Andrzej GĘBURA
Tomasz RADOŃ
Air Force Institute of Technology
RESEARCH WORKS OF AFIT
Issue 30, pp. 153÷176, 2012
DOI 10.2478/v10041-012-0010-4
FAILURES OF AVIONIC GENERATORS
AND CONTACTORS OPERATED UNDER HARSH
AMBIENT CONDITIONS
The paper outlines failures of selected avionic electric power devices operated onboard of
combat helicopters in Iraq and Afghanistan. While the authors were examining the electric
power system of aircrafts in order to prolong their operation life they paid attention to numerous symptoms of wear demonstrated by some units of aircrafts. It was peculiar that
such symptoms have never appeared during similar examinations of aircrafts operated in
our country. By all accounts, the weird wear of components results from high intensity of
flights and specific features of operating missions, but harsh climatic conditions seem to be
the crucial factor. The authors believe that many of spotted failures experienced by electric
power equipment may also happen to aircrafts operated in Poland but obviously, due to
much lower intensity of operation and mild impact of environmental factors, such failures
shall occur much later.
The authors focused their attention on two groups of electric devices and associated destructive factors:
1. Air-cooled electric rotary machines. Fine-grained sand sucked together with air leads to
very quick abrasion of protective paint coatings inside the machines. Not frequent but
intense rainfalls are the reason for appearance of corrosion pits that lead to such effects
as increase of pole shoe volumes. This, in turn, results in shearing of winding insulations with breakdowns (shorts) to ground and, as a final consequence, considerable
drop of power demonstrated by an electric machine.
2. Contactors and electromagnets, which are allegedly tight. However, dust penetrates
anyway via microfissures and disables operation of moving parts.
Keywords: technical diagnostics, thermal ageing, insulation resistance, contactor, generator, electrochemical corrosion of silicon steel plates, internal short.
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1. Introduction
While the authors of this study were examining the electric power system of
cargo helicopters operated in Iraq and Afghanistan in order to prolong their operation life they paid attention to numerous symptoms of wear demonstrated by
some electric devices of aircrafts. These symptoms have never been recorded
and were really weird in terms of size and appearance of worn parts as compared
to damage that had been occurred to date in Poland.
Specific conditions of ambient environment in Iraq and Afghanistan, such as
high temperature during the summer period (up to +55ºC in shadow), very high
dust content and flights under conditions of low atmospheric pressure present real
challenges to operation of electric power equipment. This is superposed with intensity of missions that is incomparable to domestic operation as well as with very
dynamic manoeuvres resulting from tactic and operation requirements. Accumulation of all these factors inevitably leads to premature ageing processes that
should be investigated as soon as possible. The authors believe that similar processes experienced by electric power equipment may also happen to aircrafts
operated in Poland but with much less intensity. Therefore accurate investigation
of the reason and effect chains that occur under conditions of intense operation
shall make it possible to undertake prophylactic measures in order to prevent
similar breakdowns that inevitably may happen to the equipment operated domestically.
2. The destruction process of electric rotary machines
cooled with air sucked directly from the atmosphere on
the example of the breakdown that happened to the
commutator DC generator onboard of a cargo
helicopter
According to the Arrhenius law the conditions of elevated temperature lead to
acceleration of the ageing processes affecting various materials. The higher operation temperature, the faster ageing processes take place. About a dozen years
ago the Division of Aircraft Equipment within the Air Force Institute of Technology
(ITWL) carried out investigations on accelerated ageing of insulation of aircraft
electric cables [1–3, 6]. It was demonstrated that their elasticity and abrasion resistance subject to accelerated alteration at elevated temperatures. Intensified
impact of such physical and chemical factors as high content of dust or low at-
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mospheric pressure are also conducive to speeding up the deterioration processes of materials and, in consequence, premature loss of their functional abilities.
The conditions of elevated dust content that occur on desert areas the process of
apparent sandblasting occurs in ventilation ducts of electric machinery, leading to
wear of the insulation coating of paint. The particular hazard is associated with
1
damage to insulation coating of paint on surfaces of silicon steel plates incorporated into stators and rotors of electric machinery. Both stators and rotors of electric machines have their magnetic circuits made up as a pile of silicon steel plates
with impressed silicon grains in order to mitigate the effect of eddy currents (increased impedance down the path of eddy currents) when the electric machine is
in service. The presence of silicon grains prevents from excessive heating of
magnetic circuits during operation and the machine is capable to achieve higher
efficiency.
