[3] Randy R. Schoen and Thomas G, Habetler, “A new
method of current-based condition monitoring in
induction machines operating under arbitrary load
conditions,” Electric Machines and Power Systems, vol.
25, pp. 141-152, 1997.

[4] J. Penman, H. G. Sedding, B.A, Lloyd , and W. T. Fink,
“Detection and location of interturn short circuits in the
stator windings of operating motors,” JEEE Trans. on
Energy Conversion, vol. 9, no.» 4, pp. 652-658, Dec.
1994,

[5] A. J. M. Cardoso, S. M. A. Cruz, and D. S. B, Fonseca,
“Inter-turn stator winding fault diagnosis in three-phase
induction motors by Park’s vector approach,”
Conference Record of the IEEE International Electric
Machines and Drives Conference, Milwaukee, WI, pp.
MB1-5.1-MB1-5.3, 1997.

[6] L. Pazamandi. “Stator earth-leakage protection for large
generators, IEEE Trans, on Power Apparatus and
Systems, vol. 94, no. 4, pp. 1436-1439, Jul/Aug. 1975.

{7] J. W. Pope, “A comparison of 100% stator ground fault
protection schemes for generator stator windings,” IEEE
Trans. on Power Apparatus and Systems, vol. 103, no. 4,
pp. 832-840, April 1984,

[8] D. W. Auckland, 1. E. D. Pickup, R. Suttleworth, and Y.
T. Wu, “Novel approach to alternator field winding
interturn fault detection,” IEE Proc. On Generation,
Transmission and Distribution, vol. 142, no. 2, 1995.

[9] Ronald L. Eshleman, Machinery Vibration Analysis II,
ViPress, Inc., 1996.

[10] John T. Renwick and Paul E. Babson, “Vibration
analysis - a proven technique as a predictive
maintenance tool,’ IEEE Trans. on Industry
Applications, vol. 21, no. 2, March/April 1985.

[11] S. J. Yang, Low-Noise Electrical Motors, Oxford
University Press, 1981.

[12] F, Ding and F. C. Trutt, “Calculation of frequency
spectra of electromagnetic vibration for wound-rotor
induction machines with winding faults,” Electric
Machines and Power Systems, vol. 14, pp. 137-150,
1988.

[13] R. L, Webb and C. S. Murray, “Vibration protection
for rotating machinery.” Trans. of the IEEE, vol. 63,
pp. 534-537, 1944,

(14] A. Gayland, A. Meyer, and C. Landy, ‘Acoustic
evaluation of faults in electric machines,” Conference
Record - Electric Macines and Drives, IEE Pub. No.
412, pp. 147-150, Sep. 1995,

[15] Tommy W. S. Chow, “Condition monitoring of electric
machines using third-order spectrum analysis,”
Conference Record of the IEEE Industry Applications
Society, vol. 1, pp. 679-686, 1996.

[16] S. Williamson and K. Mirzoian, “Analysis of cage
induction motors with stator winding faults," JEEE
Trans. on Power Apparatus and Systems, vol. 104, pp.
1838-1842, 1985.

{17} F. C. Trutt, “Steady-state analysis of wound rotor
induction machines with simultaneous stator and/or
rotor faults,” Electric Machines and Power Systems,
vol. 16, pp. 35-48. 1989.


Voltage Regulator

Field
to. Vi Vs
ic] 9.9

Generator Output

12 3

Generator

Armature

ay

Rectifier
Assembly

Generator
Field

Rotating Components

Fig. 3. Simplified schematic diagram of experimental “brushless” synchronous generator.

parameter is divided by its value under normal operating
conditions and then plotted as a function of the number of
shorted turns, Fig. 4 is obtained. From this figure, it may be
seen that only slight additional increases in 30-Hz vibration
were obtained as the number of shorted turns were extended
from 10 to 100. This result may be related to the difficulty in
diverting more than 2-3A from the field winding. This fault
current was relatively constant and nearly independent of the
number of shorted field winding turns. However, the
increase in the 30-Hz component of acceleration was evident
in all cases.

H

Normatzed Change (%)

&

‘S10 20 a0 a0 0
Number of Shofted Turns

Fig. 4, Bearing vibration vs. rotor winding deterioration.

