Cylindrical rotor inter-turn
short-circuit detection
by Kobus Stols, Eskom
A “strayflux probe” is commonly used in the industry to determine if any inter-turn short-circuits are present in the field winding of a synchronous
generator with a cylindrical rotor.
The most common distances vary between
1 and 3 cm. The coils typically have
between 700 and 800 turns of copper
wire with a diameter of approximately
0,05 mm. The coil is normally obtained
by winding the wire on a former with a
diameter of less than 10 mm.
The voltage signal obtained from the
probe is analysed and differences are
typically expressed as a “ percentage
error ”.It is the purpose of this paper to:


Highlight the pitfalls when analysing a
strayflux recording
To provide a tripping philosophy.
The orientation of the coil with respect to
the rotor body determines its designation.
It is therefore possible to have a “tangential
probe” (Fig. 4) as well as a “radial probe”.
Flux in the generator
“Open-circuit” flux
Fig.1 illustrates the symmetrical flux
pattern set up by the field winding under
open-circuit conditions. The flux lines are
not drawn on scale but serve merely to
indicate that the main flux lines are more
prominent than the stray-flux lines and that
they are symmetrical around the direct
axis (Xd).
The radial type probe is the most common
in industr y and this paper will focus on
this type.
Fig. 3: Basic “On-load”
field winding flux patterns.
Identifying the physical properties from a
stray flux recording
The voltage signal shown in (Figs. 6 – 8) was
obtained from a radial flux probe during a
short-circuit test. The effect of the following
physical properties is visible:

Fig.4: Tangential fluxprobe.
The main flux that results in a 50 Hz
fundamental waveform

Pole faces A and B

Damper windings


Reduced number of turns in the coils
closest to the poles
The teeth width that is not uniform
50 Hz fundamental waveform
Fig. 1: Basic “Open-circuit”
field winding flux patterns.
Fig. 5: Radial fluxprobe.
significantly reduced due to the armature
reaction, and the stray-flux component is
therefore more prominent. Analysis of the
stray-flux can be done more accurately
during the short-circuit condition.
The main flux causes a 50 Hz fundamental
voltage waveform on which the strayflux signal is superimposed. The ultimate
condition for measuring the rotor stray-flux
for analysing purposes is when there is a
short circuit applied to the terminals of the
generator, due to the following reasons:

“On-load” flux
Fig. 2: Basic “short-circuit”
field winding flux patterns.
“Short-circuit” flux
Fig. 2 illustrates the symmetrical flux
pattern set up by the field winding under
short-circuit conditions.The main flux is
The resultant flux will look different under
loading conditions due to the angle
between the flux of the rotor and stator
windings, known as the load angle. Fig 3.
clearly illustrates that there is no symmetry
around the direct axis.
Types of flux probes
A flux probe is basically a small coil that is
positioned in the air gap of a generator.
The distance of the coil from the rotor body
is not critical and can vary considerably.
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
A situation is created where the main
flux from the rotor is opposed by the
flux that results from the short circuit
current in the stator. This means that
only a small amount of flux is present
in the air-gap which may influence
the voltage peaks resulting from the
strayflux
Analysing the results is easy due to the
fact that the absolute voltage “spikes”
associated with the slots are expected
to be of similar magnitude for all slots
with the same number of turns. It is
often not practical to apply a shortcircuit to the terminals of the generator,
hence the short can applied to the
HV side of the generator transformer.
This means that the impedance of the
GENERATION
Fig. 6: Voltage signal recording from a stray-flux probe during
a short-circuit test.
Fig. 8: Ideal Strayflux without the 50 Hz fundamental waveform.
Fig. 7: Stray flux superimposed on the 50 Hz fundamental waveform.
Fig. 9: Effect from the damper winding slots on a stray flux recording.
Number of slots
38
Number of slots with field
winding coils
32
Number of turns in the
slots adjacent to the polefaces
7
Number of turns in the
other winded slots
11
Teeth width
Reduced number of turns
All not identical
Table 1: Characteristics of the rotor.
transformer will be present between
the terminals of the generator and the
applied short-circuit.