Under circumstances of elevated humidity the silicon steel plates suffer from
2
intense electrochemical corrosion when a substantial volume of ferrous structures is converted into ferrous oxides or hydroxides, which leads to volume extension of the rotor or stator teeth. Since tightly packed windings are wrapped around
each tooth, swelling of the tooth volume results in shearing of winding insulation
and shorts between electric current conducting wires occur within the rotor or
(less frequently) the stator, where shorts may happen between individual wires or
between wires and the ground (silicon steel plates). The typical symptom of internal shorts is the temperature growth of the electric machine and drop in its efficiency, which is also associated with the decline of power. In case of an electric
starter its output power may become so low that the starting procedure of the
engine would be interrupted. The alterations are clearly visible on the waveforms
[4-5, 7, 9-10, 12-16] – Fig. 1 and 2.
For a defect-free generator (with no internal shorts) one can see clear groove
pulsations at the level of the constant component (28.5V) with the shape of a
mirrored half-sine curve (Fig. 1). The pulsation amplitude is usually from 1% to
8% of the constant component value and the amplitude of those half-sine curves
is modulated (the envelope curve) by polar pulsations that impose modulation of
1
2
Silicon steel plates with the feature of electric insulations – ca. 3% of Si, the thickness
from 0.027 to 0.65 mm.
Electrochemical corrosion – the corrosion of metals caused by electrochemical processes and occurring due to the difference of electric potentials on the surface of the object
suffering from corrosion and located within the environment of an electrolyte. Such circumstances are conducive to formation of corrosion spots, where e.g. crystal grains with
the lower electric potential work as anodes whilst the ones with the higher potential – as
cathodes.
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waveforms for groove pulsations within the range from 0.2% to 1%. Small ‘humps’
are visible next to bases of each half-sine pulse, which are caused by the machine commutator (commutator pulsation). At low power (0% to 10% of the rated
power these pulsations feature with low amplitudes, as little as only few percent.
For rated loads the peak value of commutator pulsations achieves the level equal
to ca. 50% of groove pulsations, but only for the generator overloads exceeding
200% of its rated power the amplitude of commutator pulsations begins to goes
above the amplitude of groove pulsations and can reach as much as 300% under
the circumstances of a short.
When an internal short occurs inside the generator, the polar pulsations become the dominant components of the overall waveform (Fig. 2). The component
of the polar pulsation exceeds the other pulsation types by many times (the depth
of the polar modulation reaches the value of 300% to 500% of the value for the
groove pulsation) and remains steady with regard to frequency and amplitude.
The component of the commutator pulsation assumed the highest value at the
moment when the shorted turn passed above the subsequent pole of the generator stator (Fig. 2 – ‘peaks’ of waveforms). Then its amplitude decreased when the
shorted turn was passing between the poles. The foregoing relationships made it
possible to detect several shorts of the generator rotor during regular operation of
the electric power node of the aircraft (without the need to kill the main engine
and to remove the rotor).
The first tests with short phenomena were carried out on a laboratory workbench at ITWL, where rotor windings and the rotor core were shorted with use of
a copper bolt. Later on, several cases of partial or complete shorts within the
generator rotor were detected during measurements for assessment of electric
power quality carried out when the aircraft engine was running. One of such cases was found out by the authors themselves on performing of their jobs in Iraq at
the Al-Kut military base in 2007.
Description of the event. During the jobs meant to start up the engine of the
Mi-8 helicopter the driver – operator of the APA-5 ground starter reported ‘considerably elevated level of electric current’ as compared to other start-up operations.
Moreover, the engine reached the rpm level of 22% and the growth to the required level of 26% could not be achieved. The experts from ITWL were called for
troubleshooting and they found out the following:
1) the temperature of the left-hand side generator body was much higher than the
right-hand side one,
2) upon replacement of the left-hand side generator – starter the start-up procedure was carried out correctly, with no hindrances.
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Uw=28.53V
Groove pulsations
envelope – polar pulsation
Commutator pulsations
angular (rotation) speed n = 8,000 rpm; the rms value of the output voltage
reference
line
36
34
Groove pulsations
u [V]
Commutator pulsations
Fig. 1. Variations of the pulsation component for the aircraft DC generator free of shorts
32
30
28
26
t [s]
24
Fig. 2. Waveforms for the output voltage of a DC generator with damaged rotor windings
(shorts of windings in the middle of one turn)
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In addition, the historical records of the generator operation were also inspected: when the helicopter waited more than two months for delivery of the
WR-8 transmission gear it was exposed to the impact of high temperatures and
short rains. Beforehand the helicopter was used for flights under conditions of
high dust concentration, typical for the climate of Iraq.
1.
2.
3.
4.
