Energy and Power En gi neering, 2011, 3, 221-226
doi:10.4236/epe.2011.33028 Published Online July 2011 (
Copyright © 2011 SciRes. EPE
Experimental Modal Analysis of Stator Overhangs of a
Large Turbogenerator
Ram Sewak, Rajesh Ranjan, Vivek Kumar
Heavy Electrical Equipment Plant (HEEP), Bharat Heavy Electricals Limited, Haridwar, India
E-mail: {ramsewak, rranjan, vivek}
Received March 12, 201 1; revised April 10, 2011; accepted May 3, 2011
Modal analysis of engineering structure for comprehending/resolving the vibration related issues/problems
are well known. Two classical techniques-analytical (Finite Element Method-FEM) and experimental (Im-
pact testing/Natural Frequency Test-NFT/Bump test) are generally used as complementary as well as
stand-alone depending on the time, nature of structure, availability of the analysis tools, cost etc. In the pre-
sent study, experimental technique was used in mitigating the endwinding vibration problem of a turbogen-
erator. In one of the Turbogenerators of 50 Hz variant, an increasing vibration trend was observed with sys-
tem frequency sweep in almost whole of the endwinding basket particularly more on exciter end during sus-
tained short/ open circuit conditional runs. Experimental modal analysis was carried out of the overhangs.
Frequency response functions (FRFs) were generated in local and global modes. The analysis thereon indi-
cated global resonance of stator overhangs. Accordingly, appropriate remedial measures were planned and
implemented. Consequently, global resonance frequency was shifted to higher zone, which in turn, resulted
into substantial reduction in endwinding vibration levels.
Keywords: Turbogenerator, Endwinding Vibration
1. Introduction
Modal analysis is a well known technique for analysis
and resolution of vibration problems of engineering
structures. However, this has recently found use in ana-
lyzing the turbogenerator stator endwinding vibration. Its
principal co-ordinates—natural frequency, damping and
associated mode shape are the functions of structure’s
geometry, mass, stiffness, temperature, and boundary
Generator stator endwinding is one of the most inten-
sively stressed unit components of a turbogenerator.
Endwindings can be excited by electromagnetic forces
due to the stator and rotor currents in stationary and tran-
sient conditions at simple and double system frequency,
core vibrations at double system frequency, unbalances
in the rotating shaft at system frequency being transmit-
ted to the endwinding via the housing and core [1].
The present study relates to a Turbogenerator of 50 Hz
variant, where increasing vibration trend of nearly whole
endwinding basket was observed during sustained short
circuit and open circuit conditional runs, particularly
when the system frequency of prime-mover was varied
from 47.5 Hz to 51.5 Hz. The trend was more dominant
on Exciter End (EE), compared to Turbine End (TE)
(Figures 1 and 2). In view of this, it was decided to carry
out investigation of such a vibration behaviour of end-
winding structure by experimental modal analysis. This
1) Impact testing of overhang structures on specific
points and generation of FRFs in standstill condition in
local and global mode;
2) Extraction of modal parameters from FRFs gener-
ated-frequency, damping and mode shape;
3) Validating the results during operation of the gen-
erator by actually tuning the prime-mover’s frequency.
This papers deals with experimental study conducted
on overhang structures in a limited frequency span, con-
cept of theoretical deformation shape of 4-node mode/
2-lobe mode for a two-pole generator, its practical im-
plication on overall vibration behavior of endwinding
basket, rectification measures implemented and their
effectiveness validated using measured data.
2. Vibration Monitoring
Largely, piezoelectric accelerometers were used for
endwinding vibration monitoring. However, lately, Fi-
47 47.5 48 48.5 49 49.5 50 50.5 51 51.5 52
Figure 1. Endwinding vibration behaviour on Exciter End.
47 47.5 48 48.5 49 49.5 50 50.5 51 51.5 52
Figure 2. Endwinding vibration behaviour on Turbine End.
bre-Optic Accelerometers (FOA) also have been used
particularly on high voltage components like phase bar
head-joints, connections etc. For data recording and
measurement, stand-alone continuous on-line monitoring
system was used.