argument applies to the slots in the pole
faces, hence the appearance of voltage
peaks when these slots pass the flux probe.
The terminal voltage of the generator
is an indication of the amount of
flux in the core. The voltage across
the impedance of the transformer
will be “ visible” on the terminals of
the transformer, hence the levels of
flux in the machine will be low but it
will not be zero. This scenario will still
provide recordings of the strayflux that
will provide excellent results from an
analysis point of view.
Damper windings
Fig. 9 shows the voltage effects associated
with slots in the two pole faces which only
contain damper windings. This is due to the
behaviour of the flux lines when they enter
or exit the magnetic core. Flux always enter
or exits a magnetic core at angles of 90°.
The basic effect of this phenomenon with
“irregular ” shapes in the magnetic core is
illustrated in (Figs. 10 – 11).
The flux at the “irregularities” in the pole of
the magnetic core contains both radial
and tangential components. The same
It is common to find that the coils closest
to the poles have less turns than the
rest. Manufacturers use this technique to
reduce the harmonics generated by the
machine. These reduced peaks should not
be interpreted as shorted turns.
Voltage peaks “off sets”, “intervals” and the
width of the teeth.
The main flux of a cylindrical rotor is
“shaped” to produce a more sinusoidal
voltage waveform. In addition to the
reduced number of turns in the coils
closest to the pole faces, it is also common
to find that the slot spacing gets smaller
near the quadrature axis. (Figs. 12 – 15).
Analysing methodology
Numbering/labelling
The slots and the poles are labelled/
numbered as shown in Fig. 16 to illustrate
the analysing methodology.
The symmetrical point in a stray-flux
recording depends on the type of fluxprobe used. The Xq axis will be the point
of symmetry when a radial flux-probe is
used whilst the Xd axis will be the point of
symmetry if a tangential flux-probe is used.
No field winding coils are present in the
three slots on each of the pole faces. The
damper windings, which are present in
these slots, do not carry any current during
the short-circuit test because there is no
relevant movement between the rotating
stator flux and the damper winding in the
rotor because both rotate at 3000 rpm.
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Fig. 10: The effect of “irregular shapes”
on flux lines.
The slots which contain damper windings
only, i.e. pole A; slots 19, 20 and 21 and
pole B; slots 38, 39 and 1, are therefore
not shown in Fig. 16, nor will they be used
during the analysis.
Some of the characteristics of the rotor in
the example are shown in Table.1
Example of an analysis for inter-turn
problems
The symmetry method
The expected waveform for a “healthy ”
rotor is shown in Fig. 17. The waveform is
symmetrical around the Xq axes due to
the use of a radial flux-probe. The following
example proves that the Xq axes are the
point of symmetry:



The voltage peaks associated with
slots 3 and 18 are the same and they
are also comparable with the voltage
peaks of slots 22 and 37
The pattern of voltage peaks from slot
10 to slot 3 is the same as the pattern
of the voltage peaks from slot 11 to
slot 18
The pattern of voltage peaks from slot
GENERATION
Fig. 11: Effect from the damper winding slots on a stray flux recording.
Fig. 16: Labelling and numbering of the field
winding used during the analysis.
Fig. 12: Effect “narrow” teeth/slot spacing.
Fig. 17: Expected stray flux recording for a healthy rotor.
Fig. 13: Equal slot spacing.
Fig. 14: “Condensed” slot spacing near the Xq axis.
Fig. 15: The effect of “condensed” slot
spacing on the peak-to-peak voltage.
29 to slot 22 is the same as the pattern
of the voltage peaks from slot 30 to slot
37
Fi g. 1 8 s h o w s t h e a c t u a l w a v e f o r m
obtained from the radial flux-probe during
the short circuit test.
In order to illustrate the analysing
methodology more clearly, the waveform in
Fig. 18 is split into two separate recordings
as shown in Fig. 19 and Fig. 20. The
Fig. 18: Stray flux recording of a faulty rotor.
Fig. 19: Stray flux for slots 3 to 18.
presence of a shorted turn will be evident
in both the relevant slots for the specific coil
as discussed earlier. It is for this reason that
the stray flux recording of slots 3 to 18 will
theoretical be identical to those of slots 22
to 37 if the polarity of the signal is ignored.
It is already evident that possible problems
exists in slots 3, 5, and 17 by merely looking
at the symmetry of the waveform. The peaks
shown in red in Fig. 21 indicate where the
measured peak was expected to be. The
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waveform in Fig. 22 also shows that possible
problems exists in slots 23, 35 and 37. The
peaks shown in red indicate where the
measured peak was expected to be. The
problem areas indicated in the recordings
above are shown in a more familiar view
in Fig. 23.
By analysing the symmetry (or lack of it) of
the recordings, it is suggested that some
turns in certain slots are shorted with some
or no resistance.
GENERATION
Fig. 20: Stray flux for slots 22 to 37.
Fig. 21: Symmetry problems in slots 3,5 and 17.
Fig. 22: Symmetry problems in slots
23, 35 and 37.
Fig. 23: Coils with possible Interturn problems.
Slot
Side 1
(V)
Side 2
(V)
Average
(V)
3
1120
1370
1245
4
1930
1950
1940
5
1650
1750
1700
6
1800
1870
1835
7
1800
1870
1835
8
1770
1450
1610
9
1380
1420
1400
10
1350
1350
1350
11
1350
1350
1350
12
1420
1400
1410
13
1470
1770
1620
14
1870
1800
1835
15
1870
1780
1825
16
1900
1830
17
1820
18
1500
22
Coil 1
Slot 22 + Slot 18
Slot 3 + Slot 37
Coil 2
Slot 23 + Slot 17
Slot 4 + Slot 36
Coil 3
Slot 24 + Slot 16
Slot 5 + Slot 35
Coil 4
Slot 25 + Slot 15
Slot 6 + Slot 34
Coil 5
Slot 26 + Slot 14
Slot 7 + Slot 33
Coil 6
Slot 27 + Slot 13
Slot 8 + Slot 32
Coil 7
Slot 28 + Slot 12
Slot 9 + Slot 31
Coil 8
Slot 29 + Slot 11
Slot 10 + Slot 30
Table 3: Specific slots is associated
with specific coils.
Pole B
% error
Coil 1
2760 V
2510 V
9,06%
Coil 2
3560 V
3915 V
9,07%
1865
Coil 3
3705 V
3430 V
7,42%
1770
1795
Coil 4
3660 V
3700 V
1,08%
1250
1375
Coil 5
3650 V
3685 V
0,95%
1270
1500
1385
Coil 6
3240 V
32550 V
0,31%
23
1750
1780
1765
Coil 7
2820 V
2800 V
0,71%
24
1800
1880
1840
Coil 8
2725 V
2710 V
0,55%
25
1800
1870
1835
26
1780
1850
1815
27
1770
1470
1620
28
1400
1420
1410
29
1370
1380
1375
30
1370
1350
1360
31
1420
1380
1400
32
1480
1800
1640
33
1880
1820
1850
34
1900
1830
1865
Accuracy
35
1780
1680
1730
36
1980
1970
1975
37
1400
1130
1265
The relation between the number of shorted
turns and the percentage deviation are
influenced by the following factors:
The possible problematic slots are:

