All magnetic materials fall into two general classes: they are either permanently or nonper-manently magnetized. The permanently magnetized materials retain their magnetization after being placed in a magnetic field and can be used as a constant source of magnetic field. On the other hand, the nonpermanently magnetized, or soft magnetic materials, retain their magnetization only while a magnetic field is applied to the material.
The basic magnetization process and hysteresis cycle is best understood in terms of the static domain theory, which is applicable under DC excitation. However, most applications of magnetic materials are for AC use. The predominant frequency commercially used in the United States is 60 cycles. For many special applications, particularly in the aircraft and military field, 400 cycles is a common frequency. The AC losses are not simple multiples of the DC hysteresis loss, for in addition to the basic magnetic losses.
Applications
The magnetic alloys used for AC applications are used in sheet form and are most often alloyed so that maximum resistivity is obtained commensurate with the induction required, ability to hot- and cold-work the material, minimum hysteresis loss, and cost.
The number of applications of magnetic materials is so large that a complete listing is not feasible. Furthermore, even for the same general application, such as a motor, many acceptable designs could be made, each requiring different-quality levels of magnetic material. Therefore, no attempt will be made to evaluate all the uses for a given alloy. However, several typical uses will be given. In some instances very special properties of a magnetic material may be required. The most common magnetic materials with their characteristic AC and DC properties are listed in Tables M.3 and M.4.
Tables M.3 and M.4 are designed to give the reader a ready reference source to identify the kinds of materials available and a brief resume of their most important properties. After preliminary selection, reference should be made to the detailed curves available from reputable suppliers before any design is anticipated. Many of the materials listed require special handling and annealing techniques to achieve optimum properties.
a Hysteresis loop measured from Bmax. = 10,000 G. b Hysteresis loop measured from Bmax. = 15,000 G. c Permeability at H = 2 oe. d Field at which permeability is measured.
AC Magnetic Properties
a Dash indicates alloy is not usually used at this frequency.
b Induction in kilogauss at which loss is measured.
c Field annealed.
Applications
DC devices, i.e., electromagnets, relays, pole pieces
Small motors, intermittently used electrical apparatus
Small motors, fractional horsepower motors
High-quality motors, medium efficiency motors, and generators
Motors and generators, reactors, small transformers
High-efficiency motors and generators, reactors, motors
High-quality, high-power, continuous duty transformers
Aircraft motors and transformers, television transformers Magnetic amplifiers, television transformers, power aircraft
transformers Magnetic amplifiers, pulse transformers
Instrument transformers, magnetic shields, sensitive low-current relays
Magnetic amplifiers, current transformers, pulse transformers
High-temperature generators, high-temperature transformer transducers, aircraft equipment, pole pieces
FIGURE M.4 Identification of properties related to the magnetic hysteresis loop. Colored area is the magnetization energy loss in every cycle (hysteresis loss).