Chuck Yung
EASA Senior Technical Support Specialist
Historically, one emergency repair used for large, low-speed machines (motors as well as generators) was to remove the damaged/failed coils from the circuit. Cutting out a single damaged coil permitted the machine to be quickly returned to service with minimal disruption.
As long as some basic principles are followed, this method can be safely used. It is still popular with operators of large, low-speed synchronous machines. One common application for such machines is hydro power stations. There are many old hydro generators operating with a dozen or more coils bypassed. The underlying goal when cutting out coils is to minimize the negative side effects and keep coil groups intact for future removal.
WARNING: This procedure is not recommended for 2-pole machines. The odds of success are slim.
Start with the connection
The first consideration is the connection. Parallel circuits complicate matters. The best candidate is a winding connected 1-wye. With a delta connected winding, or with parallel circuits, the technician must consider the effect of circulating currents and additional heat generated in the windings. Consider the parallel paths of a 2-wye in Figure 1.
If a single damaged coil is removed from one path, there will be one fewer coil in that path than the parallel path. A circulating current will result. The mathematical calculations for estimating winding temperature are complex, but a rough estimate is to ratio the coils in the parallel paths. In this example, 4 coils paralleled with 5 coils = 20% circulating current. Expect a 15º C increase in winding temperature (1.2)1.5 in the group with fewer coils. Success, then, requires a good coil be cut out of the parallel path. With fewer coils in one phase than the other phases, slightly unbalanced current may result.
Effect of multiple parallel circuits
The more parallel circuits in the winding, the greater the number of good coils that must be removed from the winding. For example, a 20-pole machine with a 10-circuit wye with only 1 damaged coil would require that a minimum of 10 coils be removed from the circuits. Cutting out perfectly good coils runs counter to our instincts, but that step is essential for proper operation.
To take our worst-case scenario still further, if the phase current unbalance were deemed unacceptable, it would be necessary to cut out 10 coils from each of the other phases. Cutting out 30 coils because of a single damaged coil is a drastic measure.
Determine coils to remove
Assuming the winding has parallel circuits and we have to remove good coils from the circuit, the next step is to determine which coils to cut out. To minimize magnetic pull disruptions, the coils should not be in exactly the same phase position as the damaged coil (or each other). If the winding has uneven grouping (as many low-speed machines do), try to cut out coils in the groups with more coils.
In a worst case, a winding with groups of 1 and groups of 2, we should diligently avoid cutting out a group of 1 coil – that would become a phantom-pole. (The winding would then become a part-salient, part-consequent pole.) The underlying goal when cutting out coils is to minimize the negative side-effects and keep coil groups intact for future coil removal. Physical separation should be such that the cut out coils are not symmetrically spaced.
Importance of insulating and bypassing
The defective coil should be cut completely through one knuckle. Both ends of the cut should be taped to insulate them. The series connection ends should also be taped when the coil is bypassed in the connection.
When good coils must be removed from the circuits, they should only be bypassed. In other words, cut the series connection and bypass that coil. Do not cut through the knuckle or otherwise damage coils that are not defective. It may be possible to connect those coils back into the active winding if other coil failures occur in the future.
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