Machine Data: 13.2kv 8000 hp, vertical induction motor
70C rise, 307 A FLA 1182 rpm, Class F epoxy VPI Insulation,
Stress control treatments: The semicon is tape and the grading is paint.

108 stator slots
Coil Span = 1 to 14 slots, 3-circuit wye connection, 12 coils in series per leg,
11 turn coil, 2 wire wide.
6 - slots per pole per phase.

Approximately 20 years in service. In-plant periodic megger and dc step voltage tests have been satisfactory. P.D. results have been stable since pd monitoring installed in 2001. A and B phases are approx 100 mv +/-. C is approx 250 (QM+) and 150 (QM-). (We are looking at the pd data a little closer.)

Motor was removed for it's first-ever routine time-based refurbishment/inspection. During inspection, it was noted:

1 - There was a single visual indication of partial discharge (whiate powder) at location of an RTD lead (slide 1).

2 - There is a recurring pattern of partial discharge (white powder) around the stator in groups of 2-3 slots separated by 12 slots. In all cases the partial discharges is located in between coils sharing the same slot at a distance of 1"-3" above the slot on the top end of the motor (connection end). No similar pattern on the bottom end.

We are looking to map out exactly where these coils fit into the circuit and to inspect the clearances a little closer.

Any comments on the severity of this condition? What further inspection or testing would you suggest to evaluate? Do you think it can be repaired or will require a rewind? (this is a critical motor application)

This message has been edited. Last edited by: electricpete,

Presentation.ppt (1,019 Kb, 30 downloads)

Attached is map of the winding and damage areas provided by the shop. The labels in the "CONN" column ( like ML = Main lead) apply to top coil in slot. For this motor there are two series groups per path, so all groups are either a line group or a neutral group. The pd occurs where the line groups from one phase overlaps the line group from another phase, which probably shouldn't be a surprise.

I think the B and C are reversed for coils described in bottom of 56-61 and 62-67.

This message has been edited. Last edited by: electricpete,

Winding_Diagram.xls (28 Kb, 18 downloads)