3
Inspection after having the generator – starter dismantled
The GS-18MO was handed over to the hangar of the equipment division. Upon removing of brushes from their holders it was found out that that the resistance of insulation between the generator body and active components of
its electric circuits dropped to the level of about 0.6 MΩ (Table 1).
Internal components of the generator were inspected with use of a borescope.
The following findings were established:
– high corrosion degree of silicon steel plates that make up pole shoes of the
stator (Fig. 3),
– surface rust on the bottom part of the generator mounting flange,
– traces of circumferential abrasion (wear) on rotor surfaces caused by the
effect of pole shoes at the beginning and ends of rotor grooves (Fig. 4),
– traces of scratches from foreign bodies visible on the rotor surfaces,
– traces of impacts from fine foreign bodies visible on surfaces of the stator
windings,
3
– rusty weepings on nuts for bolts that fix bridges of brush cords to the
generator body, some of them nearly touched the live parts of brush holders (Fig. 5),
– microfissures on the face surface at the commutator end (green filling
agent),
– small amounts of unwoven fabric in ventilation ducts,
– traces of deep and uniformly distributed circumferential scratches (ca. 0.2
mm) on the commutator surface (Fig. 6),
– condition of insulation gaps between commutator segments was satisfying.
The generator was exposed to the effect of elevated temperature during the
test period of 24 hours (Table 2), which initially led to a slight (10%) drop of
insulation resistance with subsequent growth of the same.
The generator was placed next to a vessel with water and the entire arrangement was covered with a plastic sheet. Then the machine was exposed to daily temperature oscillations at full sunlight. After two days of the experiment the
noticeable drop of the insulation resistance was detected (Table 3), measured
between the generator body and active components of its electric circuits.
Ferrous oxides and hydroxide.
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5. After having the generator dried the initial values of the insulation resistance
(Table 4) were restored, i.e. the values before the fault /test.
6. Zdemontowano wałek giętki – bez uwag. The flexible shaft was dismounted –
no remarks.
7. When the flexible shaft was being removed a shaft bearing mounted at the
side of commutator fell out (according to the commonly assumed standard it
should remain pushed into the bearing seating). Circumferential scratches
were detected on the outer cylindrical surface of the bearing (Fig. 7). Upon the
shaft and the bearing were reassembled the test of the bearing operability [8]
was carried out –with no remarks.
Table 1
Insulation resistance of the voltage GS-18MO generator – starter with the part number 617135 for the test carried out inside an air-conditioned workshop
No.
1
2
3
4
5
6
Measuring circuit
Between insulating
parts of individual
brush holders (the
clockwise numbering
sequence starting from
the commutator side
and the generator
body
Between commutator
7 segments and the
rotor body
8
Working
denomination
T=27oC, Θ=24h,
Marking
U=600V~
on the
generator With the ‘C’ W/o the ‘C’
filter
filter
1
—
4.5
12.0
2
—
0.9
0.7
3
—
8.0
11.0
4
—
0.9
0.6
5
—
4.0
10.0
6
—
0.9
0.7
—
—
0.9
0.9
—
0.9
0.9
+
30.0
30.0
Pin ф 12 – MG
9 Between the terminal Pin ф 12- MG
plate of the generator10
Pin ф 6- MG
starter and the genera11 tor body
Pin ф 6- MG
+
30.0
30.0
Ш
1.0
1.0
12
П
0.9
0.9
Pin ф 6- MG
* The ‘starter’ terminal.
