Technical Considerations

On Operating with Turn Shorts

Although isolated shorted turns are undesirable, their existence in a rotor winding does not necessarily indicate a cancerous condition. Generatortech estimates 50% of the rotors in operation may have one or more shorted turns. Rotor windings on new generators may have one or more shorted turns after a few stop-start cycles. If the shorted turns are not significantly affecting satisfactory operating conditions, no corrective actions should be taken. Periodically monitoring for shorted turns will help establish whether new turn shorts are developing and when corrective action should be taken. 

No Such Thing as Partial Shorted Turns

Partial turn shorts can not exist for long. The very high temperatures generated at a small contact spot will cause copper melting and produce a low resistance spot weld. The low resistance short will completely bypass a complete turn. In some cases, engineering judgment must be applied to estimate the integer number of turns shorted out of a coil.

Some Turn Shorts Will Clear

Turn shorts will occasionally clear. The same turn-to-turn movement that can damage insulation and produce a turn short can sometimes break the contact responsible for the short. However, when turn shorts clear, there is a high probability that the short will re-develop in the future.

More on Thermal Sensitivity Rotor Unbalance

Slots with shorted turns operate at lower temperatures than slots without shorted turns. The I2R losses are lower, but the cooling circuits remain the same. The rotor temperature gradient to cause bows will be a function of the number of shorts and their location. Shorted turns in coils near the quadrature axis will have little effect on thermal sensitivity balance because the effected slots are 180 degrees apart. Shorted turns in the small coils 1, 2 and 3 on one pole and not the other pole will have a greater effect in causing rotor unbalance problems.