In a repair shop, a wound-rotor high-voltage motor was found to have a phase-to-phase fault in its rotor windings. A meticulous worker, while inspecting the windings, noticed a strange attraction to the small screwdriver being pulled inwards; this was actually an example of the motor shaft being magnetized.
Magnetization refers to the phenomenon where a material exhibits magnetism due to the alignment of its magnetic moments under the influence of a magnetic field. Magnetization is the process by which a material that was not originally magnetic acquires magnetism. Some objects exhibit magnetism under the influence of a magnet or electric current; this phenomenon is called magnetization.
During the operation of the motor, the energized windings generate a rotating magnetic field, and the shaft is in a magnetic field. Given the special nature of the shaft material, No. 45 steel, which is a soft magnetic material, magnetization is an inevitable phenomenon.
The magnetization effect of a shaft is that the shaft becomes magnetic due to various reasons, such as the presence of unbalanced current windings in the rotor, which magnetizes the shaft. Welding, friction, collision, and eddy current devices can all make the equipment magnetic and establish a magnetic field.
Analysis of the causes and hazards of magnetization of motor shaft
If two grounding points occur inside the motor windings or in the excitation circuit, the rotor windings, rotor core, or retaining rings may be burned by the short-circuit DC current. At the same time, the magnetic circuit asymmetry caused by some short-circuited turns will increase the vibration of the unit and may even cause the rotor body to be magnetized.
For generator rotors, the main causes of shaft current are the asymmetric effect of magnetic field line distribution and the magnetization effect of the shaft. The asymmetric magnetic field line distribution is usually caused by asymmetric gaps in the lamination layers. Besides generator rotors, other equipment can also generate shaft current due to the magnetization effect of the shaft.
The magnetization effect of a shaft occurs when a rotating shaft becomes magnetic due to various reasons. For example, unbalanced current windings in the rotor magnetize the shaft; welding, friction, collisions, and eddy current devices can all magnetize equipment and establish a magnetic field. When a rotating magnetic field cuts a conductor, it induces a potential within those components. When this potential rises sufficiently to break down the oil film, a current loop is formed. This current loop may run through the entire rotor or may only form a localized short-circuit current in the bearings or floating ring seals. The short-circuit current in the bearings or floating rings then generates a new magnetic field, magnetizing the shaft or other components. Therefore, this magneto-electric conversion creates a very strong magnetic field and a very high current within the diesel generator.
Generally, ferromagnetic materials have high magnetic permeability and can be magnetized during motor operation. In everyday life, we often observe that magnetic substances can also make ferromagnetic materials magnetic.
Electric current can generate a magnetic field, and the molecular currents within ferromagnetic materials can also generate a magnetic field. Each molecule can be considered a tiny magnet. In nature, many magnetic domain regions form within ferromagnetic materials. These domains are randomly distributed, and because the orientations of these tiny magnets are different, their magnetic properties cancel each other out, so the entire object appears non-magnetic. Once a ferromagnetic material is placed in a magnetic field, under the influence of the external magnet, the orientation of the magnetic domains will gradually converge, at which point the ferromagnetic material will exhibit magnetism. This is the principle behind the magnetism of ferromagnetic materials.
Further Knowledge – Characteristics of Soft and Hard Magnetic Materials
Soft magnetic materials are characterized by high permeability and weak remanence. They can generate strong magnetic induction under the influence of a relatively soft external magnetic field, and quickly reach magnetic saturation as the external magnetic field increases. Once the external magnetic field is removed, their magnetism essentially disappears. Commonly used materials include electrical pure iron and silicon steel sheets. Electrical pure iron is generally used in DC magnetic fields, with electromagnetic pure iron being the most common. Silicon steel sheets are classified into hot-rolled and cold-rolled types according to their manufacturing process. Cold-rolled silicon steel sheets are further divided into single-oriented and non-oriented types. Single-oriented cold-rolled silicon steel sheets, when magnetized along the rolling direction, have higher permeability and lower iron loss than those magnetized in other directions, while non-oriented cold-rolled silicon steel sheets lack directionality.
Hard magnetic materials, once magnetically saturated under the influence of an external magnetic field, retain strong and stable magnetism for a long time even after the external magnetic field is removed. Their characteristics include high remanence and stable magnetism. Their main applications are in manufacturing the pole cores of permanent magnet motors and the magnets for magnetoelectric instruments.
45# steel is a medium carbon steel, which should be considered a soft magnetic material. It can be magnetized in an external magnetic field, but it loses most of its magnetism once the field is removed. However, a slight residual magnetism may remain. Therefore, some steels need to be demagnetized after grinding, because grinding machines have an electromagnetic chuck used to hold the workpiece in place. High carbon steel has some permanent magnetic properties; the heads of some screwdrivers that can attract iron are made of high carbon steel.
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