Effect of Polarity on Erosion of Off-line Arc in Electric
Friction Couples
L M Song
1,*
, L X Jia
1,2
and R H Zhang
1
1
Department of Materials Science & Engineering, Luoyang Institute of Science and
Technology, Henan Luoyang 471023, China
2
National United Engineering Laboratory for Advanced Bearing Tribology, Henan
University of Science and Technology, Henan Luoyang, 471023, China
Abstract. Off-line arc in pantograph-catenary system of high-speed train affects contact and
current transmission. A series of comparative tests on erosion of off-line arc occurring with
break of contact between W probe and copper specimen at different polarities were carried
out on a home-made current-carrying arc tester. Experimental results were distinctly different
at different polarities. Arc erosion was serious when W probe was anode and copper
specimen was cathode. However, arc erosion was alleviated when W probe was cathode and
copper specimen was anode. Experimental results are discussed on basis of arc physical
essence. Experimental results have related with generation mechanisms of charged particles
and arc state and arc force. Proper match between polarity and physical properties of electric
friction couples can alleviate erosion. The study provides theory support for proposal of
electric arc power collection system.
1. Introduction
With the rapid development of global economy, high-speed railway is the most competitive and
advanced mode of transportation. Nowadays, the operating speed of high-speed trains has reached
350km/h and the required electric current and voltage are up to 1000A and 25kV respectively. Such
high traction power is transmitted to the moving trains from the ground by pantograph-catenary
system. Pantograph-catenary system is elastic and prone to vibrate at high speed, thus contact loss
occurs inevitably. Meanwhile, high current and voltage are transmitted by pantograph-catenary
system, so off-line arc occurs undoubtedly. Off-line arc damages contact surface of electric friction
couples and causes severe wear and makes electric power transmit unstably. Arc ever burned out
contact wire and led to interruption of power supply. In Japan and Germany, advanced manufacturing
and assembly technologies were adopted to avoid contact loss, but they failed [1, 2].
Much works have been carried out to understand the friction and wear properties and stability of
electric transmission of electric friction couples. Temperature of contact surface increases when
electric current passes through the contact surface between friction couples, High temperature leads
to a reduction of the bond energy in metal and causes softening of materials [3]. Meanwhile, high
temperature helps form oxidation film on the contact surface which may prevent direct contact
between friction couples, thus leads to a reduction of real contact area [4]. In addition, electrons can
Song, L., Jia, L. and Zhang, R.
Effect of Polarity on Erosion of Off-line Arc in Electric Friction Couples.
In Proceedings of the International Workshop on Materials, Chemistry and Engineering (IWMCE 2018), pages 357-363
ISBN: 978-989-758-346-9
Copyright © 2018 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
357

pass through thin insulating oxidation films by the quantum tunnel effect [5]. Above reports show
that appropriate temperature on contact surface plays lubrication action and contributes to low
friction and wear and good electric conductivity. However, when arc occurs with break of contact,
surface erosion is aggravated. Thicker oxidation film makes contact resistance and surface
temperature higher [6]. The higher temperature inhibits the cohesion process between oxidation film
and the base material. When oxidation film reaches critical depth, it will break [7, 8]. Because
oxidation film is harder than the substrate, the oxide debris causes severe abrasive wear [9]. In
addition, because thick oxidation film has poor electric conductivity, charged particles will gather in
the layer and form high electric field. Once the layer thickness and electric field reach a certain value,
arc occurs again [10].
Above reports show that off-line arc play an important role in service life and electric
transmission quality of electric friction couples. However less work has been carried out that arc
alone has effect on erosion and electric transmission quality. In this study, a series of tests were
carried out at different polarities on a home-made current-carrying arc tester. Experimental results
were distinctly different at different polarities. Experimental results are discussed on basis of arc
physical essence. Experimental results have related with generation mechanisms of charged particles
and arc state and arc force. Proper match between polarity and physical properties of electric friction
couples can alleviate erosion and improve electric transmission. The study provides theory support
for proposal of electric arc power collection system.
2. Experimental apparatus and experimental procedure
2.1. Principle of experimental apparatus
Figure 1. Principle diagram of current-carrying arc tester.
Experiment was carried out on a home-made current-carrying arc tester whose principle is shown in
Figure 1. The tester consists of mechanical and measure-control systems. In the mechanical system,
probe moves up and down at given speed by adjusting rotation rate and orientation of stepping motor
of Z direction. Meanwhile specimen moves back and forth at given speed by adjusting rotation rate
and orientation of stepping motor of X direction. When probe slides with specimen, probe and
specimen and electric source form a closed electric circuit and current is transmitted by the contact
surface between probe and copper. When probe separates from specimen, probe and arc and
specimen and electric source form a closed electric circuit and current is transmitted by arc.