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Fig. 3. Deeply corroded silicon steel plates of pole shoes within the GS-18 generatorstarter
Fig. 4. Traces of circumferential abrasion (wear) on rotor surfaces of the generator-starter
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Fig. 5. Rusty weepings on nuts for bolts that fix bridges of brush cords to the generator
body
Fig. 6. Traces of deep and uniformly distributed circumferential scratches (ca. 0.2 mm) on
the commutator surface of the GS-18 generator-starter
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Fig. 7. Circumferential scratches on the antifriction bearing of the GS-18 generator-starter
Fig. 8. Cleaning of the insulator for the brush holder of the GS-18 generator-starter with
use of a copper brush
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Fig. 9. Drawing of control marks on components of the bearing unit of the GS-18 generator-starter
Table 2
Insulation resistance of the voltage GS-18MO generator – starter
o
under conditions of elevated temperature (T≈80 C) and lowered humidity (η≈10%)
o
o
o
T = 80 C T = 80 C T = 80 C
after
after
after
Θ = 0h,
Θ = 1h,
Θ = 3h,
Marking
Working
on the U = 600V~ U = 600V~ U = 600V~
No. Measuring circuit
denomination
[MΩ]
[MΩ]
[MΩ]
generator
W/o the
‘C’ filter
1 Between insulating
parts of individual
2
brush holders (the
3 clockwise numbering sequence
4 starting from the
5 commutator side)
and the generator
6 body
Between commu7 tator segments
and the rotor body
W/o the
‘C’ filter
W/o the
‘C’ filter
1
—
12.0
9.0
10.0
2
—
0.7
0.6
0.6
3
—
11.0
10.0
10.0
4
—
0.6
0.7
0.7
5
—
10.0
10.5
10.5
6
—
0.7
0.6
0.6
—
—
0.9
0.9
0.9
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8
Between the terminal plate of the
10 generator-starter
and the generator
11
body
12
9
Pin ф 12-MG
—
0.9
0.9
0.9
Pin ф 12-MG
+
30.0
12.0
12.0
Pin ф 6-MG
+
30.0
11.0
11.0
Pin ф 6-MG
Ш
1.0
0.65
0.65
Pin ф 6-MG
П
0.9
0.65
0.65
Table 3
Insulation resistance of the voltage GS-18MO generator – starter
o
o
under conditions of cyclical temperature oscillations (T ≈ 25 C to 55 C)and elevated
humidity (η≈95%)
η ≈ 95%
η ≈ 95%
η ≈ 95%
after
after
after
Θ
=
2h,
Θ
=
11h,
Θ
= 24h,
Marking
Working
U
=
600V
U
=
600V
U
=
100V=
~
~
No. Measuring circuit
on the
denomination
[MΩ]
[MΩ]
[MΩ]
generator
W/o the
‘C’ filter
1
2
3
4
5
6
Between insulating
parts of individual
brush holders (the
clockwise numbering sequence starting from the commutator side) and
the generator body
Between commuta7 tor segments and
the rotor body
8
W/o the
‘C’ filter
W/o the
‘C’ filter
1
—
10.0
4.5
0.1
2
—
0.6
0.45
0.1
3
—
10.0
6.5
0.1
4
—
0.7
0.45
0.1
5
—
10.5
7.0
0.1
6
—
0.6
0.5
0.1
—
—
0.9
0.05
0.1
—
0.35
0.05
0.1
+
12.0
7.0
0.1
+
11.0
7.0
0.1
Ш
0.65
0.65
0.1
П
0.65
0.65
0.1
Pin ф 12 - MG
Between the termi9
Pin ф 12- MG
nal plate of the
10 generator-starter
Pin ф 6- MG
and the generator
11
Pin ф 6- MG
body
12
Pin ф 6- MG
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Table 4
Insulation resistance of the voltage GS-18MO generator – starter
o
under conditions during ‘drying’ at T ≈ 25 C and humidity η ≈ 60%
W/o the ‘C’ W/o the ‘C’
filter
filter
W/o the ‘C’ filter
Marking on the
generator
η ≈ 60%
η ≈ 60%
after
Θ=4
Θ = 24h,
days,
U = 100V= U = 100V=
[MΩ]
[MΩ]
Working denomination
η ≈ 60%
η ≈ 60%
after
after
Θ = 0,1h, Θ = 0,3h,
U = 100V= U = 100V=
[MΩ]
[MΩ]
1
—
0.1
0.2
177.0
∞
2
—
0.1
0.1
0.6
0.7
3
—
0.1
0.2
157.0
∞
4
—
0.1
0.1
0.6
55.0
5
—
0.1
0.2
40.0
140.0
6
—
0.1
0.1
0.6
139.0
—
—
0.2
0.3
0.6
0.6
8
Pin
ф 12 - MG
—
0.1
0.1
0.8
0.8
9
Pin
ф 12- MG
+
0.2
0.2
106.7
∞
Pin
ф 6- MG
+
0.2
0.2
98.8
116.0
Pin
ф 6- MG
Ш
0.6
0.6
0.6
0.8
Pin
ф 6- MG
П
0.1
0.1
0.6
0.7
No.
Measuring
circuit
1 Between insulating parts of indi2
vidual brush
3 holders (the
clockwise num4 bering sequence
5 starting from the
commutator
side) and the
6
generator body
Between commutator seg7
ments and the
rotor body
Between the
terminal plate of
10 the generatorstarter and the
generator body
11
12
Analysis
1. Traces of circumferential scratches on the rotor surface caused by the effect of
pole shoes as well as deeply corroded pole shoes (at the manufacturing factory
they are coated with paint with high resistance to abrasion), traces of circumferUnauthenticated
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2.
3.
4.
5.
6.
7.