Generally, three neutral/near neutral points of genera-
tor stator endwinding bar head-joints are chosen on each
side of TE and EE for mounting six piezo-electric accel-
erometers in radial and tangential/circumferential direc-
tions. Two probes-in radial and tangential directions are
screwed on to a single patented epoxy insulated base-
plate which in turn, is glued to bar head-joint with putty.
The probe-cables are taken out over the involute over-
hang from inside of the generator casing to a monitoring
instrumentation set-up via specially designed lead-in-
plates [5].
The generator was operated on no-load i.e. sustained
short circuit condition (SCC) at rated In and open circuit
condition (OCC) at rated Vn. Vibration measurements
were carried out on the above conditions for all the
12-points simultaneously. The average winding tem-
perature in SCC was about 60˚C while in OCC, it was
about 45˚C. Also, vibrations were recorded continuously
during system frequency sweep of prime-mover from
Copyright © 2011 SciRes. EPE
47.5 Hz to 51.5 Hz (Figures 1 and 2).
The stator endwindings are subjected to three different
vibratory forces [2].
1) Pulsating forces during start-up, shutdown and
normal operation due to forced vibration
of the entire end-winding system, induced by bearing
and stator frame vibrations (excitation frequency de-
pending on th e rotor speeds)
of the entire end-winding system induced by core
vibrations (double frequency excitation)
of each single bar, induced by electromagnetic forces
(double-frequency ex citation);
2) Forces resulting from restrained thermal expansion
which depends upon generator load and coolant tem-
3) Electromagnetic forces of double system frequency
resulting from a high D.C. component arising from ab-
normal operating conditions especially during short cir-
cuit conditions during synchronization.
Resulting endwinding vibration, largely, is a combina-
tion of electromagnetic vibrations of windings (self ex-
cited vibration) and core induced magnetic vibration
which is independent to stator current.
3. Modal Analysis
Modes of vibration are inherent properties of a structure.
These are determined by the material properties (mass,
stiffness and damping), geometric configuration, and
boundary conditions of the structure. Each mode is de-
fined by a natural (modal or resonant) frequency, modal
damping, and a mode shape. If either the material prop-
erties or the boundary conditions of a structure change,
its modes will change. At or near the natural frequency
of a mode, the overall vibration shape of a machine or
structure will tend to be dominated by the mode shape of
the resonance.
There are two methods for the excitation of the struc-
1) Impact method where sudden stimulus is given to
the structure and corresponding response is recorded as a
ratio of output and input signals-called FRF (frequency
response function);
2) Harmonic excitation by a shaker.
Both approaches have advantages and disadvantages,
while the former is quick, less expensive and easy to
operate. Im pact test i ng was us ed in our present study.
3.1. Impact/Bump Test
Frequency Response Functions (FRFs) were generated
using light /heavy-duty impact hammers B & K type
8201 and 2304, an ICP accelerometer and a 2-Ch. OROS
34 FFT Analyzer. This test was done on complete wound
stator overhang on both the endsTE/EE in local and
global mode.
3.1.1. Local Mode
This test determines individual bar head-joint vibration
modes. There were 54 bar-head joints. Each bar-head
joint was impacted by a light instrumentation hammer in
radial and tangential directions separately and corre-
sponding directional responses was recorded as FRFs
one at a time in a limited frequency range of 0 - 200 Hz.
This was done on both the ends—TE and EE.
3.1.2. Glob al Mode
This test determines the vibration modes of entire over-
hang structure. Entire involute overhang of 54 bar-head
joints was radially divided into 18-points. One bar
head-joint was taken as a reference point. The impact
was given on steel bracket near to this point. The probe
was mounted on nose joint in radial direction. Frequency
Response Functions (FRFs) were recorded by impacting
the structure on single ref erence point and corresponding
responses in radial direction was recorded as FRF on
designated locations one by one. Thus, measurements on
all the 18-points were carried out on both the TE and EE.
3.2. Modal Parameter Extraction
The natural frequency and associated mode shape were
extracted manually from individual FRF both for local
and global mode of measurements. Typical global natu-
ral frequencies in a span of 200 Hz are summarized be-
In local mode, the frequency distribution amongst 54
bar-head joints was quite varied given the fact that indi-
vidual bar head-joint had different local tightness and
other boundary conditions. In global mode, the said
variation was very little between point to point as
brought out in Figure 3.
