The magnetic circuit of an ac generator, as with other electric machines, is made up of the air gap, the stator teeth and backiron, the rotor poles, and the shaft section. Each of these elements has an effect on machine rating and operation. The function of the magnetic circuit is to carry flux that links the armature conductors to produce voltage.

Air gap.
The air gap constitutes the division between the rotating part of the machine—the rotor, which carries the field winding—and the stationary part of the machine—the stator, which carries the armature winding. In ac generators, the air-gap dimension is determined by the electrical characteristics of the machine.

There is a trade-off between excitation mmf (toward a small air-gap dimension) and armature reaction flux (toward a large air-gap dimension). This trade-off generally results in an air gap, which is substantially larger than mechanical considerations such as machining tolerances or windage loss would dictate.

Stator Teeth and Backiron.
The armature magnetic circuit carries alternating flux and is always laminated, either with complete ring laminations (for small machines) or with overlapping segmented laminations. The material most commonly used is sheet steel, of an alloy containing about 3.5% silicon, in sheets of thickness between about 0.35 and 0.65 mm.

Grain-oriented steel, with reduced losses and improved permeability in the direction of rolling, is often used in large turbogenerators. Orientation in the circumferential direction is advantageous in such machines because of the large proportion of steel and moderate flux densities in the backiron.

At high flux densities characteristic of the armature teeth, the advantage of grain orientation becomes less pronounced.

The active region of the armature constitutes the alternation of stator teeth and slots carrying the armature winding. The division between teeth and slots is a compromise between flux-carrying capability and current-carrying capability.

The trade-off generally results in a division that is about half slots and half teeth. Flux densities in the stator teeth are usually high enough to result in moderate saturation of the magnetic material.

Rotor Iron.
The magnetic flux in the rotor is nearly constant, varying in the main only slightly with changes in load and terminal voltage and with small higher frequency components due to time and space harmonics of armature flux.

This allows the rotor magnetic circuit to be made of solid steel. In turbine generators, the rotor is typically made of a single-piece forging of steel with slots for the field winding cut by machining.

The losses caused by harmonic driven eddy currents in the solid steel pole faces can be problematic, and are reduced by making the air gap larger, by increasing the number of stator slots and, by choosing a suitable (short-pitch) coil throw for the armature.

Salient-pole machines may have solid or laminated poles. In many cases, laminated poles are necessary to control eddy current losses. Pole laminations are commonly made of low carbon steel, 1.5 to 2 mm thick.

Thinner steel, sometimes with silicon content, may be used where further control of eddy current losses is required. The shaft, or inner portion of the rotor of salient-pole machines, is often a solid forging, or in large machines such as hydroelectric generators may be fabricated from structural steel pieces.

Magnetic Materials. 
Typical magnetization characteristics of steel materials used in the magnetic circuit of ac generators are shown in Fig. 7-14.

Fig 7-14 Magnetization curves of commonly used steels.

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