Case Study: Experimental Modal Analysis to Mitigate High Vibrations of Generator Stator End Windings

Praveen Gupta, Manager- Reliability Engg. & Asset Management, Special Oilfield Services Co. LLC


Generator Stator End Windings (SEW) are one of the most intensively stressed unit components of a turbogenerator. Endwindings can be excited by electromagnetic forces due to the stator and rotor currents in stationary and transient conditions at simple and double system frequency, core vibrations at double system frequency, unbalances in the rotating shaft at system frequency being transmitted to the end winding via the housing and core.

Experimental Modal Analysis (EMA) was carried out on the GTG #2B Generator at one of the power plants in Oman, to assess the condition and behaviour of end winding vibrations which had a history of  fractures of the bar insulation, banding cracking related failures. The operational condition and stresses on the end winding may loosen the ties & blocks and may cause the windings to vibrate. The natural frequencies of end winding should not be equal to the system natural frequency or operational frequency (95-110 Hz) of the generator, which may cause high amplitude due to resonance, increasing stresses on the machine.

The EMA was repeated after the Generator repairs to assess the efficacy of the repairs and analyse the vibration behavior of the generator stator after the repairs. EMA post repairs indicated a reduction of 20% amplitude of vibrations and shift of around 10% in the natural frequencies. The EMA was carried out with the methodology of Roving Hammer Test in which the structure was impacted at the winding structure at 25 slot locations for Endcap vibrations and 12 slots for Endwinding vibrations with an Impact Hammer and response being measured by accelerometer mounted at one location.

This paper highights the Turbo Generator design details, test methodlogy, instrumentation used, test results pre and post repairs with discussion on the test results.


Presently, Mr. Praveen Gupta is heading Special Oilfield Services Co. LLC’s Reliability Engineering & Asset Management (REAM) business unit, providing solutions and services for reliability improvement, condition monitoring & asset integrity of Plant Assets in Oman & Middle East. Praveen is an experienced professional with 20 years of rich experience & expertise in Condition Monitoring & Reliability with Vibration Analyst Cat III Certification from Mobius Institute, Australia and CMRP certification from Society of Maintenance & Reliability Professionals (SMRP), US. He is an Electronics Engineer with Management Post Graduation from India, having worked for various reputed organisations viz. Indira Gandhi Centre for Atomic Research (IGCAR) in India, IRD Mechanalysis etc. He has been at Pruftechnik Germany & Meggitt Switzerland on Machine Protection & Condition Monitoring Systems. He has been a regular speaker in National & International Conferences on Reliability & Asset Management in India and abroad and has published more than 20 technical papers in leading National Conferences & journals.