Inter Turn Fault in the Rotor

i am not qualified in this particular area, but i have used the Tinsley Short-turn tester on contactor coils. Very good, even one shorted turn in more than 20000 shows up. As you may be aware, even one turn acts as a shorted secondary of a transformer and will lead to eventual failure in AC coils of contactors.

No problem if the coil is DC.

Which prompts me to ask...won't the rotor in synchronous machines see only DC? If so, what is the problem if there are shorted turns ?

i have used an impulse-voltage based shorted-turn detector for assessing probability of vulnerable insulation...you may get some help at this weblink :Impulse_Tester

Thank you for the information.The weblink :Impulse_Tester suggests use of 'Winding Turns Tester'. This needs further study.

A "running short" can be detected with an oscilloscope.

Thank you for advising specific solution: A "running short" can be detected with an oscilloscope.

In this connection, IEEE Xplore Digital library refers to a paper:

"Fault diagnosis of the inter-turn short circuit in rotor windings based on wavelet analysis"

This paper appears in: "Electrical Machines and Systems, 2008. ICEMS 2008. International Conference"

"Abstract

A new fault diagnosis method of inter-turn short circuit detection in generator rotor

windings based on wavelet transform is presented. The detection method is based on the singularity characteristics of the induced electromotive force signal of search coil in the air gap. Electromagnetism characteristic of turbo-generator rotor windings is analyzed. Wavelet transform was applied to the detection of the abruptly change signal based on detecting coil method, extracts the fault characteristics of the induced electromotive force signal and excludes noises, thus realize the detection of inter-turn short circuit fault in generator rotor windings. Simulation indicates the wavelet analysis method is suited to detecting fault of inter-turn short circuit in generator rotor windings. "

It will be useful if we can access the full text of this paper.

I suppose the paper is talking about injecting an RF signal and observing the reflection. Sort of a "coil radar".

The winding is held firmly in place by insulation and coatings of varnishes, epoxy and the like (orignally liquid to solidify later), it is therefore practically impossible that centrifugal force acting on rotor to have created such an event.

Hypothetically speaking if the interturn fault does occours due in rotation, the circulating current in the faulty coil section would cause arcing and weld the defective spot, when the machine is brought to a standstill the staus quo is maintained.

"practically impossible" You evidently, are not familiar with real motor faults caused by damage/deterioration of this insulation system, or you are a member of BP's board of directors.

GA.

These methods can also be used to detect air gap eccentricity.

Dear nvmani,

Interturn fault in rotor may be detected instandstill condition by using Electric Motor Checker Instrument. In case of interturn fault, by comparing % inductance value with other good one ( in defective winding the value will be less) interturn fault may be detected; also you may use TDR ( Time Domain Reflectro meter) instrument for detecting interturn fault. At the fault point, a slight inverted loop can be seen in display giving length of the conductor where fault occurs; velocity factor should be chosen very carefully and wisely to get correct result.

It is not possible turns get shorted only when rotor starts rotating, since rotor winding is comprise of conductor,insulation, and liquid insulating vernish which are put in baking oven during manufacturing process and solidify after end of process and cool down.

Your suggestion on the method for detection of fault is quite informative.

Regarding such scenario of fault only at running condition, an abstract of paper presented during an IEEE conference is quoted below, which mentions of such possibilty.

Abstract

The electromagnetic characteristic and rotor vibration characteristic of turbine-generator are analyzed when rotor winding inter-turn short circuit fault has happened. This paper reveals that exciting magnetic force F_f is constant in a fixed condition whereas the exciting current I_f increases in case of rotor inter-turn fault. This paper also finds relevant characteristic parameters. Based on the theory, we can get training patterns without doing destructive tests. Then BP (back propagation) neural network can be adequately trained and diagnosis rotor winding inter-turn short circuit. BP neural network is independent on mathematic models and parameters of turbine-generator. Finally practically acquired dynamic experiment data of the MJF-30-6 generator, the results of verification show that the theory analysis is right and the method is efficient and accurate.

Ref: IEEE Xplore digital library "Fault diagnosis of rotor winding inter-turn short circuit in turbine-generator based on BP neural network"

hi, I would want to know if it's possible to detect short circuit with typical tdr like "canoga perkins 1401"? Below a brochure. I appreciate your help.

www.at2.com/downloads/documents/canoga/1401e.pdf

It would be necessary to have full mapping (percentage length verses slot number) of the rotor winding to locate the fault. In cylindrical rotors, the length of coil varies due to variable overhang portion. So in order to locate inter turn fault, one should know in which slot (or pole) the fault lies. Otherwise unnecessary removal of slot wedges and then lifting of the coil may result. Generally manufacturer keeps reference wave response curves (with purposely created faults in different slots) for comparison with actual fault.

It means that I can detect short circuits in rotors with this tdr tester but if I'd want to know in wich pole the fault lies, I need more information. Right? Thanks for the information and your time.