In large motors and generators, the air gap needed to separate the revolving rotor from the stator should be as small as possible. This reduces the magnetizing power requirement which needs to be large enough to prevent contact between the two, despite manufacturing tolerances on their dimensions or movement resulting from mechanical deflection or looseness in supporting bearings. The gap should also be uniform; a non-uniform gap causes increased noise and vibration.

Since large generators and motors are usually quite expensive, it’s advisable to monitor the gap to prevent catastrophic failure should the rotor or stator expand due to heating and operating conditions.¬† Automated shut down or load reduction can save millions of dollars and possibly even save lives.