Generator Rotor Shorted Turn Analyzer

Shorted turns in generator cylindrical rotor field windings can contribute to vibration problems due to rotor thermal bending from the uneven heating associated with non-symmetrical dc current flow and watt losses in faulted windings. Shorted turns can also cause unbalanced magnetic flux in the air gap that can also aggravate vibration problems.

Since vibration signature analyses for rotor shorted turn problems is not always an exact science, it is desirable to have confirming data from other testing before proceeding with very costly disassemblies and repairs of large machines. Additional tests for confirming the existence of shorted turns in generator rotor field windings are commonly performed before committing to expensive repairs. At this time, the following three test procedures are generally used in the industry to help verify whether or not generator field windings have shorted turns:

1. Thermal Stability Testing - involves changing generator-operating parameters (watts, vars, and cooling) and recording and analyzing the impact on rotor vibration signatures.

2. Flux Probe Analysis - utilizes an installed air gap probe to measure and analyze the magnetic flux from each rotor slot as it passes by the location of the sensor. Some generators are permanently equipped with flux probes and many are not. Installing the probe normally requires a unit outage, especially with hydrogen-cooled machines.

3. RSO (Repetitive Surge Oscilloscope) Testing - which is the applied principal for the "Sumatron Generator Rotor Shorted Turn Analyzer" will be discussed in more detail in the following text:

It should be noted that none of the foregoing testing (vibration analyses, thermal stability, flux probe analyses, and RSO testing by themselves provide absolute certainty that there is a shorted turn problem in the generator rotor. However, when confirmed by other testing the probability of the field winding being the cause of the vibration problem increases significantly. Shorted turn anomalies can be masked if they are near the center of the winding or otherwise balanced, if there are multiple shorted turns, if they are intermittent, if there are grounds, and if there are other contributors to the overall machine vibration levels.

RSO testing has some advantages over other testing in that it can be used periodically during rewinds to verify that windings are free of shorts and on both at rest and spinning de-energized rotor windings.

The instrument simultaneously injects fast rise time low voltage repetitive surges on each collector ring. The reflected wave forms are displayed on a dual trace scope. If there are no anomalies the reflected wave forms will be identical. The wave forms will be different if shorted turns are present. The instrument can be used with analog or digital dual trace scopes that have bandwidths of 20 megahertz or greater and a voltage resolution of at least 0.5 volts per division. The instrument case is designed to carry the instruction manual and all necessary test leads. The following test leads are included with the instrument:

5 ea. BNC to BNC cables (scope and rotor connections)
2 ea. BNC to 1/2 inch alligator adapter (for connecting to collector rings)
1 ea. Green banana plug to 1 1/16 inch alligator (generator rotor ground)
1 ea. Green banana plug to banana plug
(optional scope ground)
1 ea. Computer type power cable
2 ea. Banana plug to banana plug (for test simulator)
2 ea. BNC to single banana jack adapter (for test simulator)