The mode shapes were generated semi-manually using
recorded FRFs in a MS Excel sheet for all the global
natural frequencies of EE. One such mode shape at 110
Hz is shown in Figure 4. From this, it is clear that the
shape is of 4-Node/2-lobe-mode.
3.3. 4-Node/2-Lobe Mode of Overhang
The electro-magnetic forces in the electrical machines
(generator and motors) are of special significance. These
have been explained with the help of example of a
two-pole motor. In case of higher number of poles only
the complexity of form is raised, without producing ba-
sically any effects.
Copyright © 2011 SciRes. EPE
Figure 3. Global natural frequency distribution on over-
hangs at TE & EE.
-80 -60 -40 -20020406080
Figure 4. Expe rimentally generated 4-node/2-lobe deforma-
tion of an overhang of a turbogenerator.
Figure 5 points out the magnetic forces with which
the pole of the rotor acts on the stator core packet and
deforms it to an oval form. The original circular form is
superimposed with 4-node points (with red)—where no
exact deformation take place. These forces occur as soon
as the rotor is excited, that is even in no-load. When the
rotor rotates, then the ovality also follows it and stimu-
lates a vibration with the double the rotation frequency
Figure 6 describes the front view of an overhang. The
plotted current distribution corresponds to the operation
with power factor 1, pure active power. Current exit in
upper region of winding from the active part of the stator
and accordingly again enters in the lower part. Bar invo-
lutes transport the current from the top to bottom half
(black arrows). The opposite axial components of current
tend to collide with each other, while tangential compo-
nents exercise the attracting forces (green arrows). They
Figure 5. Magnetic forces of rotor.
Figure 6. Current forces within the core packet & over-
together make the overhang oval again. It may be noted
that the deformation takes place exactly opposite to the
deformation in core packet. If additional reactive power
is driven and the power factor COSØ becomes less than
1, then the oval of overhang deformation rotate by angle
Ø till it exactly shows the same direction, just as that of
the core packet deformation. Overhang forces are pro-
portional to the square of stator current [3,4].
Both the quantities superimpose the electro-magnetic
forces at load during operation. While the forces act at
different positions of stator, the result of superimposition
strongly depend on the place viewed. In general, it is
applicable, that the electromagnetic deformation show
doubles the number of nodes, as the no of poles. Fre-
Copyright © 2011 SciRes. EPE
quency is just the double grid frequency for all number
of poles. Experimental mode shape generated from
global frequency measurements are shown in Figure 4.
4. Analysis
1) Since the generator overhang including core vibrate at
double the rotor frequency, the natural frequencies near
to the system frequency of 50 or 60 Hz and twice the
value, 100 or 120 Hz, are of special interest [1].
The generator was put on operation in SSC and OCC
successively for about 4 to 5 hour in each regime, the
average winding temperature recorded to about 60˚C and
45˚C respectively. Further, in stabilized conditions, when
the drive motor frequency swept slowly from 47.5 Hz to
51.5 Hz, the vibration level on EE shot up sharply (Fig-
ures 1 and 2), particularly from 50 Hz onwards. This
behavior can be explained on the basis of structural dy-
namics. In standstill condition i.e. at normal temperature
condition, global natural frequency was obtained as 110
Hz (Table 1). As the generator run, the temperature of
the overhang raised which brought down the natural fre-
quency in the region of nearly 102 to 103 Hz. Since
overhang excites at double the fundamental rotor fre-
quency (2 × 50 Hz = 100 Hz) in a 2-pole machine, thus
resonance phenomenon might have been occurred when
the prime-mover’s frequency was swept from 50 Hz to
51.5 Hz. The reduction of natural frequency at elevated
temperature from normal temperature has been reported
by others as well [6].
However, the machine could not be operated beyond
51.5 Hz due to system’s limitations, otherwise exact
value of resonance frequency would have been detected.
On TE, generally, this phenomenon was not observed
probably due to higher global natural frequency (Table
In addition to above, 4-node/2-lobe mode shape (Fig-
ure 4) of generator ov erhang obtained at 110 Hz by mo-
dal analysis might have been coincided with the theo-
retical mode shape. This further aggravat ed the vibration
behavior of ov e rhang.
