Analysis of major parameters
(1) Turns of coil and magnet wire diameter (other parameters remain unchanged)  
We know from 5.1 that the potential constant KE increases when the turns of coil increase. Motor speed n is
therefore lowered. On the contrary, when the turns of coil decrease, the motor speed increases. 
When the diameter of the magnet wire increases, the rotor resistance r reduces. Back EMF of the rotor increases
(E=V-2△U-I*r). The motor speed n therefore increases. On the contrary, when the diameter of the magnet wire
decreases, the motor speed n decreases. 
The current at stall is in inverse proportion to the resistance r. Turns of the coil and diameter of the magnet wire
restrict each other under the space limit of the lamination slot. We should clearly understand such relationship
when we try to adjust the motor parameters.
(2)  Magnetic flux (other parameters remain unchanged)
Magnets with higher magnetic flux density and longer lamination sheets will both increase the magnetic flux Φ.
From 5.1 and 6.2 we know that speed n decreases. At the same time, load (T2) has less influence over speed n.
The characteristic of the motor is thus called hard. On the contrary, if we use magnets with lower magnetic flux
density and shorter lamination sheets, the characteristic of the motor is called soft. 
(3)  Air gap
See figure 12, the magnetization curve of the air gap
Φδ=-μ0*(Sδ/δ)*Fδ
Φδ: Air gap flux
Sδ: Air gap area
Δ: Air gap length
Fδ: Air gap magnetomotive force(magnetic EMF)
Permeance angle: α=tg-1[μ0*(Sδ/δ)]. 
We can see that when δ is longer, α is smaller, air gap flux Φδ is smaller. Motor speed will increase if the other parameters remain unchanged. On the contrary, when δ is shorter, α is larger, air gap flux Φδ is larger. Motor speed will decrease. We will see the same result as we see in 7.2. We usually pursue the maximum possible value of (Φδ*Fδ) in motor design. 
(4)  Effective volume D2*L
Motor torque is proportional to D2*L.             [D: diameter of the rotor   L: length of the rotor]
Motor power is proportional to D2*L *n.