Motor slot wedges mainly come in various types, including 3240 epoxy cloth board slot wedges, pull-out slot wedges, magnetic slot wedges, silicone slot wedges, 4330 molded slot wedges, and diphenyl ether slot wedges. Among them, magnetic slot wedges are quite special and are widely used in high-voltage or high-efficiency motors. Today, Ms. Can will discuss the characteristics of magnetic slot wedges with you.
Magnetic slot wedges and motor losses
As the name suggests, magnetic slot wedges are magnetic. Adding magnetically conductive materials (such as steel wire, thin steel plates, iron powder, etc.) to the materials used to manufacture slot wedges can create magnetic slot wedges.
An asynchronous motor is an electromechanical conversion device that converts electrical energy into mechanical energy, and losses are generated during the conversion process. The proportion of these losses is related to the motor capacity, type, number of poles, etc.: motors with fewer poles (higher speed) have a larger proportion of mechanical losses; no-load stray losses increase with the number of poles of the motor, while load stray losses decrease with the number of poles of the motor.
The iron loss of an electric motor measured under rated voltage and frequency when it is not under load is called no-load iron loss. It consists of basic iron loss and additional iron loss. The basic iron loss is mainly the hysteresis and eddy current loss generated by the main magnetic flux in the stator core teeth and stator yoke. Since the rotor frequency is extremely low during normal operation, the hysteresis and eddy current losses caused by the main magnetic flux in the rotor are negligible.
The no-load additional iron loss is generated in the stator and rotor cores by harmonic flux induced by the cogging effect within the motor. The high-frequency additional iron loss induced in the stator and rotor cores is called pulsation loss. In addition, the stator and rotor teeth are sometimes aligned and sometimes misaligned, causing changes in the magnetic flux of the tooth clusters, which can induce eddy currents in the tooth surface layer, generating surface losses.
According to experimental data, pulsation loss and surface loss are collectively referred to as high-frequency additional losses, which account for 70-90% of the stray losses of motors. The remaining 10-30% is called load additional losses, which are generated by leakage flux.
Using magnetic slot wedges can effectively reduce losses.
Using magnetic slot wedges can reduce surface losses and pulsation losses, which are the main types of losses in no-load iron losses. The wider the slot opening of the motor and the smaller the average length of the air gap, the greater the surface loss. Using magnetic slot wedges significantly reduces the pulsation amplitude of the air gap magnetic flux density, thus reducing the motor's surface losses. Furthermore, because a portion of the magnetic flux passes through the magnetic slot wedges, the in-tooth magnetic flux density is reduced, further decreasing pulsation losses.
Energy-saving analysis using magnetic slot wedges
●Motor efficiency can be improved due to reduced motor losses.
●If the output power remains unchanged, the reduced losses will decrease the motor's input active power and the input current. If the input active power remains unchanged, the motor's output power will increase, which is equivalent to increasing the motor's capacity.
●Because a portion of the magnetic flux can be conducted inside the magnetic slot wedge, the magnetic flux distribution in the air gap tends to be uniform, which can reduce motor noise and vibration. Reduced motor vibration can also extend bearing life.
● Placing a magnetic slot wedge in the slot is equivalent to shortening the effective air gap length of the motor. This reduces the excitation current and lowers the no-load current.
● Because motor losses are reduced, the amount of heat generated by the motor decreases, thus reducing the motor's temperature rise and extending its service life.
Application examples of magnetic slot wedges
A high-voltage motor sold by a motor factory was causing production disruptions due to overheating. After using magnetic slot wedges, the motor temperature dropped significantly, and the noise level also decreased markedly, ensuring normal operation during the summer.
It is evident that modifying motors with magnetic slot wedges is an effective energy-saving method. However, the application of magnetic slot wedges must be moderate to avoid excessive increase in magnetic leakage at the slot opening, which could lead to insufficient starting capacity.