Measurement-control system consists of voltage sensor and current sensor and high-speed camera. In
Arc current
Arc voltage
Arc photograph
Synchronous trigger signal
Measurement and control part
Mechanical part
Computer
Data acquisition
Motion control
X-direction motor
Y-direction motor
Z-direction motor
Electric source
Voltage sensor
Current sensor
Z-direction slipway
Probe
Arc
Specimen
Y-direction slipway
High-speed camera
X-direction slipway
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addition all these data are collected synchronously by data acquisition card and are displayed on the
computer screen after treatment of software.
2.2. Experimental procedure
In the experiment probe was tungsten alloy which was made up of W and ThO
2
and the content of
ThO
2
was 0.7~0.99%. W probe was 2.4mm in diameter and 45mm in length and its end was
processed into cone which was 30 degree in angle. Specimen was pure copper which was 80mm in
length and 40mm in width and 8mm in thickness and copper specimen and W probe would be treated
with #800 metallographic abrasive papers. Electric source adopted JP50100D electric source of direct
current which could supply constant current or voltage. Experimental current was set 20A and
experimental voltage was set 25V, 30V, 35V, 40V, 45V. W probe moved up and down at 1mm/s
along Z direction and copper specimen moved back and forth at 15 mm/s along X direction. All
above parameters would be preseted before experiment.
Arc current and arc voltage were measured by current sensor and voltage sensor respectively. Arc
pictures were shot by high-speed camera. Arc dimension was obtained by measuring pixel numbers
of arc picture with the help of Image-Pro Plus software. Microstructures on the erosion surface of
electric friction couples were available by SEM.
3. Test results
3.1. Arc burning processes at different polarities
Figure 2. Arc burning processes at different polarities (30V) (a) W Probe: anode, Copper specimen:
cathode; (b) W Probe: cathode, Copper specimen: anode.
0 50 100 150 200 250 300 350
0.0
20.0k
40.0k
60.0k
80.0k
100.0k
120.0k
W probe: Cathode
Copper specimen: Anode
Arc area
A(Pixel number)
Time t(ms)
W probe: Anode
Copper specimen: Cathode
Figure 3. Variation of arc area with time at different polarities (30V).
b
Ignition
Arc burning steadily
Extinction
a
Ignitio
n
Arc burning steadily
Extinction
Effect of Polarity on Erosion of Off-line Arc in Electric Friction Couples
359

Figure 2 shows arc burning processes at different polarities. The common characteristics were firstly
arc grew up rapidly and then burned steadily along with metallic vapour and at last arc extinguished
rapidly. But arc burning processes have distinct differences at different polarities.
When W probe was anode and copper specimen was cathode, arc burned strongly with a great
deal of metallic vapours and arc was bright white. However, arc burned unsteadily and spatter flied
out, as shown in Figure 2 (a). Meanwhile arc area was large, as shown in Figure 3. When W probe
was cathode and copper specimen was anode, arc burned with a little metallic vapour and arc was
green. However, arc burned steadily and spatter flied out occasionally, as shown in Figure 2 (b).
Meanwhile arc area was little, as shown in Figure 3.
Figure 4. Micrographs of current-carrying arc erosion at different polarities (40V). (a) W probe as
anode, (b) copper specimen as cathode, (c) W probe as cathode, (d) copper specimen as anode.
3.2. Erosion of off-line arc at different polarities
Figure 4 shows SEM micrographs of off-line arc erosion of W probe and copper specimen at
different polarities. When W probe was anode and copper specimen was cathode, W probe surface
was rough and eroded badly and the surface of copper specimen form erosion pit, as shown in Figure
4 (a) (b). However, when W probe was anode and copper specimen was cathode, W probe surface
was smooth and eroded slightly and the surface of copper specimen melted only and no erosion pit
formed, as shown in Figure 4 (c) (d).
The experimental results show arc erosion was serious when W probe was anode and copper
specimen was cathode. While arc erosion was slight when W probe was cathode and copper
specimen was anode.
4. Discussions
4.1. Effect of generation mechanisms of charged particles
Current density passed by W probe is high due to its small diameter and then lots of resistance heat is
produced which makes W probe with high melting point reach high temperature. Copper specimen is
d
c
b
a
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prone to melt under the high temperature of arc due to its low melting point, but copper could not
reach high temperature due to high heat conductivity.
When W probe is anode and copper specimen is cathode, W probe and copper specimen provide
anode ions and electrons respectively. W probe is impacted by electrons and kinetic energy of
electrons turns into thermal energy, and thus W probe evaporates and a great deal of metallic vapour
flows out, as shown in Figure 2 (a). Meanwhile, arc erosion of W probe is serious which is shown in
Figure 4(a). Anode ions are provided by thermal ionization in anode region of W probe. Copper
specimen is impacted by anode ions and thermal energy produces, however it cannot reach high
temperature due to high heat conductivity, and thus copper specimen surface melt alone and no
distinct evaporation happens. Because of low temperature of copper specimen surface and little
metallic vapour, it is impossible to produce enough electrons by thermal emission and ionization.
