor
The EMI affecting the electronics system of HDD is a key factor to be con-
sidered while selecting the spindle motor. Particles released inside the drive
enclosure and the range of speed are two other important points to consider.
Moreover there are some special requirements on the spindle motor, which are