Induction motor over speed limitation

Reputable manufacturers are required to design the mechanical system of an induction motor (or generator) to be mechanically safe at a minimum of 25 percent above listed nameplate maximum speed. In some specific instances, a user may have defined something even higher, but the chance of this is vanishingly small.

What "mechanically safe" means is that the rotating components of the machine will not "break" under the higher centrifugal / centripetal loading resulting from the higher rotational speeds. It does NOT mean that the machine will be capable of being under power at the higher speeds; nor does it mean that the induction motor can be successfully operated AFTER such an over speed event.

The reason the 25 percent limit is in effect goes back to the days of mechanical (gear and spring) over speed devices. By design, the machine must be capable of operating successfully at 10 percent above top nameplate speed; the higher mechanical "safe" limit was to allow the mechanical device to notice the condition and subsequently signal other (typically mechanical) devices to remove the incoming power from the induction motor terminals - or disconnect the mechanical input for a generator.

As noted, over speed will potentially result in higher vibrations (associated with unbalanced masses moving at higher peripheral velocities) which can damage bearings and - possibly - cause shaft flexure such that the rotor contacts the stator. This same vibration may be at a frequency which results in premature shaft failure (high frequency torsion, for example). If the variable speed drive is not operating in a field-weakened condition, the higher terminal voltage that comes from constant V/Hz will potentially cause damage to the stator winding insulation, leading to a costly repair. In addition - operating at elevated frequencies will require more care in the design and implementation of the equipment grounding to prevent unwanted bearing damage.

All that being said - contact the induction motor manufacturer of the rotating equipment to determine the ACTUAL limit for over speed . . . regardless of how that over speed is achieved.

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