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Core Manufacture
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
lengths 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
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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.
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- 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.
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