Cold Rolled Sheet 


Cold Rolled Motor Lamination
Application Considerations

The consideration of the above-discussed efficiency issues has led to today's highly sophisticated manufacturing practices for manufacturing motors, generators, and transformers. As discussed previously, the core is constructed of layers of thin sections of steel sheet to decrease current losses caused by eddy currents.

Thus, the manufacture of a motor involves the following processing steps on the coil received from the steel mill:

1.	slitting into narrow strips,
2.	punching the slit coil length into "lamination sheets" of a specific size and design for a particular product,
3.	annealing the sheet to optimize the magnetic characteristics,
4.	stacking the "lamination sheets" as required for the application, and
5.	completing the winding of the wire coil around the core.

Steel Issues Affecting the manufacturing Process

	Punching To obtain laminations of acceptable shape and dimensions, Cold Rolled Lamination Sheet must be flat and punchable. Good flatness means that the sheet has minimal amounts of waviness throughout the entire coil from end to end and edge to edge. For good punchability, the steel sheet should have a relatively high hardness and a high yield strength to tensile strength ratio.
	Annealing Annealing, the heating of steel to elevated temperatures to impart the desired properties, of punched laminations, is critical for developing good magnetic properties, i.e., low core loss and high permeability. The annealing conditions such as time, temperature, atmosphere composition, and dew point are controlled to accomplish two main objectives: decarburization and grain growth. Decarburization is intended to decrease the amount of carbon in the lamination steel to less than 0.005%. Remember that carbon adversely affects magnetic properties. Grain growth during annealing is related to the amount of temper mill extension or cold work imparted during the temper rolling process prior to shipment from the steel manufacturing plant. Considerably higher amounts of temper mill extension than is normally applied to conventional cold rolled sheet steel are required to obtain the desired grain growth in CRML steel during lamination annealing. Another important factor in achieving the desired grain growth and magnetic properties during lamination annealing is the steel purity or cleanliness. Levels of the particle forming elements of oxygen, nitrogen and sulfur should be as low as possible such that grain growth and magnetic domain inhibiting particles are minimized.  Also, it is important that during the lamination anneal, magnetically favorable crystallographic textures be developed. Crystallographic texture refers to a preferred orientation of crystal structure within and across the steel grains forming the lamination steel. Magnetically favorable textures are therefore those crystallographic textures that result in easy magnetization of the steel laminations. The processing conditions used during hot rolling, the coil annealing temperature and the percent cold reduction and temper mill extension are important variables that influence the formation of magnetically favorable textures. During the lamination annealing process, the surface oxide layer is developed which acts as an insulator and provides resistance to eddy-current flow between the laminations. Lastly, the surface roughness of the steel sheet, normally a fairly rough matte finish, is an important parameter to be controlled during the temper rolling process. The rough matte finish is used to prevent the laminations from sticking together during the annealing process.
	Stacking To produce the final laminated-core structure, annealed laminations are stacked to the desired height (defined as the core length) and held together by bolting, welding, or other means of interlocking.
	Coil Winding and Assembly The finished electromagnet is produce by winding insulated copper or aluminum wire onto the core. Such electromagnets are components ready for assembly into a finished item of electrical equipment.

 

Typical Applications Application Considerations Tolerances