Slots 17 and 23 (Pole A – Coil 2)

Slots 5 and 35 (Pole B – Coil 3)

Slots 3 and 37 (Pole B – Coil 1)
Comparison of the respective coils
method
Another approach is to compare the
“absolute value” of the voltage peaks
for the opposing coils as in Fig. 24 . The
values of the two sides of each voltage
peak were measured to calculate the
average value for the peak as shown in
the mentioned table.
The voltage peaks for the various slots were
measured and are indicated in Table 2.
Fig. 25: Measuring the other side
of the same voltage peak.
Pole B
Pole A
Table 2: Stray flux peak voltages as measured.
Fig. 24: Measuring the one side
of a voltage peak.
Pole A
A shorted turn will have no current when
compared to the corresponding “healthy ”
turns. This also implies that the ampereturns for the coil with the shorted turns
will be less than the ampere-turns for the
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Table 4: Comparison of the relevant coils.
“healthy ” coil. It is for this reason that the
coil with the highest value is assumed to
be the “healthy ” coil. The analysis also
takes into account the number of turns
per slot. The relevant slots for the rotor
should be compared as indicated in
Table 3 to determine the presence of interturn problems (Fig. 23).




Reduced number of turns in the coils
closest to the poles
The width of the teeth that is not
uniform (see Fig. 15)
The width of the teeth is such that
it results in a offset that effects the
strayflux signal (see Fig. 16)
The flux in the air gap is not 100%
sinusoidal.
It is therefore not possible to get a 100%
accurate relation between the % error
and the number of shorted turns. It is
especially the offset due to the width of
the teeth that can greatly exaggerate the
error when expressed as a percentage
difference between the relevant peak-topeak voltages.
Effects of Inter-turn short circuits in a field
winding
Inter-turn short circuits in the field winding
(rotor winding) can cause high levels of
vibration in a generator. Both thermal and
magnetic unbalances are commonly
associated with inter-turn shorts.
GENERATION
Thermal balance
The heat of the rotor body is distributed
in such a way that the rotor expands
uniformly.
The current in the field winding and the
resistance of the winding results in the
generation of heat (I2R), which is transferred
to the rotor forging. A turn that is shortcircuited does not carr y any current;
hence temperature of such a turn will
be significantly lower than its healthy
counterpart.
A shorted turn with no current will be cooler
than its counterpart which is associated
with the opposite pole. The pole with the
shorted turn will therefore be slightly colder
than the healthy, hence hotter pole. The
difference in temperatures will result in
the “bowing” of the rotor. More than one
shorted turn near the Xq axis may have
less of an impact on the delta T of the two
poles than one shorted turn near one of
the pole faces.
Magnetic balance
A shorted turn will also result in a different
flux density in the one pole when compared
to the other. This effect will also have a
negative impact on the vibrations of the
machine. The vibration effect will be higher
the closer the shorted turn is to one of the
pole faces.
Fig. 26: Excitation requirements.
More than one shorted turn near the Xq
axis may therefore affect the machine less
than one shorted turn near the pole face.
Excitation requirements
inter-turn short circuits exceed any one of
the following limitations of the machine:


Vibration levels
Stability of the machine in the reactive
power import region
A specific number of ampere-turns is
required to set up the flux required for
a specific quantity of reactive power. It
is therefore necessary for the excitation
system to compensate for a shorted turn
by increasing the excitation current to get
to the same ampere-turns value.
If the symptoms of inter-turn faults don't
exceed the mentioned limits of the
machine, it poses no direct risk to the
machine and does not require automatic
tripping.
Recommendations
Contact Kobus Stols, Eskom,
Tel 011 800-8632,
kobus.stols@eskom.co.za 
Tripping is required if the symptoms of
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