Major causes of noise in a motor are

• Electromagnetic phenomenon
• Mechanical structure
• Aerodynamic behavior

Causes for electromagnetic phenomeon:

• Cogging torque
• Unbalance magnetic pull
• Reluctance torque
• Commutation torque ripple
• Phase unbalance
• Input current distortion
• Magnetic saturation
• Magnetostriction

Cogging torque is one of the major electromagnetic sources for motor noise. Reducing cogging torque results in a quieter motor.

Slot Per Pole Combination

Optimal combination of slot per pole is one with:

• Fractional slot per pole
• Function 1 (F₁) : Highest [Least Common Multiple (Slots, Poles)]
• Function 2 (F₂): Smaller (Poles×Slots/F₁ )

Skewed Magnet / Magnetization

• Skewing reduces the rate of change of reluctance seen by magnet flux
• The optimum skew angle for minimum cogging torque is Function 3 (F₃): ( 360/F₁ )
• The optimum skew angle for 10-Slot/4-pole motor is F1=20, F₃= 18^o mechanical

Effect of skew angle on 10-Slot / 4-Pole motor cogging torque

Magnetization Profile

• Preferred magnetization profile depends on selection of number of poles as well as magnet thickness
• Halbach magnetization is preferred in a motor with higher pole number and thicker magnet
• For magnet with fewer poles, radial magnetization with/without skew is preferred

Effect of magnetization profile on 12-Slot / 14-Pole motor cogging torque