Hardware Reference
In-Depth Information
Chapter 4
Spindle Motor Control
4.1 Magnetic Field Fundamentals
An electric machine is an electromagnetic (EM) system. It relies on the re-
actions of the magnetic
fi
eld in the machine to realize the electromechanical
energy conversion. Knowledge about the fundamentals of electromagnetic the-
ory is, therefore, essential for the design, analysis and applications of electric
machines.
In the operation of electric machine, the frequency of variation of the mag-
netic
fi
eld is normally below the range of kHZ. Therefore, the displacement
current, which must be taken into consideration for analyzing high frequency
electromagnetic
fi
eld, can be neglected. This makes it possible to use of some
simpli
fi
ed EM models to describe and analyze electric machines.
In this section, the basic concepts of the magnetic
fi
eld are introduced
fi
rst,
followed by the concepts of magnetic circuit that can be treated as a useful
tool to simplify the magnetic
fi
eld analysis in the machine.
4.1.1 Flux
Flux is a scalar, normally denoted by the symbol Φ. It is used to describe
the total amount of magnetic
fi
eld in a given region. Flux is depicted as lines
(or
fl
ux lines) which can be visualized by a
fl
ow circulating out of the N-pole
and into the S-pole of a magnet in a circulating path. Figure 4.1 shows an
illustration of the
fl
ux lines generated by a permanent magnet. It should be
noted that these lines do not cross one another. The SI unit of
fl
ux is Weber.
In the electric machine analysis,
fl
ux can be used as a quantity to describe
the global performance of the magnetic system, or the global effect of the
magnetic
fi
elds within a certain area, e.g., the
fl
ux linked with a winding.
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