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:
where H is the magnetic field intensity produced by current Inet. Current I is measured in 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
Hlc = Ni
where lc is the mean path length of the core. The magnetic field intensity H is a measure of 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
H represents effort exerted by current to establish a magnetic field
1- represents relative ease of establishing a magnetic field in a given material In SI, the units are as follows: H ampere-turns per meter; 1- henrys/meter (H/m); B
webers/m2, known as teslas (T). And 1- is the permeability of free space. Its value is The relative permeability compares the magnetizability of materials. For example, in mod- ern machines, the steels used in the cores have relative permeabilities of 2000 to 7000. Thus, for a given current, the flux established in a steel core is 2000 to 7000 times stronger than in a corresponding area of air (air has the same permeability as free space). Thus, the metals of the core in transformers, motors, and generators play an essential part in increasing and concen- trating the magnetic flux in the device. The magnitude of the flux density is given by
