Hardware Reference
In-Depth Information
Figure 4.33: UMP generated in the motor operation: P
x
- UMP component in
X direction and P
y
: UMP component in Y direction
Normally, UMP is detrimental for the application of spindle motor as it
produces acoustic noise and vibration. The quality control of components and
production is critical for reducing UMP. Selecting the suitable match between
the slot number and magnetic poles can avoid the intrinsic UMP. In [16], it is
proven that, if the slot number is even, the intrinsic UMP can be eliminated.
The orders of UMP harmonics are even.
4.2.5 Generation of back-EMF
When a coil rotates in a magnetic
fi
eld, electromotive force (EMF) is induced
in the coil, and the induced EMF can be expressed as
e = −N
dφ
dt
(4.54)
where, φ is the
fl
ux going through the area surrounded by the coil, and N is
the number of turns of the coil.
When the coil is in an evenly distributed magnetic
fi
eld and rotates at a
constant angular speed Ω shown in Figure 4.34, the
fl
ux linked with the coil is
afunctionoftime,
φ = BS cos(θ)=BS cos(Ωt +α),
(4.55)
