STATOR AND ROTOR CONSTRUCTION OF AC GENERATORS


Stator Construction


Armature cores are built up of thin laminations, produced as either segments or complete rings, depending on the size of the generator. Successive layers or groups of layers of the segmented laminations are staggered to minimize the effect of the joints in the magnetic circuit.

The core is clamped between pressure plates and fingers to support it with sufficient pressure to prevent undue vibration of the laminations. Especially in long cores, the clamping arrangement may include some provision to compensate for compacting of the core after initial assembly.

The armature windings are fitted tightly in the slots and secured radially by wedges driven into suitable notches at the air gap end of the slots. It is necessary that the stator coil ends be able to resist the abnormal forces associated with short circuits.

A supporting structure may be employed for this purpose. There are many variations of support design; most of them provide filler blocks between the coil sides, strategically located to transmit the circumferential forces from coil to coil, and additional structure to counteract the radial forces.

Coil supports ordinarily are designed to suit the need of a particular machine. Large 2-pole machines require a quite elaborate structure; the combination of large short-circuit currents and coil ends inherently flexible because of their long length makes these machines particularly susceptible to coil-end movement.

Low-speed machines with stiffer coil ends require less support; in the smallest ratings the coils may be capable of withstanding the short-circuit sources without any additional support.

Stator frames, sometimes called casings, are commonly fabricated from structural steel, designed to support the core in alignment with the rotor and to suit the ventilating scheme used. In large machines with 2-pole or sometimes 4-pole construction, the stator core is mounted on springs to isolate core vibration from the machine frame.

Rotor Construction


The pole pieces of salient-pole alternators may be built up of steel laminations, both as manufacturing convenience and a means of limiting the loss in their air gap surfaces due to pulsations in air gap flux. The field coils, wound directly on the poles or preformed and then mounted on the poles, are suitably insulated from the poles for the voltages associated with normal and transient operation.

The pole-and-coil assembly is bolted, dovetailed, or otherwise attached to the rotor body. It is the limitation of this attachment which usually dictates when round-rotor construction must be used rather than salient-pole construction.

The rotor body for a salient-pole machine may be a solid forging or assembly of heavy steel plates, for high speed designs, or a spider-and-rim assembly for low-speed designs. The shaft may be integral with the body, as in the case of a forging, or may be bolted to or inserted into the body.

When the spider-and-rim construction is used, the entire assembly may be an integral weldment or casting, or the rim may be separate from the spider, as in the case of large waterwheel-driven generators.

A common construction for this latter case is a rim built up of thin steel laminations, assembled around a cast or fabricated spider, bolted together between steel end plates and keyed to the spider.

The rotor of a round-rotor machine is cylindrical in shape with axial slots provided in its body for the field coils. The body is usually a steel forging with integral shaft ends. In special applications, other constructions may be used, with this same general configuration.

The field coils are wound in axial slots in the rotor body, held in place by heavy slot wedges and by retaining rings over the coil ends.

Rotors are designed for operation at overspeeds, which depend on the characteristics of the prime mover. The overspeed limit (the speed above which the unit is no longer capable of safe operation) may be as low as 20% for a steam-turbine-driven unit or as high as 125% for some adjustable-blade, axial-flow hydraulic turbine-driven units.

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