STATOR AND ROTOR LAMINATION ANNEALING BASIC INFORMATION

Stator Lamination Annealing

Semiprocessed lamination sheet is received from the producing mill in the heavily temper-rolled condition. This condition enhances the punchability of the sheet and provides energy for the metallurgical process of grain growth that takes place during the annealing treatment.Annealing of the laminations is done for several reasons. Among them are the following.

Cleaning. Punched laminations carry some of the punching lubricant on their surfaces. This can be a water-based or a petroleum-based lubricant. It must be removed before the laminations enter the high temperature zone of the annealing furnace to avoid sticking and carburization problems. This is done by preheating the laminations in an air or open-flame atmosphere to 260 to 427°C (500 to 800°F).

Carbon Control. Carbon in solution in steel can form iron carbides during mill processing, annealing, and electromagnetic device service. These carbides have several effects on properties—all detrimental. They affect metallurgical processing in the producing mill, degrading permeability and, to some extent, core loss.

They pin grain boundaries during annealing, slowing grain growth.They pin magnetic domain walls in devices, inhibiting magnetization and thus increasing core losses and magnetizing current. If the carbides precipitate during device use, the process is called aging.

Because of these problems, the amount of carbon is kept as low as is practical during mill processing. The best lamination steels are produced to carbon contents of less than 50 ppm. Steels of lesser quality can be produced with up to 600-ppm carbon, but in the United States, 400 ppm is presently a practical upper limit.Laminated cores cannot run efficiently with these high carbon contents, so the carbon is removed by decarburization during annealing.

The annealing atmosphere contains water vapor and carbon dioxide,which react with carbon in the steel to form carbon monoxide.The carbon monoxide is removed as a gas from the furnace.

This process works well for low-alloy steels, but for steels with appreciable amounts of silicon and aluminum, the same water vapor and carbon dioxide provide oxygen that diffuses into the steel, forming subsurface silicates and aluminates.

These subsurface oxides impede magnetic domain wall motion, lowering permeability and raising core loss.

Grain Growth. The grain diameter that minimizes losses in laminations driven at common power frequencies is 80 to 180 μm.As the driving frequency increases, this diameter will decrease. Presently, the temper-rolling percentage and the annealing time and temperature are designed to achieve grain diameters of 80 to 180 μm.

Coating. Laminations punched from semiprocessed steel are uncoated, while those punched from fully processed sheet are typically coated at the steel mill with a core plate coating. This coating insulates laminations from each other to reduce interlamination eddy currents, protects the steel from rust, reduces contact between laminations from burrs, and reduces die wear by acting as a lubricant during stamping.

The semiprocessed steel laminations are also improved by a coating, but economics precludes coating them at the steel mill. Instead, they are coated at the end of the annealing treatment when the laminations are cooling from 566°C to about 260°C (1050°F to about 500°F). The moisture content of the annealing atmosphere is controlled to form a surface oxide coating of magnetite.

This oxide of iron is very adherent and has a reasonably high insulating value. Therefore, it can be used for the same purposes as the relatively expensive core plate coating on the fully processed steel laminations. This magnetite coating is referred to as a blue coating or bluing because blue to blue-gray is its predominant color.

Rotor Lamination Annealing

Sometimes rotors are annealed with the stators, but often they are only given a rotor blue anneal.This is similar to the end of the stator anneal,mentioned previously.

The rotors are heated to about 371°C (700°F) in a steam-containing atmosphere to form a magnetite oxide on their surface. They are then die-cast with aluminum to form conductor bars and end rings. The magnetite oxide prevents adherence of the aluminum to the steel laminations and thereby reduces rotor losses.Stator Lamination Annealing

Semiprocessed lamination sheet is received from the producing mill in the heavily temper-rolled condition. This condition enhances the punchability of the sheet and provides energy for the metallurgical process of grain growth that takes place during the annealing treatment.Annealing of the laminations is done for several reasons. Among them are the following.

Cleaning. Punched laminations carry some of the punching lubricant on their surfaces. This can be a water-based or a petroleum-based lubricant. It must be removed before the laminations enter the high temperature zone of the annealing furnace to avoid sticking and carburization problems. This is done by preheating the laminations in an air or open-flame atmosphere to 260 to 427°C (500 to 800°F).

Carbon Control. Carbon in solution in steel can form iron carbides during mill processing, annealing, and electromagnetic device service. These carbides have several effects on properties—all detrimental. They affect metallurgical processing in the producing mill, degrading permeability and, to some extent, core loss.

