Page 29 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

INTRODUCTION TO MACHINERY PRINCIPLES

2.2 CHAPTER TWO

THE MAGNETIC FIELD

Energy is converted from one form to another in motors, generators, and transformers by
the action of magnetic fields. These are the four basic principles that describe how mag-
netic fields are used in these devices:

Production of a Magnetic Field
Ampere’s law is the basic law that governs the production of a magnetic field:
H dl I
net
where H is the magnetic field intensity produced by current I . Current I is measured in
net
amperes and H in ampere-turns per meter. Figure 2.1 shows a rectangular core having a
winding of N turns of wire wrapped on one leg of the core. If the core is made of ferromag-
netic material (such as iron), most of the magnetic field produced by the current will remain
inside the core.
Ampere’s law becomes
Hl Ni
c
where l is the mean path length of the core. The magnetic field intensity H is a measure of
c
the “effort” that the current is putting out to establish a magnetic field. The material of the
core affects the strength of the magnetic field flux produced in the core. The magnetic field
intensity H is linked with the resulting magnetic flux density B within the material by
B H
where H magnetic field intensity
magnetic permeability of material
B resulting magnetic flux density produced
Thus, the actual magnetic flux density produced in a piece of material is given by the product
of two terms:

FIGURE 2.1 A simple magnetic core.

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