torque. However, such torque is not desired in the operation of HDD spindle
motor, which will be explained later in section 4.3.
Besides the electromagnetic torque, which is tangential to the rotor sur-
face, the magnetic fi eld in the airgap of the motor also induce radial force.
The local radial force in the airgap of the motor is usually much higher than
the local tangent force that produces the electromagnetic torque. When the
geometric center of the rotor is aligned with the center of the electromagnetic
fi eld, the local radial forces balance each other and the resultant force caused
by these radial forces is zero. However, if the two centers are not aligned, the
resultant radial force is non-zero, which is known as the Unbalanced Magnetic
The UMP may also be introduced due to problems with component quality.
For example, when the magnet ring is not symmetrically magnetized or when
the stator core dimensions are not correct, UMP is generated. The unbalance
can also be induced if the motor is not correctly assembled. The UMP caused
by these factors is called extrinsic UMP . On the other hand, the UMP
may produced by the electromagnetic structure itself. Figure 4.32 shows the
magnetic fi eldinaspindlemotorwith9statorslots and 4 magnetic pole-pairs.
This structure shows an unbalance between the left and right sides of the EM
fi eld, inducing the UMP. As the local fi eld changes with different position of
the rotor, the UMP also changes as shown in Figure 4.33. This UMP is not
caused by the qualities of the components and production, but by the motor
structure itself, and it is thus known as the intrinsic UMP.
Figure 4.32: Magnetic fi eld in a 9-slot/4-pole-pair spindle motor (obtained