They pin grain boundaries during annealing, slowing grain growth.They pin magnetic domain walls in devices, inhibiting magnetization and thus increasing core losses and magnetizing current. If the carbides precipitate during device use, the process is called aging.

Because of these problems, the amount of carbon is kept as low as is practical during mill processing. The best lamination steels are produced to carbon contents of less than 50 ppm. Steels of lesser quality can be produced with up to 600-ppm carbon, but in the United States, 400 ppm is presently a practical upper limit.Laminated cores cannot run efficiently with these high carbon contents, so the carbon is removed by decarburization during annealing.

The annealing atmosphere contains water vapor and carbon dioxide,which react with carbon in the steel to form carbon monoxide.The carbon monoxide is removed as a gas from the furnace.

This process works well for low-alloy steels, but for steels with appreciable amounts of silicon and aluminum, the same water vapor and carbon dioxide provide oxygen that diffuses into the steel, forming subsurface silicates and aluminates.

These subsurface oxides impede magnetic domain wall motion, lowering permeability and raising core loss.

Grain Growth. The grain diameter that minimizes losses in laminations driven at common power frequencies is 80 to 180 μm.As the driving frequency increases, this diameter will decrease. Presently, the temper-rolling percentage and the annealing time and temperature are designed to achieve grain diameters of 80 to 180 μm.

Coating. Laminations punched from semiprocessed steel are uncoated, while those punched from fully processed sheet are typically coated at the steel mill with a core plate coating. This coating insulates laminations from each other to reduce interlamination eddy currents, protects the steel from rust, reduces contact between laminations from burrs, and reduces die wear by acting as a lubricant during stamping.

The semiprocessed steel laminations are also improved by a coating, but economics precludes coating them at the steel mill. Instead, they are coated at the end of the annealing treatment when the laminations are cooling from 566°C to about 260°C (1050°F to about 500°F). The moisture content of the annealing atmosphere is controlled to form a surface oxide coating of magnetite.

This oxide of iron is very adherent and has a reasonably high insulating value. Therefore, it can be used for the same purposes as the relatively expensive core plate coating on the fully processed steel laminations. This magnetite coating is referred to as a blue coating or bluing because blue to blue-gray is its predominant color.

Rotor Lamination Annealing

Sometimes rotors are annealed with the stators, but often they are only given a rotor blue anneal.This is similar to the end of the stator anneal,mentioned previously.

The rotors are heated to about 371°C (700°F) in a steam-containing atmosphere to form a magnetite oxide on their surface. They are then die-cast with aluminum to form conductor bars and end rings. The magnetite oxide prevents adherence of the aluminum to the steel laminations and thereby reduces rotor losses.

GENERATOR STATOR CONSTRUCTION BASIC INFORMATION

The stator core is constructed from a stack of thin steel sheets or laminations which are stamped to shape and insulated electrically from each other, either by a thin coating or by an oxide layer which is produced during heat treatment.

The steel used has a low silicon content; this increases the resistivity of the steel and therefore reduces the losses due to eddy currents. The steel is carefully processed in order to minimize the hysteresis losses because the whole stator core is subjected to alternating magnetic flux.

In large turbogenerators the core is built up in segments and grain-oriented steel is used to reduce the losses further.

The stator windings are located in axial slots in the stator core which are formed by the shape of the laminations. Except in high-voltage machines, the individual coils of the winding are wound with copper wire covered with a layer of polyester/polyamide enamel which is about 0.05 mm thick.

The slots are lined with a tough insulating sheet, usually about 0.25–0.5 mm thick; a popular material is a laminate of Mylar and Nomex.

The coils are impregnated when in place with a resin to give the winding mechanical strength as well as to improve the heat transfer from the copper to the cooling air.

Windings operating at different voltages, such as the three phases, have a further sheet of insulating material separating them in the end-winding region. The mounting of the stator core in its frame differs according to the size of the machine, but for a majority of medium-sized generators, the arrangement is as shown in Fig. 5.18

Fig. 5.18 Cut-away section of an ac generator (courtesy of Newage International)

At either end of the frame are bearing housings which locate the rotor shaft. These housings or end bells are cast in smaller and medium size machines, and fabricated in larger sizes. The generator is often mounted directly onto the engine, and in this case it is usual to eliminate the drive-end bearing, using the rear bearing of the engine to locate the generator shaft.

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.