2) Role of core vibration was also studied by carrying
out impact test on it. Dominant natural frequency was
found around 84 Hz which seemed to be quite safe from
the resonance point of view.
So, from the above description, it appeared that the
Table 1. Typical global natural frequencies (Hz) obtained
on overhangs.
Global Mode (Hz) Local Mode (Hz)
Turbine End Exciter End Turbine End. Exciter End
119.6, 146.6, 156.6 110, 122, 133117, 133, 146 110, 125, 148
global natural frequency and its associated 4-node mode
of vibration seemed to be the principal cause for higher
endwinding vibration on EE.
5. Rectifying Measures
In order to arrest the vibration tend, suitable measures
were planned and implemented. These measures pro-
vided additional stiffness and overall strength to over-
hang structure. Some of them are given below:
1) Continuous resin rope was introduced encircling all
the bar-head joints in order to integrate the overall over-
hang basket (Figure 7);
2) Additional bracing was provided, so as to increase
the stiffness of overhang within phase and also at phase
3) Concerned process bottlenecks were removed by
suitable technological means.
6. Evaluation of Measures
Subsequent to rectification and testing, following results
were obtained:
Overall global natural frequency was found to be in-
creased about 5 Hz on EE and 10 Hz on TE from the
un- rectified situation;
Endwinding vibration level was reduced to nearly 1/3
to its previous level (Figure 8);
No resonance like phenomenon was observed in any
testing /operational regime.
7. Conclusions
1) Experimental modal analysis of overhang structures
and subsequent operational runs of generator indicated
global resonance phenomenon as the most likely cause
for sharp increase endwinding vibration levels. Further,
4-node/2-lobe mode shape obtained by modal analysis
seemed to have aggravated vibration behavior, thus, de-
Figure 7. Rectification measure: a continuous resin rope
integrating bar-head joints.
Copyright © 2011 SciRes. EPE
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8. References
[1] K. Senske, S. Kulig, J. Hauhoff and D. Wünsch “Vibra-
tional Behaviour of the Turbogenerator Stator End
Winding in Case of Electrical Failures,” Conférence In-
ternationale des Grands Rèseaux Electriques, Yokohama,
29 October 1997.
[2] D. Lambrecht and H. Berger “Integrated End-Winding
Ring Support for Water-Cooled Stator Winding,” IEEE
Transactions on Power Apparatus and Systems, Vol.
PAS-102, No. 4, 1983, pp. 998-1006.
[3] L. Intichar, “Natural Frequency, Mode Shape Determina-
tion of Turbogenerator Overhangs,” Symposium-Vibra-
tion Diagnostics on Power Plant Turbogenerators, Pots-
dam Sanssouci, 22-24 March 2006, pp. 371-391.
Figure 8. Endwinding vibration levels obtained before and
after rectificatio n.
[4] E. D. Frerichs, “Monitoring of Endwinding Vibration of
Generator with Fibre-Optics Accelerometers,” Sympo-
sium-Vibration Diagnostics on Power Plant Turbogen-
erators, Sanssouci, 22-24 March 2006, pp. 373-428.
monstrating its practical implication on overall structural
dynamic response;
2) Appropriate rectification measures planned and im-
plemented on overhangs and associated assembly had
desired effect in arresting the endwinding vibration
[5] R. Sewak, R. Ranjan and A. K. L. Rao, “Intricate Aspects
of Turbogenerator Endwinding Vibration Monitoring
Based on Data Analysis,” International Conference on
Condition Monitoring and Diagnosis, Beijing, 21-24
April 2008, pp. 130-135.
3) The complicated nature of both the forces and the
responding structure make the exact relationship between
cause and effect quite complex in various practical con-
ditions including the present one. Hence, the study con-
ducted and inferences drawn, do not claim to have com-
prehended all the aspects of endwinding vibration be-
[6] D. Shally, M. Farrell and K. Sullivan, “Generator End
Winding Vibration Monitoring,” 43rd International Uni-
versities Power Engineering Conference, Padova, 1-4
September 2008, pp. 1-5.