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..If low-carbon iron or other ferromagnetic material is inserted in the solenoid, the situation depicted in FIG. 13B will result. The density of the magnetic lines of force emanating from, and returning to, the polar regions is now greatly increased. In other words, a stronger field is available for the production of either motor or generator action in rotating machines. So-called soft magnetic material is generally used so that initial current flow will not produce a permanent magnet. “Hard” magnetic materials would result in high magnetic retentivity and would deprive the solenoid of its electrical control feature. Generally, the strength and polarity of the magnetic poles is desired to be a function of the current flowing in the winding. Additionally, the use of a “hard” ferromagnetic material would result in inordinately high hysteresis loss when such a solenoid was energized by alternating current. Magnetic hysteresis is a friction-like phenomenon deriving from the sluggish efforts of magnetic domains to continually realign themselves in response to the changing field produced by an alternating current. (Each domain comprises millions of atoms with net magnetic fields arrayed in the same direction.)
FIG. 13 Increasing the magnetic field strength of a solenoid.
A. The air-core solenoid
B. Same solenoid with core of iron or other ferromagnetic material.
A characteristic of the iron-core solenoid is that the magnetizing force (current times the number of turns) cannot be increased indefinitely in the quest for more- powerful magnetic fields. Depending on the material, an area of diminishing return is reached as the magnetizing force is increased. In this area of operation, the material is said to be magnetically saturated. All ferromagnetic materials are characterized by saturation, including ferrites, their alloys, and even those that actually contain no iron. For many years, it appeared that the saturation behavior of ferro magnetic materials imposed an upper limit on the strength of the magnetic fields available for motors and generators. This is no longer true.
Seemingly, nature has exacted a price for the gains accompanying the use of solenoids. Although magnetizing force (ampere-turns) increases directly with the number of turns, it turns out that inductance increases as the square of the number of turns. Inductive effects are generally of nuisance value in rotating machines. They interfere with commutation and slow down response time when electronic controls are employed. A dc shunt motor, which at first might be expected to operate on alternating current, will not do so because of the high inductance in its field winding. Motor action is lost because the magnetic forces from the armature and those from the field winding do not occur simultaneously.