Ordinary slot wedges are non-magnetic, such as common bamboo wedges and slot wedges made of epoxy glass cloth; while magnetic slot wedges are magnetically conductive, which are formed by adding magnetic materials to the materials used to manufacture the slot wedges.
Why do electric motors use magnetic slot wedges?
● Energy saving. Using magnetic slot wedges is equivalent to shortening the effective air gap length of the motor, which can reduce the excitation current, improve the power factor, reduce the no-load current, and increase efficiency by 1%~2%.
● Extends motor lifespan. Using magnetic slot wedges makes the magnetic flux distribution in the air gap more uniform, effectively reducing core losses, motor temperature rise, electromagnetic noise and vibration, and extending bearing life. For motor products, even a 1% increase in motor efficiency can generate significant socio-economic benefits. Especially for high-efficiency motors, replacing insulating slot wedges with magnetic slot wedges is an effective energy-saving measure.
However, according to collected technical data from manufacturers and accident reports from multiple user plants, the performance of magnetic slotted wedge motors in practical applications is unsatisfactory. High-voltage motors employing magnetic slotted wedges frequently experience large-area wear and detachment of the wedges, leading to escalating accidents and unit shutdowns, posing a significant threat to safe production.
Analysis of the causes of magnetic slot wedge detachment: 1. Force acting on the magnetic slot wedge during operation
● The electromagnetic force resulting from the interaction between the magnetic field in the iron core tooth region and the current flowing through the magnetic slot wedges. The eddy current induced in the magnetic slot wedges under the action of the alternating magnetic field, or the transverse current passing through the slot wedges and the magnetic circuit of the silicon steel sheet, interacts with the air gap magnetic field to generate a large pulling force. The direction of the pulling force is towards the air gap between the stator and rotor; the larger the current, the stronger the air gap magnetic field, and the greater the pulling force.
● The magnetic pull of the magnetic core tooth region on the magnetic slot wedge. The magnetic pull is an inherent characteristic of the magnetic slot wedge with a permeability μ>1. The pull is directed towards the air gap between the stator and rotor. When designing a motor, if magnetic slot wedges are used, the magnitude of this force must be considered.
● The force transmitted from the mechanical vibration of the iron core teeth to the magnetic slot wedge. Due to the presence of the air gap magnetic field, and because the stator and rotor iron cores cannot be aligned during actual production, the air gap magnetic field of the motor generates a large axial magnetic pull on the stator iron core, which produces periodic mechanical vibration in the iron core teeth and is transmitted to the magnetic slot wedge, causing damage to the straight section of the magnetic slot wedge at the motor end.
Process assembly reasons
The fit between the slot wedge and the slot opening is crucial. Many motor manufacturers specify that epoxy adhesive should be applied to the contact surface between the slot wedge and the iron core for bonding. If the fit between the slot wedge and the slot opening is too loose, the epoxy adhesive, if too concentrated, will have difficulty flowing into the gap between the slot wedge and the iron core; if too diluted, it will have difficulty filling the gap. Both situations result in a smaller actual bonding area and an unsatisfactory bonding effect.
When the slot wedge fits too tightly with the slot opening, it increases the difficulty of assembly. When forcefully driven in, the squeezing force on the slot wedge will exceed the design limit, causing internal damage to the magnetic slot wedge. It cannot withstand long-term operation and is prone to falling off even after being coated with epoxy glue.
The design and manufacturing standards of magnetic slot wedges do not meet the actual operating conditions.
The requirements for the mechanical properties, temperature resistance, and magnetic properties of magnetic slot wedges are not clearly defined during the design and manufacturing process, which fails to meet the actual operating conditions and is also an important reason for the detachment of magnetic slot wedges.
●Mechanical Properties. Magnetic slotted wedges must withstand various forces during assembly and operation. During assembly, due to dimensional tolerances, they are subjected to impact forces. This necessitates that the magnetic slotted wedges possess sufficient bending strength, modulus, adhesive strength, and impact toughness. If these requirements are not met, wedge detachment during operation becomes inevitable.
●Temperature resistance. With the development of motor technology, temperature resistance levels are becoming increasingly higher, and the operating conditions of the windings and cores are becoming more severe, which places higher demands on the temperature resistance of magnetic slot wedges. In addition, the solvent-free resin VPI process used in motor manufacturing also imposes strict requirements on the temperature resistance of magnetic slot wedges. All of these factors require magnetic slot wedges to have high-temperature heat resistance and high-temperature stability.
●Magnetic properties. Due to the influence of the magnetic field in the iron core tooth area on the magnetic pull of the magnetic slot wedge, when selecting the magnetic properties of the magnetic slot wedge, unilaterally pursuing the improvement of motor performance, slot wedges with high magnetic permeability were selected, which increased the factors of magnetic slot wedge detachment.
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