Application analysis of Magnetic circuit
As a hook, we hereby analyze some commonly met phenomenon using the above knowledge.
(1) In production, when the motor speed is higher than specification, we usually adjust the magnetizing voltage to make the motors conforming to specification. Why?
Figure 2 is the theoretical drawing of a magnetizer. Different magnetizing voltage results in different magnetizing current. Different magnetic potential results in different remanence (figure 11). Finally influences the remanence (Br) of the motor. According to the formula:  
n＝(V﹣I*r)/(K_E*Φ)
when (V﹣I*r) & KE remain unchanged, different Φ value results in different speed n value. So adjusting value of Φ is able to adjust the value of n

(2) Why the motor speed increases when we remove its flux yoke?
Removing the flux yoke means reducing the cross-sectional area of the stator yoke. Its magnetic resistance Rmy1 increases. According to formula (5), when magnetic potential keeps unchanged, decreased magnetic flux increases motor speed n＝(V﹣I*r)/(K_E*Φ).

(3) Why there is so-called axial pulling force in some motors?
Magnetic line of force has the feature to choose the shortest path. When the center of stator magnetic field is not aligned with the center of rotor magnetic field, the magnetic lines are stretched longer. The axial force FX is therefore generated to pull the centers aligned and shorten the magnetic line.