The air gap Lg (Fig. 5.100) should be chosen to give the greatest torque per stroke. This approach will, however, tend to make the air gap length small. T. J. E. Miller (1993) recommends that it be about 0.05 percent of the rotor diameter. This tends to produce air gaps in the 0.007- to 0.010-in range for small motors. This can lead to manufacturing problems and increased costs in commercial-type motors because tight tolerances will be required on many parts. Holding a 0.010-in air gap is very feasible in most manufacturing situations and is a good initial selection. Small air gaps increase torque but also increase audible noise.
Sizing the Air Gap (Electric Motors)