8.
Andrzej GĘBURA, Tomasz RADOŃ
ential scratches visible on the rotor surface and caused by foreign bodies may
serve as the evidence for the destructive effect of sand that passes through the
machine body. That ‘sandblasting’ effect is probably liable for wear of paint from
pole shoes.
It is possible that some large sand grains (with the diameter exceeding the air
gap between the stator and the rotor) caused momentary jamming and engine
stalls that magnified values of momentary dynamic forces acting on bearings.
Circumferential scratches on the outer cylindrical surface of the bearing for the
generator rotor may serve as the proof for multiple undesired rotation exercised
by the outer ring of the bearing inside the bearing seating due to increased resistance of the rolling friction.
Insulators (textile laminate) of brush holders after ‘sandblasting’ of their surface
became more susceptible to penetration of water and /or coal dust.
Rust on nuts for bolts that fix pole shoes to the generator body could have appeared due to the effect of moisture.
Microfissures on the face surface at the commutator end (green filling agent)
probably appeared due to premature ageing of the material due to elevated ambient temperature and insufficient moisture.
Small amounts of unwoven fabric in ventilation ducts could have been probably
sucked from the pipe that connect the helicopter fan with the air intake for the
generator cooling system. That fabric could have contributed to less efficient
cooling of the generator-starter.
Traces of deep circumferential scratches (ca. 0.2 mm) on the commutator surface may serve as the evidence of insufficient running-in of the commutator and
the effect of sand. Such a condition of the commutator could have substantially
contributed to increase of voltage drops across the contacts between the commutator segments and the carbon brush, that entailed drop of the magnetic flux
within the rotor during the start-up operation of the engine.
Conclusions
1. Sand that passes through the air ducts of the machine together with cooling air
deteriorates painted surfaces and other protective coatings. Consequently, the
pole shoes of the stator (assembled as a pile of silicon steel plates) become unprotected and subject to rapid corrosion processes, due to even short-term impact of moisture at elevated ambient temperature. Since the corrosion process
leads to formation of ferrous oxides with much higher volume than the primary
material, swelling of pole shoes could have led to arising of compression forces
acting onto stator windings and local punctures or mechanical wear of insulation
on those windings. Eventually the current-carrying paths arose, which resulted in
shorts between live wires of windings and the generator body.
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2. It is highly probably that sand dust penetrates onto interiors of bearings leading to increased resistance of rolling friction and even stalling of the bearing
cage when the torque gradient builds up during the start-up of the engine. In
consequence, the external thrust race of the bearing, normally pushed into the
bearing seating, could have been pulled out and the entire bearing rotated inside the seating.
3. Elevated moisture resulted in the effect of electrochemical corrosion of nuts for
bolts that fix brush holders to the generator body and rusty spots frequently
expanded onto the insulator surface. Since ferrous oxides are electrically conductive materials, the rusting process may lead to drop of insulation resistance
or even shorts across the brush holders.
4. Elevated moisture is also the reason for the drop of insulation resistance demonstrated by insulators for brush holders as well as resistance of stator windings.
5. Premature ageing under conditions of elevated ambient temperature and reduced humidity lead to formation of microfissures in layers of cast insulation
filler on the face surface of the commutator end.
6. Insufficient running-in of the commutator as well as the effect of sand resulted
in formation of deep circumferential scratches (ca. 0.2 mm) visible on the
commutator segments that also could have contributed to increased start-up
torque of the engine.
Suggested recommendations
1. Inlets of the cooling air for generators should be protected with metal meshes with
the mesh size ca. 0.6 mm in order to disable penetration of large sand particles.
2. During the maintenance inspections that are carried out every 25 hours of
operation one has to dismount inlets of cooling air and use a copper brush or
a bore brush to clean the sensitive parts. Remove rust from nuts that fix brush
holders to the generator body and dust from insulators for brush holders. It is
particularly important at locations where air vortexes may appear (Fig. 8).
3. Each time, during the scheduled maintenance No. 2, carry out wearing-in of
brushes and the commutator, regardless the actual condition of brushes.
4. Each time, during the scheduled maintenance No. 2, unscrew eyelets of capacitors and remove brushes form brush holders in order to check insulation
resistance between the body of the GS-18 generator-starter and each pin on
the terminal plate as well as between rotor panels and the rotor axis.
5. With a felt-pen draw reference marks on the bearing seating, the external bearing
race, the bearing cage, the internal bearing race and the journal (Fig. 9). Then rotate the generator rotor and count the number of revolutions until the reference
mark on the bearing cage reaches again its starting position. When the number
of rotor revolutions is less than 2 replace the bearing with a new one [8].
