Diameter from the middle of the airgap to the middle of the airgap on the opposite side.
The airgap diameter is defined as the distance between the middle of the airgap on two opposite sides, refer to Fig. 1. The motor in Fig. 1 was designed for a concrete cutter and has a two-layer concentrated winding with 8 poles and 6 slots.
Fig. 1 Illustration of the airgap diameter (airgap thickness is exagerated).
The rotor diameter of an inner-rotor motor or the stator diameter of an outer-rotor motor is consequently the airgap diameter minus the airgap thickness. Accordingly, the inside stator diameter of an inner-rotor motor or the inside rotor diameter of an outer-rotor motor is the airgap diameter plus the airgap thickness.
Generally, you would like to have the airgap diameter as large as possible, since the torque of the machine is proportional to the square of the airgap diameter. For a given maximum motor diameter, the outer-rotor surface-mounted permanent magnet machine often allows to have the largest airgap diameter and consequently torque density.
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