IV. CONCLUSION

In order to develop reliable systems for the detection and
classification of deterioration in ac machines, it seems logical
that all sources of information should be utilized. This is
particularly true since each predictor will have its own unique
characteristics and could contribute data that may not
otherwise be available. In this paper, theoretical and
experimental motivation has been presented for the
consideration of mechanical vibration predictors as an
indicator of electrical winding deterioration in rotating
machines. Based upon the results of this study, it is
recommended that the relationships between electrical
winding faults and mechanical vibration be investigated in
greater detail and that the potential application of such
predictors in continuous monitoring systems be considered.

REFERENCES

(1] G. B. Kliman, W. J, Premerlaini, R. A. Koegl, and D,
Hoeweler, “A new approach to on-line turn fault
detection in ac motors,” Conference Record of the IEEE
Industry Applications Society, vol. 1, pp. 687-693, 1996.

[2] Hamid A. Toliyat and Thomas A. Lipo, “Transient
analysis of cage induction machines under stator, rotor
bar and end ring faults,” JEEE Trans, on Energy
Conversion, vol. 10, no. 2, pp. 241-247, June 1995.



Detection of A-C Machine Winding Deterioration using Electrically Excited

Vibrations
F.C. Trutt J. Sottile J. L. Kohler
University of Kentucky University of Kentucky National Institute of Occupational
Lexington, KY 40506 USA Lexington, KY 40506 USA Safety and Health
Pittsburgh, PA 15236 USA.

Abstract - Presented in this paper is a theoretical
review of the relationships that should exist between
electrical winding parameters and the mechanical
vibration of a-c machine elements under normal and
faulted operating conditions. Also included is data from
an experimental study that relates stator vibration and
bearing vibration to selected winding faults in a
synchronous machine. Consideration of these results
indicates a significant relationship between electrical
deterioration and mechanical vibration and thus provides
the motivation for additional study and a basis for future
applications,

1. INTRODUCTION

Studies relating to the detection of electrical winding
faults in rotating machines have normally been oriented
towards the measurement and analysis of electrical
parameters such as current, voltage and magnetic flux [1-8].
Conversely, efforts to apply mechanical vibration technology
in the condition monitoring of ac motors and generators have
generally been focused on areas relating to unbalance,
bearing condition, eccentricities, and other mechanical
phenomena [9,10]. However, theory predicts that current
changes due to electrical winding deterioration in rotating
machines will alter internal magnetic forces which will then
cause a modification in vibration characteristics [11,12]. The
monitoring of mechanical vibration should therefore be a
useful indicator of electrical winding condition,

While the application of these concepts in the protection
of rotating machinery have been considered for many years
(13-15), the major emphasis has been to study the
relationships between winding faults and electrical
parameters. However, an understanding of the relationships
between mechanical vibrations and electrical winding
deterioration could provide a means for supplemental
monitoring of electrical. winding integrity as well as
information that might be used to discriminate between
electrical and mechanical problems.

This paper begins with a theoretical review of the
relationships that should exist between electrical winding
currents and the mechanical vibration of machine elements
under normal and faulted operating conditions. Data from an

experimental study that relates stator vibration and bearing
vibration to selected winding faults in a synchronous machine
are then presented. Results demonstrate a measurable
relationship between electrical deterioration and mechanical
vibration and thus provide the motivation for additional study
and a basis for monitoring applications.

TI. THEORETICAL JUSTIFICATION

The theoretical motivation for the use of mechanical
vibration measurements for the detection of electrical
winding faults follows that given in [11] and will be
presented: by considering the total airgap mmf to be
composed of the sum of the individual mmfs of each coil in
the machine. In this manner, the mmfs that would exist
during either normal or faulted conditions may be
represented. Since the flow of current in a single coil of a
machine produces a series of space harmonic waves, the
airgap mmf due to the nth stator coil may be represented as

Fo SKN isCosllny+9,) a

where K, is a magnitude. coefficient, N, is the number of
series turns in the coil, p is the number of pole pairs, k is the
order of a space harmonic, @, is a phase angle representing
the space position of the coil axis, y is the space position in
the airgap, and i, is the time varying current flowing in the
nth stator coil. In general, this current may consist of a
fundamental plus time harmonics. For mmfs due to current
in a rotor coil, the effect of rotor velocity must be included
and (1) is made to rotate at the rotor mechanical speed a,.