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Andrzej GĘBURA, Tomasz RADOŃ
6. Each time, during the scheduled maintenance No. 2, open the bearing cover
at the commutator side, unlock the cover of the generator shaft and unbend
the nail that locks the washers and prevents the inner sleeve against spontaneous loosening. Next unscrew the inner sleeve and rotate the generator to
have the commutator at the bottommost position. The bearing should firmly sit
inside its bearing cage. If the bearing falls out it should be substituted with a
new one. The contact point between the bearing cover and the generator
body should be locked and sealed with paint.
7. Insulate bolts (M5) that fix brush holders to the generator body by pulling a
heat-shrink sheath on them (diameter φ = 5 mm and length l = 14 mm) in order to provide electric separation of these bolts from the generator body.
8. Each overhaul of the engine should include hot impregnation of the stator
winding and the generator rotor with use of the Epidian 3 agent. The impregnation process must be carried out at negative pressure (in an autoclave).
9. Each overhaul of the engine should include replacement of the green cast filling applied onto the face surface of the commutator end.
10. Each overhaul of the engine or the airframe should include dismounting of insulators for brush holders with surface impregnation of them.
2. Contactors and electromagnets operated under condition
of high dust concentration and elevated air temperature
The switching and control equipment comprise switches, selectors, relays
and contactors. Under condition of high or buoying temperatures, low pressure or
high dust content these devices suffer from high fault rates that chiefly refers to
contactors. Contactors are used to switch electric devices with high consumption
of electric currents, from 20A to 600A or even more [11]. Contactors are actuated
by application (or not) of control voltages to their control contacts electrically connected to electromagnet coils (Fig. 10). Power supply circuit of the controlled
high-power device is opened or closed by means of main contacts placed on a
shuttle armature (detail 2, Fig. 10) that is capable to connect the source of electric
power to the device. Commutation armatures are displaced (to close main contacts) by pulling a dedicated steel core into the coil interior, where that core and
the commutating armature are made as a whole unity. However, the capacity of
the electromagnet to move the steel core is limited, so as the force that enforces
the movements. When substantial amount of dust is deposited inside the device it
is much more difficult for the electromagnet to move the steel core. Sometimes
the mechanical resistance becomes so large that the contactor is no longer capaUnauthenticated
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ble to work correctly, the controlled electric circuit cannot be closed and actuation
of the turbine starter proves to be infeasible.
The completed measurements made it possible to reveal that the resistance of
main contacts within such a contactor features with large deviations that exceeds
10% of the average value for sequential measurements and is higher than the applicable standards allow. Sometimes one can spot as well that the contactor closes the
electric circuit with substantial delay (Tables 5 and 6). In practice, due to incorrect
operation of contacts the start-up time of the engine was prolonged or the engine
could not be kicked off at all, which led to the wrong decision that the turbine starter
had to be replaced.
4.
5.
3.
6.
2.
7.
16.
1.
8.
9.
15.
14.
13.
12.
11.
10.
Fig. 10. Cross-section (schematic) of an avionic contactor:
1 – enclosure of main contacts, 2 – steel core, 3 – commutating armature, 4 – movable main contacts
(of the commutating armature), 5 – enclosure of the electromagnet, 6 – movable contacts of the auxiliary commutating circuit, 7 – textile laminate plate inserted into the housing of auxiliary contacts, 8 –
sand and dust, 9 – housing of the auxiliary commutating circuit, 10 – fixed contacts of the auxiliary
commutating circuit, 11 – electric wires that connect fixed contacts of the auxiliary commutating circuit
with the electromagnet coil, 12 – electromagnet coil, 13 – control contacts of the electromagnet coil,
14 – electric wires that connect fixed contacts with the controlled device and the power source, 15 –
fixed main contacts, 16 – movement directions of the steel core.
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Andrzej GĘBURA, Tomasz RADOŃ
Table 5
Voltage drops measured for contactors within the circuits responsible for actuation
of the turbine starter for the cargo helicopter No. 1.
Examined contactor,
marking on the dia- Required limits
gram
TKS101KOD
„7/1” nr 881
(RK commissioning
AI-9W from Mi-17 No.
6102) prior to cleaning
TKS101KOD
„7/1” nr 881
(RK commissioning
AI-9W from Mi-17 No.
6102) after cleaning
∆UNśr ≤ 300 mV
UZAŁ ≤ 18V
UWYŁ ≤ 6,5V
(∆UP deviations
from the average
TKS101KOD
value for meas„7/1” nr 764
urements must
(RK commissioning
never exceed
AI-9W from Mi-17 No. 10%)
6101) prior to cleaning
TKS101KOD
„7/1” nr 764
(RK commissioning
AI-9W from Mi-17 No.
