Technical FAQs
What is a flux probe?
A flux probe test determines the existence of rotor winding shorted turns by measuring the magnetic flux produced by each coil in a generator's rotor.
What is a shorted turn?
Shorted-turns are the result of failed insulation between individual windings in generator rotors. The impact of operating with shorted turns can result in abnormal vibration, load limits, higher operating temperatures and forced outages.
The amount of magnetic flux produced by a coil is the product of the current and number of active turns in the coil as measured in amp-turns or magnetomotive force (MMF). If a shorted turn is present, the number of active turns in a coil is reduced and the resulting decrease in magnetic flux produced by the coil can be determined by analysis of the flux probe waveform.
In order to get the best sensitivity for detecting shorted turns in all rotor coils, Generatortech records a series of data points as the load on the unit is varied.
What are the advantages of testing for shorted turns while unit is online and at speed?
Generatortech Rotor Winding Shorted Turn Flux Probe testing is the most comprehensive testing performed while your generator is online and at speed.
What is the prevalence of rotors in operation with shorted turns?
Although isolated shorted turns are undesirable, their existence in a rotor winding does not necessarily indicate a call to action. Generatortech estimates ~50% of the rotors in operation today may have one or more shorted turns. In fact, 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 performance, 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.
What about “partial” shorted turns?
Partial turn shorts cannot exist for long. The very high temperatures generated at a small spot will effectively cause 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.
The slight relative movement of windings during natural start-stop cycles may temporarily heal the shorted turn but chances are the next cycle will reproduce the shorted turn.
Can Generatortech acquire data from flux probes from other manufacturers?
As long as the flux probe was designed correctly to have a adequate probe-to-rotor distance (neither too small or too large) and is sensitive to radial magnetic flux, the Generatortech software will analyze the waveforms correctly.
My company bought the Generatortech Analysis System many years ago. Recently, I have inherited responsibility for the annual testing of our generators. Can I call for help?
Yes! Generatortech will be happy to answer your questions about testing, installation or to advise regarding data analysis. Contact Us via phone or email.
Do we have to buy a Generatortech Analysis Package in order to test our generators?
We have two options for our customers who have not purchased a Generatortech Analysis Package:
You can rent a Generatortech Portable Analysis system, record data files according to our testing protocol, return the equipment and we will analyze your data and prepare a formal report.
Generatortech personnel can travel to your site, record test data and provide you with a formal report.
We bought the Generatortech Analysis Package so we can test our machines. Will Generatortech still do the analysis?
Yes. Generatortech can be contracted for data analysis and formal reports.
What are the common causes of shorted turns?
Shorted-turns are the result of failed insulation between individual windings in generator rotors. Stop-Start cycles, line disturbances, contamination, moisture, manufacturer error, and damage during retaining ring installation are some of the reasons insulation fails.
What’s the impact of operating with shorted turns?
The impact of operating with shorted turns ranges from no-problem to abnormal vibration, load limits, higher operating temperatures and forced outages.
The location and number of shorted turns in a rotor determines the magnitude of the effects of running a rotor with shorted-turns.
As generator units age, shorted-turn problems are more likely to be experienced. The stresses involved in each Stop-Start cycle play an especially important role. Occasionally, newly rewound fields will have shorted-turns.
Shorted-turns in the field winding can cause operating conditions which may limit unit loads. If conditions are extreme, forced outages may occur.
What is the advantage of the G50 Monitoring System?
With the G50 your generator’s flux probe is permanently connected to a server which can be accessed via Internet or your company intranet using a secure VPN connection. The G50 allows test data to be acquired automatically and/or manually. So, you no longer need to travel to distant power stations for testing. Also, Generatortech can be contracted to be responsible for your regularly scheduled testing and analysis via the secure VPN connection.
How do I set up a shorted-turn condition monitoring program?
When planning the installation of permanent probes, critical machine dimensions such as rotor outer diameter (O.D.), retaining ring O.D., and stator core inner diameter (I.D.) need to be supplied to Generatortech so the flux probe can be custom manufactured before assembly. Generatortech will provide a drawing showing which dimensions must be supplied. If desired, Generatortech personnel will take the dimensions.
If plans are being made to assemble an air-gap probe we recommend contacting Generatortech as early as possible.
What software do you use to collect and analyze the test data?
Generatortech provides a Windows-based program that combines data acquisition with the display and analysis of rotor shorted turns. The system uses a high-speed, 32-bit data acquisition board to record the flux probe waveforms. The program provides an oscilloscope-like display while recording the data. The waveform can be analyzed immediately with Generatortech's software. The program automatically identifies and labels the waveform features and measures the appropriate values to provide estimates of turn shorts.
Generatortech's Acquisition and Analysis Program has been designed to quickly and easily acquire and save data from flux probes.
Who are our typical customers?
Power generating utilities and co-generation industries worldwide including turbine and hydro generators.
Service companies who provide retrofit installations and use our probes & analysis software for spin pit testing.
Original Equipment Manufacturers (OEM) who install Generatortech equipment in new generators and as retrofit applications during major outages.