The airgap permeance is represented as a sum of time
varying sinusoidal space functions in order to allow for
saliency. This permeance function has a component due to
rotor slotting that moves with the rotor and the total airgap
permeance may be represented as

Fad BA CosliK,Z, £5K,2,)y-iK,,}

where i and j are integers, @, is the rotor speed, Z, and Z, are
rotor and stator slot numbers, and the K, K,, and Pj are


appropriate constants. It should also be noted that dynamic
rotor eccentricity and/or dissymmetries in rotor and/or stator
shape may also be included in this function.

The airgap flux density is equal to the product of the mmf
and the permeance. The individual coil mmfs given in (1)
may then be multiplied by the airgap permeance (2) and this
result may be summed over all the coils in the machine to
obtain the resultant airgap flux density. Thus, for example, a
three phase machine having 100 distributed coils in each
phase of the stator would have 300 terms as given in (1), each
term having enough space harmonics to appropriately
represent the coil’s rectangular mmf space distribution.
Rotor coils must also be included in the summation. The
resulting representation of the airgap flux density therefore
includes many terms, each of which has a time varying
amplitude and is sinusoidally distributed in space.

The electrically excited force wave acting on the the
machine members is given as

We7 Q)

where B is the airgap flux density discussed in the previous
paragraph. Evaluation of (3) yields a complicated function of
the space position in the airgap, the individual coil currents,
machine saliency, and the space harmonics of each coil mmf.
This expression is given in [11] and [12] and will not be
repeated here. For our purposes, it is sufficient to realize that
the many terms in this force function are each sinusoidal in
nature and that each term represents a force component acting
over a particular space distribution and at a certain frequency.
These forces act upon the rotor and the stator of the machine
and the resulting mechanical responses to these forces
constitute its electrically excited vibration spectrum. Since
these forces are dependent upon the individual coil currents,
changes in these currents with winding faults or deterioration
as demonstrated in [16] and [17] should produce
corresponding changes in the airgap flux density and hence
the electrically excited forces acting on the machine
members. The manner in which the mechanical structure
responds to these force changes should be a critical factor in
the use of vibration monitoring for the detection and
evaluation of winding deterioration.

The dynamic response of the machine members at the
frequencies present in the electrically excited force wave
depends upon a number of parameters including their
elasticity, geometry, natural frequencies, damping
characteristics, and vibration modes. Although some of these
quantities are difficult to determine accurately for a given
machine, it is known that forces acting at a frequency near a
natural frequency of a structure will produce more dramatic
motion responses than those that may act. at more highly
damped frequencies. Increases or decreases in force
magnitudes should also be observable.

Based upon these theoretical considerations, its seems
clear that the changes in winding and coil currents due to
electrical winding faults or deterioration will be reflected in

the spectrum of the forces acting on the stator and rotor of a
rotating machine. Measurement of the mechanical vibrations
of the machine structure in response to these changes
therefore provides a potential means for the monitoring of
electrical winding condition. This method of condition
monitoring could be particularly useful if the force changes
due to winding deterioration are of large magnitude or if they
occur near a natural frequency of a machine member.

TIL, EXPERIMENTAL STUDY

The experimental study was carried out on two machines
that were constructed to allow for the simulation of stator
and/or rotor winding faults and the subsequent measurement
of stator and bearing vibration. The first of these is a
universal laboratory machine (ULM). The second system is a
specially constructed three-phase “brushless” synchronous
generator.

The universal laboratory machine set is composed of a
two-pole universal machine that is mechanically connected to
adc dynamometer. For the purposes of this investigation, the
ULM was connected as a 2-kva 115-V 60-Hz 3600-rpm
three-phase synchronous alternator. The dynamometer was
connected as a 110V de shunt motor and the motor field
current was adjusted for a mechanical speed of 3600 rpm.

The stator winding of the universal laboratory machine
consists of 12 coils that may be interconnected in various
ways in order to create different stator winding
configurations. For the experiments to be described, four of
these stator coils were connected in series to form each phase
of a balanced wye-connected three-phase armature. With
these connections, the machine can be operated as a normal
3600-rpm alternator or the junction points between coil
connections can be used to simulate turn-to-tum stator
winding deterioration as shown in Fig. 1. Phase-to-phase
faults may also be simulated. While the rotor is accessible
through brushes and slip rings as a single coil per pole, there
is no effective method to simulate rotor field winding
deterioration in this 2-pole machine and the rotor excitation
therefore consisted of a constant value of dc current applied
through the brushes.