6101) after cleaning
Measured
value
Average value
calculated for
the rated
current
∆UP
IP [A]
[mV]
∆UNśr
[mV]
Control
voltage
UZAŁ
[V]
25.3
31.3 10
Remarks
UWYŁ
[V]
525.3
7.85
fails to meet
2.70 the requirements
32.3
9.15
3.26
101.0
32.0
32.7 10
meets the
requirements
32.3
14.9
20.5 10
178.0
17.9
fails to meet
14.57 1.10 the requirements
6.8
5.8 10
60.7
5.39
0.74
meets the
requirements
5.6
Findings: deteriorated values of the parameters for the TKS101KOD „7/1” contactor No. 881
(RK commissioning AI-9W Mi-17 No. 6102):
– closing of contacts delayed to ca. 0.5 s,
– opening of contacts delayed to ca. 2÷3 s,
– excessive voltage drop across main contacts – as high as 525.3 mV (only 300 mV is permitted),
– deviation for sequential measurements of voltage drops much exceeds 10% of the average value
Findings: deteriorated values of the parameters for the TKS101KOD „7/1” contactor No. 764
(RK commissioning AI-9W Mi-17 No. 6101) – deviation for sequential measurements of voltage
drops much exceeds 10% of the average value.
Presence of fine-grained dust and corrosion of the armature was detected after dismantling of the
cover for auxiliary contacts.
Follow-up recommendations:
Clean interior of the TKS101KOD No. „7/1” contactor (RK commissioning AI-9) from sand (finegrained dust), wash the armature with a penetrating agent (to remove corrosion products) and blow
the interior with compressed air. Seal the contactor interior (housing) at the side of auxiliary contacts
with use of electroinsulating varnish, stick the insulating disk (textile laminate) inside the cover of
auxiliary contacts, take care to enable unrestricted movements of the contactor armature.
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171
Table 6
Voltage drops measured for contactors within the circuits responsible
for actuation of the turbine starter for the cargo helicopter No. 2.
Examined
contactor,
marking on the
Average
Measured value calcuvalue
lated for the
rated
current
Required limits
∆UP IP
[mV] [A]
diagram
TKS201KOD
„3/1”
(RK commissioning AI-9W)
TKS101KOD
„7/1”
(RK commissioning AI-9W)
∆UNśr
[mV]
Control
voltage
Remarks
UZAŁ
[V]
UWYŁ
[V]
8.6
∆UNśr ≤ 300 mV
UZAŁ ≤ 18V
UWYŁ ≤ 6,5V
(∆UP deviations
from the average
value for measurements is less
than 10%)
9.3 10
182.0
5.7 1.18
meets the
requirements
9.5
25.3
31.3 10
101.0
525.3
7.85
2.7
fails to
meet the
requirements
Findings: deteriorated values of the parameters for the TKS101KOD „7/1” contactor
(RK commissioning AI-9W):
– closing of contacts delayed to ca. 0.5 s,
– opening of contacts delayed to ca. 2÷3 s,
– excessive voltage drop across main contacts – as high as 525.3 mV (only 300 mV
is permitted),
– deviation for sequential measurements of voltage drops much exceeds 10% of the
average value
Follow-up recommendations:
Substitute the TKS101KOD „7/1” contactor (RK commissioning AI-9W) with a technically operative one (that meets requirements).
Routine measurements of contactor parameters carried out by ITWL within
the test program to approve prolongation of the service life for several helicopters
operated in Afghanistan, revealed persistent imperfectness of TKS101KOD „7/1”
contactors within the circuit for actuation of the AI-9W auxiliary motor. The following deficiencies were found out:
– excessive voltage drops across contacts,
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Andrzej GĘBURA, Tomasz RADOŃ
– excessive deviation of voltage drops across contacts at subsequent actuation of the contactor,
– closing of contacts delayed up to ca. 0,5 s,
– opening of contacts delayed up to ca 3 s, intense heating of the contactor
enclosure during measurements.
Upon unscrewing the cover at the side of auxiliary contacts the following findings were established:
4
– presence of fine-grained sand within the region of the auxiliary contacts,
– loose protective disk of textile laminate, some amount of sand trapped between the cover of the auxiliary contact and the textile laminate disk could
restrict the movement range of the contactor armature
– traces of corrosion on the steel core of the contactor,
– presence of sand between the contactor sleeve and its steel core.
