The magnetic network model of hybrid step motor is established according to the air gap late permeance method, and the expression of the harmonic back EMF is deduced.

The theoretical calculation result fits well with the experiment result, and the mathematical method adopted is feasible and valuable for the flux prediction of the air gap membrane distillation.

According to the theory of robustness, this paper takes a robustness processing in air gap discharge test data when the up-down method is employed.