Rotor Winding Description and Types of Turn Shorts
Figure 1 - Rotor cross-section with 7 Coils/Pole and a rotor showing end-turns of winding after the retaining ring was removed. Note the blocking used between the coils to maintain separation.
Rotor Winding Description
The rotor windings used in round rotors have turns that are arranged in multiple coils (Figure 1 shows a two-pole rotor cross-section with 7 Coils/Pole). Each coil has two machined slots that carry the copper turns from one end of the rotor to the other. In a coil, the turns spiral up or down before connecting to the next coil. The copper turns do a U-turn under each retaining ring. The retaining rings support the weight of the end-turns of the coils and the rotor wedges support the weight of the turns in the coil slots.
Figure 2 shows a large two-pole rotor after it was pulled from a generator stator. The yellow overlay lines suggest the current flow around the turns in the visible pole face. The DC current used to excite the rotor winding is provided by some type of excitation system, In this case, the excitation current gets to the rotor winding through stationary carbon brushes that slide on the two collector rings.
The current travels from one collector ring down a copper conductor along the rotor shaft (axial lead, Figure 3) and then passes out through the shaft to the bottom turn of Coil 1 - Pole A (Coil 1A, Figure 4). The initial turn travels along a Coil 1A slot to the turbine-end retaining ring section and curves around to enter the other Coil 1A slot. The copper turn travels back along rotor to the exciter-end retaining ring section and spirals on top of the first turn (starting the next turn in Coil 1A) The turns continue to spiral up the Coil 1A slots until the slots are filled with the desired number of turns and then a coil-to-coil connector allows current to travel from the top turn of Coil 1A to the top turn of Coil 2A (Figures 5 and 6). The current then spirals down the turns used for Coil 2A until it reaches the bottom turn and a coil-to-coil connector allows current to get to the bottom turn of Coil 3A. The process is repeated, with the current alternately spiraling up and then down the turns in each coil until the current reaches the top turn of Coil 7A. A pole-to-pole connector is used to carry current to the top turn of Coil 7-Pole B (Coil 7B) and the whole process is repeated in reverse until the current arrives back at the other collector ring. All of the rotor turns in both poles are connected in one long series loop, from one collector ring to the other.
Figure 2 - Large two-pole rotor with a collector rings. Current flows from one collector ring down the shaft (Figure 3) to where it connects to Coil 1A and traverses all the turns used in each coil in Pole A. A pole-to-pole connector gets the current to Pole B, where it travels through each turn in that pole before heading back up the shaft to the other collector ring.
Figure 3 - A rotor shaft opened to see the copper conductors (axial leads) that carry current between the retaining rings and the initial turns connecting to the each pole’s windings.
Figure 4 - Main lead connection from shaft copper to bottom turn of Coil 1.
Figure 5 - Diagram showing how coil-to-coil and pole-to-pole connectors link the coils into one series loop. In this case, the coil-to-coil connectors between Coils 1-2, Coils 3-4 and Coil 5-6 are between the top turns of those coils and are visible when the retaining ring is removed (Figure 5). Not shown in the diagram: for each turn in each coil, the turn travels down the rotor body to the turbine-end area in one coil slot and makes a u-turn to return to the collector-ring area in the other coil slot.
Figure 6 - Coil-to-coil connections in a 7 Coil/Pole rotor winding where the connection from the shaft copper axial leads used the bottom turns of the #1 coils. The bottom coil-to-coil connectors and the pole-to-pole connector are not visible in this image.
Turn-to-Turn Short
The most common type of short involving a rotor winding is the turn-to-turn short (99% of detected shorts), where insulation damage or movement allows contact between two adjacent turns making up part of the coil stack. The hundreds to thousands of amps used by large generator rotors means that the initial point contacts between turns will produce copper melting resulting in a low-resistance spot weld. The size of the weld is proportional to the current carried by the winding. The low resistance of the short means that 99.9% of the current will go through the short, effectively removing one active turn from the winding. Turn-to-turn shorts are common and often produce no operational issues for the generator. Whether problems arise from turn-to-turn shorts is dependent on the percentage of turns shorted out of the winding and the location of the shorts within the winding. The problem with these “silent” shorts is that they raise the temperature of the rotor winding, which makes the additional shorts more likely to develop. Therefore, it is important to track the increase in turn shorts even if they are not yet affecting operation.
If turn shorts are affecting operation, the flux probe test can tell which coils are affected and how many shorts are in each affected coil, facilitating repair efforts.
Figure 7 - Coil winding schematic with five total turns - The short between Turns 3 and 4 removes a complete turn (gray) from the coil, leaving 4 active turns. It doesn’t matter where the short is located along the turn since it will always bypass a complete turn.
Figure 8 - Alternate diagram illustrating a turn-to-turn short in Coil 3A. The short contact (red T-T) carries the full excitation current, bypassing a complete turn in that coil.
Coil-to-Coil Short
Coil-to-coil shorts are a much more serious type of short, but they occur relatively rarely (1% of detected shorts). A coil-to-coil short involves contact between two adjacent coils in the end-winding region under the retaining rings. If unlucky, a coil-to-coil shorts will bypass all of the turns used in those two adjacent coils, removing 10-20% of the total turns in the winding at one stroke. Coil-to-coil shorts often cause a forced outage to repair since they normally create serious operational issues (high vibration that can trip the unit, excitation limits that prevent unit from reaching rated load, overheating of the rotor winding that quickly causes additional shorts).
Figures 9-13 show an example of a coil-to-coil short in a rotor where end-turn distortion cause contact between the top turns of Coil 2B and Coil 3B, bypassing all 26 turns used in those two coils. The unit could not reach over 50% of full load before vibration rose above trip level.
Figure 9 - Lead Slot Overlay Graph that shows the Coil 2B-Coil 3B short. Higher loads would have shown the coil-to-coil short much better than this, but rotor vibration was limiting load to less than 50% full load.
Figure 10 - Rotor cross-section showing location of the 26 total turns bypassed due to Coil 2B-to-Coil 3B short.
Figure 11 - After the retaining ring was removed, the flux probe test prediction was confirmed. Top turn distortion in Coils 2B and 3B made contact between those two turns, allowing current to bypass all of the turns used in those two coils (Figures 11 and 12).
Figure 12 - Diagram showing how contact between top turns of Coils 2B and Coil 3B allowed current to bypass all 26 turns used in those two coils.
Figure 13 - Alternate diagram illustrating a coil-to-coil short between top turns of Coil 2B and Coil 3B allow current to bypass all the turns in the two coils.