Due to the forgoing deficiencies the following remedies were applied:
– the contactor was opened at the both sides, i.e. at the side of the main
contacts and the side of auxiliary contacts,
– mechanical components of the contactor were flushed with the WD-4 penetrating agent,
– interior of the contactor was blown with a stream of compressed air at the
working pressure of 1÷2 atm.,
– contacts and moving mechanical parts were washed with spirit,
– the enclosure of main contacts was closed,
– auxiliary contacts were safeguarded with use of adhesive tape,
– a layer of electroinsulating varnish was applied to seal the gap between
the housing of the auxiliary contact and the contactor body,
– the cover of the auxiliary contact was reinstalled,
– a layer of electroinsulating varnish was applied externally to provide additional sealing of the contactor enclosure.
Upon completion of the foregoing operations the contactor parameters were
rechecked. It was found out that:
– voltage drops across contacts were reduced by 3 to 10 times,
4
The aforementioned deficiencies of the TKS101KOD „7/1” contactors installed within the
start-up circuit of the auxiliary AI-9W engine may certainly be a reason for delay of the
auxiliary engine start-up and for recording of incorrect parameter values during the startup process (exceeded start-up time, elevated temperature, etc.). Such problems formerly
served as the reason to replace, often unnecessarily, the start-up controller or even the
entire turbine starter.
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173
– deviation of voltage drops during subsequent actuations of the contactor
was diminished below the limit of 5% (the maximum permissible deviation
is 10%),
– the values for tripping and deactivation time of the contactor were reduced
below the normative limit (7 to 18 ms).
The authors established that the events when sand is sucked into the contactor enclosures are more frequent in Afghanistan than in Iraq. Probably size of
sand grains in Afghanistan is less than in Iraq due to wider annual amplitude of
temperature variations. Moreover, due to flights in mountains and associated
large pressure jumps, suction of sands into the contactors is also more intense.
After a helicopter rises to a high altitude (which is imposed by requirements of
combat missions) the air pressure inside the contactor drops down. In turn, when
a helicopter rapidly lowers for landing, where high lowering rates are required by
tactical needs, air together with dust from the landing field is sucked into the contactor.
3. Recapitulation and conclusions
The very specific way how selected components of electric power systems
installed onboard of cargo helicopters subject to wear and tear under conditions
of military missions in Iraq and Afghanistan is determined by harsh climatic and
environmental circumstances. Significant gradients of both daily and annual temperature variations lead to premature ageing of insulation materials. High content
of dust in ambient air, particularly during takeoff and landing operations is the
reason for wearing (‘sandblasting’) of insulating and protective parts within the
areas of ventilation ducts inside electric generators. Rare but very intense rainfalls
initiate the process of electrochemical corrosion of bare steel magnetic cores of
electric generators deprived of protective coatings due to the aforementioned
abrasion processes. Due to progress of the corrosion process the volume of
magnetic cores increases (ferrous oxides and hydroxides are of higher volume
than the parent material), which leads to shearing of windings and occurrence of
electric shorts. The Workshop of Avionic Equipment of the Division of Aircraft
Equipment within the Air Force Institute of Technology (ITWL) has developed
methods suitable to detect the very beginning of the shirt-circuit process (incomplete short) even before the DC generator-starter subjects to the breakdown
(complete short). The method consists in monitoring of interrelationships between
amplitudes of three pulsation types, namely groove, commutator and polar pulsations.
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Andrzej GĘBURA, Tomasz RADOŃ
At the same time some measures were suggested, suitable for passive protection against the adverse effect of dusty air onto aircraft generators and generator-starters.
Attention was paid to the high fault rates of bearings operated in electric machinery. Fine-grained sand penetrates into the bearing internal areas and causes
temporary stalls and pulls out the external bearing ring from its seat. The authors
proposed an innovative, but very simple and easy for implementation method that
makes it possible to check operability of the bearing under field conditions. The
method consists in measurement of the current rolling factor of the bearing and
comparison of the measurement result against the reference rolling factor calculated from geometrical parameters of the specific bearing.
Another topic that is addressed in the paper refers to deficiencies of contactors, where the hypothetical suction process of fine-grained sand into the contactor interiors is outlined. It was proposed how to detect presence of sand inside the
contactor housing without the need to fully disassemble the contactor. Some simple prophylactic measures and maintenance procedures are suggested to restore
operability of such contactors.
Authors believe that extensive studies on premature wear of components
within the electric power systems of aircrafts operated under heavy duty environmental conditions shall enable to undertake prophylactic actions to prevent similar
faults in future as such faults shall inevitably, but perhaps in later times, affect
also the equipment operated domestically.
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