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Effect on Motor Design Parameters on Cogging Torque – Part 1

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= 18o 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