Technical Considerations

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 spot will cause copper melting and a low resistance spot weld. The low resistance short will completely short out one turn. Test data which calculates a partial shorted-turn condition are, therefore, not correct. They are defining a limit in the test sensitivity. Engineering judgment must be applied to estimate the integer number of turns shorted. Start-stop cycles have resulted in healing shorted turns due to slight relative movements of the turns. However, the chances are high that the next start-stop cycle will reproduce the shorted turns.

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.

Generator Electrical Losses from Shorted Turns

When shorted turns occur, higher field current is required for a specific load. The amp turn relationship: A_S = A_NT_N/T_S. The field winding loss for a specific load will increase by the same ratio (T_S/T_N). This can be calculated from the winding I²R loss formula. "R" is lower by T_S/T_N, but "I²" is increased by (T_N/T_S)². Two turns shorted out of one hundred turns will result in a 4% higher hot spot rise over cold gas. The table below is an example for one specific load.

No turn shorts

2% shorted-turns

10% shorted-turns

Cold Gas

46.0 C

Hot Spot Rise

84.0 C

87.5 C

101.6 C

Hot Spot

130.0 C

133.5 C

147.6 C

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