Magnetic slot wedges are made by adding magnetic materials to the materials used in ordinary slot wedges, followed by hot pressing and curing. They are mainly composed of thermosetting matrix resin, reinforcing glass fiber, and magnetic powder. The matrix resin and reinforcing fiber are used to improve the mechanical and heat resistance properties of the slot wedge, while the magnetic material improves its electrical and magnetic conductivity.
High-efficiency motors represent the practical application of environmental protection and energy conservation in their production and promotion. Traditional methods for improving motor efficiency include increasing iron length, using more and better silicon steel sheets to reduce iron losses, employing appropriate slot fits, and using low-harmonic sinusoidal windings. Experimental data demonstrates that using magnetic slot wedges instead of traditional insulating slot wedges is an economical and effective method.
Because the magnetic slot wedge has a large permeability, it increases the effective cross-sectional area of the stator teeth and reduces magnetic resistance; moreover, it reduces the air gap coefficient of the motor, which is equivalent to shortening the effective air gap of the motor, thereby reducing the surface loss and pulsation loss of the motor, improving the motor efficiency, reducing the winding temperature rise, and greatly reducing vibration and noise levels, thus extending the service life of the motor.
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Additional losses of the motor
The additional losses in three-phase asynchronous motors arise from different causes and can generally be divided into two categories: fundamental frequency additional losses and high-frequency additional losses. Fundamental frequency additional losses are mainly generated by the fundamental current, including hysteresis and eddy current losses caused by leakage flux at the winding ends in structures such as the core surface, end covers, and fans. This portion of the loss accounts for a relatively small proportion of the total additional losses; the total additional losses are dominated by high-frequency additional losses. High-frequency additional losses are mainly caused by air-gap harmonic flux in the stator and rotor cores and rotor bars.
The effect of magnetic slot wedges on additional losses
The magnitude of the additional losses depends on the amplitude of the slot magnetic field. Therefore, it is expected that larger additional losses will occur in motors that require open slots in the windings for manufacturing reasons. This assumption has been confirmed in practice on numerous occasions. It has been found that poorly designed open-slot motors suffer from unsatisfactory performance due to increased additional losses, resulting in significantly reduced efficiency and unacceptable heat generation. Therefore, when using open slots in asynchronous motors, due to the relatively small air gap, it is often necessary to suppress the increase of additional losses.
The structure of the high-order harmonic magnetic field in the air gap can be significantly disrupted by reducing the slot width: after the winding is inserted, the slot is sealed with a magnetic slot wedge made of partially magnetic material instead of the commonly used wooden, fibrous or plastic slot wedges.
Considering that closing the magnetic flux through the tooth tip would significantly increase the slot leakage flux and thus impair the motor's power factor, it is necessary to maintain a sufficiently low permeability of the entire slot mold or to use a directional magnetic material with low tangential permeability and high radial permeability.
Although the problem of magnetic slot wedges has been solved in many ways in recent decades and many patents have been obtained in this area, the results are not entirely satisfactory, either because the production cost of magnetic slot wedges is high, or because they age rapidly, causing their magnetic properties, especially their mechanical properties, to deteriorate rapidly.
However, despite this, the use of magnetic slot wedges has resulted in a significant reduction in additional high-frequency losses. Measurements of motors employing different types of directional magnetic slot wedges have shown that stray losses have been reduced by approximately 50% while maintaining the original motor performance. Slot wedges with uniform magnetism, made by dispersing iron powder in epoxy resin, have reduced additional losses by up to 85%, along with a 15% to 25% reduction in heat generation.
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