The clearance between stator and rotor of Mini brushless motor is called air clearance. If the air gap is too large, the reluctance and excitation current are increased, thus reducing the power factor of the brushless motor.

But the air gap can not be too small, too small will appear fixed rotor coincidence phenomenon. The air gap of MIni brushless motor is generally between 0.2 and 2.0mm.

The reasons for uneven air gap distribution are as follows:

1, the machining accuracy of parts is poor, such as the inner diameter size of the stator, the abnormal tolerance, the outer diameter size of the rotor, the abnormal tolerance.

2. The accuracy of the assembly process is low, such as the bearing is not in the middle.

Mini brushless motor for magnetic circuit system. The larger the air gap between the rotor and stator, the greater the reluctance. According to magnetic circuit Ohm's law, when magnetic potential is constant, magnetic resistance increases and magnetic flux decreases. Accordingly, the magnetic flux becomes smaller; The rotor generation current is also small; The induced current is small and the resulting torque is small. Therefore, the size of the air gap will affect the mechanical properties of the whole motor, making the mechanical properties of the motor worse and softer.

The stator and rotor air gap of Mini brushless motor is much smaller than the DC motor of the same capacity, generally only 0.2-1.5mm, the size of the air gap has a great impact on the operation performance of induction motor. When the air gap is large, the excitation current (lagged reactive current) supplied by the grid is large, which makes the power factor of the motor worse. However, if the air clearance is too small, assembly will be difficult, operation is not reliable, and the high-order harmonic magnetic field will be enhanced, which will increase the additional loss and make the starting characteristic worse.