Rotor Overspeed Effects on Stator Damage

Rotor overspeed can have significant direct or indirect impacts on the stator, and in severe cases may even lead to stator failure. From the perspective of electromagnetic coupling, rotor overspeed causes the rotational speed of the air-gap magnetic field to far exceed its rated value. As a result, the electromotive force (EMF) and current induced in the stator windings rise sharply. This not only leads to stator core overheating but also accelerates the aging of insulation materials.

From a mechanical stress standpoint, rotor overspeed tends to aggravate rotor imbalance, triggering severe vibration of the unit. Such vibration is transmitted directly to the stator, potentially causing loosening of the stator core and deformation of the windings. In extreme cases, it may even result in the failure of end rings. More dangerously, extreme overspeed may cause rotor components to detach or the rotor to expand beyond allowable limits, leading to friction or collision with the stator and causing direct and serious damage.

Effects on the Stator at Different Levels of Overspeed

Slight Overspeed (1.05–1.1 × rated speed):
At this stage, the stator is mainly affected by minor electromagnetic fluctuations. Rotor overspeed slightly increases the speed of the air-gap magnetic field, resulting in small rises in induced EMF and current in the stator windings. The stator core temperature may experience a brief increase but remains within safe limits. Insulation aging is only marginally accelerated, and no obvious mechanical vibration is transmitted. The stator core and windings remain structurally stable, protection devices typically do not operate, and normal operation can usually be resumed after shutdown and inspection.

Moderate Overspeed (1.1–1.2 × rated speed):
Both electromagnetic and mechanical effects begin to appear and become increasingly evident. Induced current and EMF in the stator windings increase significantly, intensifying core overheating and accelerating insulation aging. Prolonged operation under these conditions can lead to irreversible degradation of insulation performance. Meanwhile, increased rotor imbalance amplifies vibration levels, which, when transmitted to the stator, may cause slight loosening of the stator core, minor deformation at winding ends, and increased stress on end rings. At this point, protection systems are likely to issue warning signals, and continued operation will further increase the risk of structural damage to the stator.

Severe Overspeed (1.2–1.3 × rated speed):
The stator faces serious electromagnetic and mechanical damage. Stator winding current and temperature rise sharply, and localized insulation overheating or carbonization may occur, potentially leading to inter-turn short circuits. Intense vibration can expand the area of stator core loosening, worsen winding deformation, and cause cracks in end rings. In some cases, increased non-uniformity of the air gap between rotor and stator induces fluctuating electromagnetic forces, further deteriorating stator stress conditions. Protection devices will typically trip and force shutdown. If shutdown is delayed, localized stator damage may occur.

Extreme Overspeed (above 1.3 × rated speed):
The stator is exposed to catastrophic damage risks. Rotor overspeed may cause rotor components to break off or excessive rotor expansion, resulting in direct friction or collision with the stator. This can lead to severe structural failures such as shattered stator cores, burned windings, and fractured end rings. Even without direct contact, the ultra-high-frequency air-gap magnetic field can induce extremely large currents in the stator windings, instantly breaking down insulation, triggering phase-to-phase short circuits, and ultimately rendering the stator completely scrapped—often accompanied by serious safety incidents.