The rotor of a DC motor consists of a laminated piece of electrical steel. When the rotor rotates in the motor's magnetic field, it generates a voltage in the coil, which generates eddy currents, which are a type of magnetic loss, and eddy current loss leads to power loss. Several factors affect the effect of eddy currents on power losses, such as the electromagnetic field, the thickness of the magnetic material, and the density of the magnetic flux. The resistance of the material to the current affects the way eddy currents are generated, for example, when the material is too thick, the cross-sectional area increases, resulting in eddy current losses. Thinner materials are needed to minimize the cross-sectional area. To make the material thinner, manufacturers use several thin sheets called laminations to form the armature core, and unlike thicker sheets, thinner sheets produce higher resistance, which results in less eddy current.
The choice of material used for motor laminations is one of the most important considerations in the motor design process, and because of their versatility, some of the most popular choices are cold-rolled motor laminated steel and silicon steel. High silicon content (2-5.5 wt% silicon) and thin plate (0.2-0.65 mm) steels are soft magnetic materials for motor stators and rotors. The addition of silicon to iron results in lower coercivity and higher resistivity, and the reduction in thin plate thickness results in lower eddy current losses.
Cold rolled laminated steel is one of the lowest cost materials in mass production and is one of the most popular alloys. The material is easy to stamp and produces less wear on the stamping tool than other materials. Motor manufacturers anneal motor laminated steel with an oxide film that increases interlayer resistance, making it comparable to low-silicon steels. The difference between motor laminated steel and cold-rolled steel is in the steel composition and processing improvements (such as annealing).
Silicon steel, also known as electrical steel, is a low carbon steel with a small amount of silicon added to reduce eddy current losses in the core. Silicon protects the stator and transformer cores and reduces the hysteresis of the material, the time between the initial generation of the magnetic field and its full generation. Once cold rolled and properly oriented, the material is ready for lamination applications. Typically, silicon steel laminates are insulated on both sides and stacked on top of each other to reduce eddy currents, and the addition of silicon to the alloy has a significant impact on the life of stamping tools and dies.
Silicon steel is available in various thicknesses and grades, with the optimum type depending on the allowable iron loss in watts per kilogram. Each grade and thickness affects the surface insulation of the alloy, the life of the stamping tool, and the life of the die. Like cold-rolled motor laminated steel, annealing helps strengthen the silicon steel, and the post-stamping annealing process eliminates excess carbon, thereby reducing stress. Depending on the type of silicon steel used, additional treatment of the component is required to further relieve stress.
The cold-rolled steel manufacturing process adds significant advantages to the raw material. Cold-rolled manufacturing is done at or slightly above room temperature, resulting in the grains of steel remaining elongated in the rolling direction. The high pressure applied to the material during the manufacturing process treats the inherent rigidity requirements of cold steel, resulting in a smooth surface and more precise and consistent dimensions. The cold rolling process also causes what is known as "strain hardening", which can increase the hardness by up to 20% compared to non-rolled steel in grades called full hard, semi-hard, quarter hard and surface rolled. Rolling is available in a variety of shapes, including round, square and flat, and in a variety of grades to suit a wide range of strength, intensity and ductility requirements, and its low cost continues to make it the backbone of all laminated manufacturing.
The rotor and stator in a motor are made from hundreds of laminated and joined thin electrical steel sheets, which reduce eddy current losses and increase efficiency, and both are coated with insulation on both sides to laminate the steel and cut off eddy currents between the layers in the motor application. Typically, the electrical steel is riveted or welded to ensure the mechanical strength of the laminate. Damage to the insulation coating from the welding process can lead to a decrease in magnetic properties, changes in microstructure, and the introduction of residual stresses, making it a great challenge to compromise between mechanical strength and magnetic properties.
Post time: Dec-28-2021