For the first series of tests that will be presented, the ULM.
was run as a loaded synchronous generator at synchronous
speed. Stator coil-to-coil winding deterioration was
introduced as shown in Fig. 1 and the resistance Ry was
varied in order to adjust the deterioration current I. For all
the levels of deterioration that were simulated, the machine
still appeared to be running normally and no abnormal
heating was apparent.

In order to collect vibration information, one
accelerometer was mounted on a rotor bearing and another
accelerometer was mounted on the stator stack of the
machine under test. Since the frequencies of interest were
less than 1kHz, magnetic mounts were utilized with anti-
aliasing filtering having a cutoff frequency of 1.125kHz.
Filter outputs were then sampled at a rate of 5128 samples/s


Ke

kh
Fig. 1. ULM stator winding with simulated deterioration.

and this data was transferred to a laptop computer for
analysis. .

An acceleration frequency spectrum measured at the
stator stack by one of the magnetic mount accelerometers was
then analyzed for both normal operation and for a moderate
amount of deterioration current 1, Results showed
measurable differences, particularly at 180Hz and 480Hz. To
further investigate this situation, testing was conducted for
levels of deterioration current from 0-2A. While no major
changes in output voltages or currents were observed,
increases in stator vibration occurred as shown in Fig. 2. In
this figure, the stator accelerations at 180Hz and 480Hz plus
the negative sequence load current are plotted as a percentage
of their values with no deterioration vs. deterioration current.
The 200% increase in measured stator acceleration at 480Hz
and the 90% increase at 180Hz seem to indicate a clear
relationship between these parameters and stator coil-to-coil
deterioration. These changes appear particularly significant
when compared with the 10% increase that was observed in
the negative sequence component of the load current.

Similar results may be observed for other types of stator
faults in the ULM but limitations of the universal machine for
this investigation included the inability to simulate rotor
winding deterioration as well as the requirement that 25% of
a stator phase winding must be involved in a fault simulation.
In an effort to circumvent these difficulties, a specialized
alternator system was constructed.

Major components of this specialized alternator system
include a 4-pole 6.25kva 240V three-phase generator with a
“brushless” excitation system that is directly coupled to a
7.Shp 1800rpm 60Hz brushless synchronous motor. Also
included is the appropriate exciter for each unit as well as the
circuitry for alternator voltage regulation and motor speed
control. The stator of the alternator is wye connected with
two parallel paths per phase. Each stator winding has
externally accessible taps at 1, 2, 3, 4, 5, 10, 21, 42, and 68%

Normalized Change (%)

Of 04 08 08 1 42 ta 18 48 2
Deterioration Curront, 1, (A)

Fig. 2. Measured parameters vs. deterioration in the ULM.

of the coil. Thus internal winding or phase-to-phase stator
deterioration involving as little as 1% in one parallel path of a
winding may be simulated.

On the rotor, taps are available at 0, 5, 10, 25, 50, 100,
200, 350 and 500 turns on one pole of a rotor winding that
has 500 series turns/pole (2000 total turns in series). These
taps are externally accessible when the rotor is stationary
such that rotor deterioration may be introduced prior to
startup. Although the structure of the alternator excitation
system is brushless in nature, brushes and slip rings have also
been provided so that access to the generator exciter output,
rectifier diodes, and the actual field voltage and current is
available. Thus these parameters may be measured while the
alternator is running and hence the “brushless” connotation.
The structure of this alternator system is illustrated in Fig. 3.

While it has been possible to show similar results for
stator winding deterioration with the three phase “brushless”
experimental generator, it is more interesting to investigate
the consequences of rotor winding deterioration in this
alternator since such a study was not feasible using the
universal machine. For this experiment, deterioration was
simulated by shorting 0-100 turns on the tapped rotor: pole.
In this manner, rotor winding short circuits involving a
maxitnum of 5% of the total rotor winding were created.
The actual de current that was shunted from the field winding
through these short-circuit connections was limited in
practice to the range of 2-3A because of connection
resistances and other factors. Thus it was only possible to
divert 80-90% of the normal dc field current using this
approach. Once again, no appreciable changes in the output
voltages or currents were observed as rotor-winding
deterioration was added.

An acceleration frequency spectrum measured by an
accelerometer located at a rotor bearing was then analyzed
for normal operation and varying amounts of rotor winding
deterioration. Results showed that a significant increase in he
magnitude of bearing vibration occurs at a frequency of 30Hz
when the deterioration is added. If this acceleration