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.