The air gap is the space between the stator and rotor of a motor . During motor operation, the stator does not rotate, while the rotor needs to rotate relative to the stator; the air gap is essential for motor operation. The size of the air gap varies depending on the type of motor. Generally speaking, asynchronous motors have smaller air gaps, while synchronous motors have larger air gaps.
The Importance of the Air Gap in Motors: Motors have many geometric dimensions, including core dimensions, winding dimensions, overall dimensions, mounting dimensions, and the dimensions of various other structural components. However, which are the main dimensions? The electromagnetic processes of a motor mainly take place in the air gap, and the conversion of its energy form is carried out through the "main magnetic flux of the air gap."
Therefore, the main dimensions are necessarily closely related to the air gap. Practice has shown that the armature diameter and the effective length of the core, which are close to the air gap, are the main dimensions of the motor, while the air gap can be considered the third dimension. From a geometric point of view, once these dimensions are determined, the other dimensions are largely determined, and many electromagnetic properties are basically influenced by or slightly varied from them.
The air gap in a motor should be appropriate. The air gap is a crucial part of the motor's magnetic circuit and significantly impacts its performance. For a well-manufactured asynchronous motor, the air gap should not only be uniform but also of appropriate length. A larger air gap results in a larger excitation current, more reactive power consumption, and a lower power factor. Therefore, to reduce excitation current and improve the power factor, the air gap should be as small as possible.
However, an excessively small air gap can increase additional losses in the motor and even cause stator-rotor rubbing. To reduce additional losses caused by magnetic field pulsation and harmonic leakage flux, while ensuring ease of manufacturing, assembly, and reliable operation, the air gap cannot be too small. Therefore, all requirements must be considered. The air gap of medium and small asynchronous motors is typically 0.2~1.5mm.
The air gap characteristics of permanent magnet synchronous motors (PMSMs) and their main dimensions can be determined by the required maximum torque and dynamic response performance. The dynamic response performance of a PMSM is reflected in the time required for the motor to accelerate from rest to its turning speed under maximum electromagnetic torque. While ensuring the motor's response characteristics, the maximum value of the stator inner diameter is determined. This, in turn, determines the stator core length.
The air gap length of a permanent magnet synchronous motor (PMSM) is generally larger than that of an induction motor of the same specifications. Speed-regulating PMSMs use high-performance rare-earth permanent magnet materials, so slightly increasing the air gap will not alter the motor's performance. The selection of the air gap depends not only on the rotor magnetic circuit structure used but also on the motor's field weakening and speed-enhancing capabilities. In speed-regulating PMSMs with surface or rotor magnetic circuit structures, the tile-shaped magnets on the rotor core require protective sleeves. To reduce magnetic leakage, the protective sleeves are generally made of non-magnetic materials, resulting in a larger effective air gap for the PMSM. For speed-regulating PMSMs with an internal rotor magnet structure and requiring a certain constant power operating speed range, the air gap should not be too large; otherwise, the direct-axis inductance of the motor will be too small, resulting in insufficient field weakening capability and an inability to reach the required maximum speed.
When determining the stator outer diameter of a permanent magnet synchronous motor, the general principle is to increase the stator outer diameter to improve the efficiency of the permanent magnet synchronous motor or to reduce the manufacturing cost of the motor, provided that the motor has sufficient heat dissipation capacity, depending on the specific circumstances.
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