3. Magnetic Circuit
The Magnetic Field
Magnetic fields are the fundamental mechanism by which energy is converted from one form to
another in motors, generators and transformers.
First, we are going to look at the basic principle – A current-carrying wire produces a magnetic field
in the area around it.
Production of a Magnetic Field
Ampere’s Law – the basic law governing the production of a magnetic field by a current:
where H is the magnetic field intensity produced by the current Inet and dl is a differential element of
length along the path of integration. H is measured in Ampere-turns per meter.
Consider a current currying conductor is wrapped around a ferromagnetic core;
mean path length, lc
Applying Ampere’s law, the total amount of magnetic field induced will be proportional to the
amount of current flowing through the conductor wound with N turns around the ferromagnetic
material as shown. Since the core is made of ferromagnetic material, it is assume that a majority
of the magnetic field will be confined to the core.
The path of integration in Ampere’s law is the mean path length of the core, lc. The current
passing within the path of integration Inet is then Ni, since the coil of wires cuts the path of
integration N times while carrying the current i. Hence Ampere’s Law becomes,
Lecture 3 & 4
In this sense, H (Ampere turns per metre) is known as the effort required to induce a magnetic
field. The strength of the magnetic field flux produced in the core also depends on the material of
the core. Thus,
B = magnetic flux density (webers per square meter, Tesla (T))
µ= magnetic permeability of material (Henrys per meter)
H = magnetic field intensity (ampere-turns per meter)
The constant μ may be further expanded to include relative permeability which can be defined as