White Paper: Test & Measurement

On Track for Growth: Why Spin Testing Matters for eDrive Systems


Optimizing the amount of air gap in a motor is one of the key design concerns for electrical motor engineers. The gap between the rotor and the stator, of course, is a necessary feature to separate the spinning rotor from the stator. Depending on the speed and the size of a motor, the amount of the air gap could range from 0.2~5 mm.

The amount of gap must be large enough to accommodate the relative movement between the rotor and the stator due to:

  • Manufacturing tolerances
  • Movement resulting from the looseness in the support bearings
  • The movement caused by shaft/rotor deflection due to unbalance or magnetic pull force
  • Growth (or expansion) of the rotor diameter due to centrifugal loads
  • Cooling air flow requirement to manage the heat

The undesirable effect of the larger air gap is the high resistance to the magnetic flux, which results in an associated electrical loss. For the higher speed motors, the air gap tends to be larger. Though in some cases a larger air gap is intentionally designed to reduce the “armature reaction,” it is generally desirable to minimize the gap to lessen the magnetic flux resistance and to achieve a better efficiency.

Traditionally, electric motors are designed to operate at relatively low speed (2000-5000rpm).

Nascent high-speed/high-power density motors operates above 15,000rpm, posing a serious structural engineering concerns akin to those of turbomachines.

Don't have an account? Sign up here.