Why does the motor use magnetic slot wedge?

2019-05-11 17:09:00

The magnetic slot wedge is made by adding magnetic conductive material into the material of common slot wedge, which is formed by hot pressing and solidification. It is mainly composed of thermosetting matrix resin, reinforced glass fiber and magnetic powder. Matrix resin and reinforcing fiber are used to improve the mechanical properties and heat resistance of slot wedge, while magnetic materials improve the conductivity and magnetic properties of slot wedge.

High efficiency motor is the production and promotion of environmental protection and energy saving in practical application. Traditional methods to improve motor efficiency include: increasing iron length, using more and better silicon steel sheets to reduce iron loss; using appropriate slot matching; using low harmonic sinusoidal windings, etc. The test data prove that using magnetic slot wedge instead of traditional insulating slot wedge is an economical and effective method.

Because of the large permeability coefficient of the magnetic slot wedge, the effective cross-section area of the stator teeth is increased and the magnetoresistance is reduced; moreover, the air gap coefficient of the motor is reduced, which is equivalent to shortening the effective air gap of the motor, thus reducing the surface loss and pulse vibration loss of the motor, improving the efficiency of the motor, reducing the temperature rise of the winding, greatly reducing the level of vibration and noise, and prolonging the service life of the motor. 。

Knowledge Development of Electrical Machinery

Additional Loss of Motor

The causes of additional losses of three-phase asynchronous motors are different. Generally, they can be divided into two categories: basic frequency additional losses and high frequency additional losses. Additional loss of fundamental frequency is mainly caused by fundamental current. Hysteresis loss and eddy current loss are produced by leakage flux of winding end in core surface, end cover and fan structure. This part of loss accounts for a small proportion of total additional loss. The main additional loss is high frequency additional loss. High frequency additional loss is mainly caused by air gap harmonic flux in stator, rotor core and rotor guide bar.

Effect of Magnetic Slot Wedge on Additional Loss

The magnitude of additional loss depends on the magnitude of slot magnetic field. Therefore, it can be expected that larger additional loss will occur in motors that have to use open slots for production reasons and put them into windings. This idea has been proved many times in practice. It is also found that the performance of the open slot motor which is not properly designed will not meet the requirements because of the increase of additional losses, which will lead to a significant reduction in efficiency and consequently an inadmissible heating. Therefore, when open slots are used in asynchronous motors, because the air gap is relatively small, it is often necessary to restrain the increase of additional losses.

The structure of high-order harmonic magnetic field in air gap can be significantly disturbed by the reduction of slot width: after winding insertion, the slot is sealed by magnetic slot wedge made of partially permeable material instead of wood, fibre or plastic slot wedge.

Considering that the closure of the flux through the tip of the teeth will significantly increase the leakage flux of the slot, which will damage the power factor of the motor, it is necessary to keep the permeability of the whole slot mode low enough or use directional magnetic materials, which have low tangential permeability and high radial permeability.

Although in recent decades, the problem of magnetic slot wedges has been solved by many methods and patents have been obtained in this respect, the results of operation are not entirely satisfactory, either because of the high production price of magnetic slot wedges or because of their rapid aging, resulting in the rapid deterioration of their magnetic properties, especially in mechanical aspects.

Nevertheless, the use of magnetic slot wedges has resulted in a significant reduction in additional high frequency losses. According to the measurement of different types of directional magnetic slot wedge motors, the stray loss can be reduced by about 50% while the original performance of the motor is maintained. The additional loss has been reduced by up to 85% by using a uniform magnetic wedge made of iron powder dispersed in epoxy resin and by 15% to 25% by heating.