Because no enough electrons keep balance with anode ions, surplus anode ions gather above copper
specimen and high electric field intensity is formed. Electrons can be produced by electric field
emission.
When W probe is cathode and copper specimen is anode, W probe and copper specimen provide
electrons and anode ions respectively. W probe is impacted by anode ions and thermal energy is
produced. Under the thermal energy and resistance heat, W probe reaches easily high temperature
which helps W probe emit electrons by thermal emission. Meanwhile, electron emission consumes
much energy, and thus W probe surface is cooled. So erosion on the surface of W probe is slight, as
shown in Figure 4(c), and little metallic vapours are flowed out which is shown in Figure 2(b).
Copper specimen is impacted by lots of electrons and is heated, but copper specimen cannot
evaporate strongly due to its high thermal conductivity. No enough anode ions are produced by heat
ionization alone. Because no enough anode ions keep balance with electrons, surplus electrons gather
above copper specimen and high electric field intensity is formed. Anode ions can be produced by
electric field ionization.
4.2. Effect of arc state
Boddy et al. [11] found that arc between switch contacts would undergo two stages. The first stage is
described as metallic vapour state arc which mainly burns in metallic vapour. The second stage is
described as gas state arc which burns in little metallic vapour and surrounding air takes part in arc
burning. When arc transfers from metallic vapour state to gas state, arc voltage would jump.
0 20 40 60 80 100 120 140 160 180 200 220
0
4
8
12
16
20
24
28
32
36
W probe: Cathode; Copper specimen: Anode
W probe: Anode; Copper specimen: Cathode
Metallic Vapor
State
Gas State
Gas State
Arc Voltage
U(V)
Time t(ms)
Metallic Vapor
State
Figure 5. Variation of arc voltage with time at different polarities.
Effect of Polarity on Erosion of Off-line Arc in Electric Friction Couples
361

Figure 5 is variation of arc voltage with time at different polarities. When W probe is anode and
copper specimen is cathode, off-line arc is mainly metallic vapour state. When arc is in metallic
vapour state, charged particles are produced by ionization of metallic vapour. Metallic vapour is
easily ionized due to low ionization voltage, so a great deal of charged particles can keep arc burning
strongly and help current-carrying efficiency increase. However charged particles which are
produced by metallic vapour are heavy and move slowly, thus arc heat concentrates and arc erosion
gets serious. When W probe is cathode and copper specimen is anode, off-line arc quickly turns into
gas state. When arc is in gas state, air takes part in ionization. Charged particles are ionized
difficultly because of high ionization voltage of air, thus arc burns weakly. Charged particles which
are produced by air are light and move quickly, thus arc heat disperses and arc erosion is alleviated.
5. Conclusions
 Electric erosion of current-carrying arc was distinctly different at different polarities.
 The forming mechanisms of charged particles were different at different polarities. When W
probe was anode and copper specimen was cathode, W probe produces anode ions by
thermal ionization and copper specimen emitted electrons by thermal emission and electric
field emission; When W probe was cathode and copper specimen was anode, W probe emits
electrons by emitted electrons by thermal emission and copper specimen produced anode
ions by electric field emission.
 Arc state was different at different polarities. When W probe was anode and copper
specimen was cathode, current-carrying arc was mainly metallic vapor state and thus arc
burned easily but erosion was serious; When W probe was cathode and copper specimen was
anode, current-carrying arc was mainly gas state and thus arc burned difficultly but erosion
was light.
 Proper match between polarity and physical properties of electric friction couples can
alleviate erosion and improve electric transmission. The study provides theory support for
proposal of electric arc power collection system.
References
[1] Gao Z B, Wu G N, Lu W, He C H, Zhou L and J 2009 Research review of arc phenomenon
between pantograph and catenary in high-speed electrified railway High Voltage Apparatus
45:104-108
[2] Lei D, Wu G N, Zhang X Y, Wang W G and He C H 2008 Research of a method of inhibition
of electric arc between pantograph and catenary in high-speed electrified railway Electric
Railway 5:1-4
[3] Holm R 1967 Electric Contacts Germany: Springer-Verlag 7
[4] Zaidi H., Chin K J and Frene J 2001 Analysis of surface and subsurface of sliding electrical
contact steel/steel in magnetic Surface and Coatings Technology 148: 241-250
[5] Fisher J C and Giaever I 1961 Tunneling through thin insulating layers Journal of Applied
Physics 32(2): 172-177
[6] Mansori M EI, Paulmier D, Ginsztler J and Horvath M 1999 Lubrication mechanisms of a
soliding contact by simultaneous action of electric current and magnetic field Wear
225~229:1011-1016
[7] Wang Y A, Li J X, Yan Y and Qiao L J 2012 Effect of electrical current on tribological
behavior of copper-impregnated metalized carbon against a Cu-Cr-Zr alloy Tribology
International 50:26-34
[8] Csapo E, Zaidi H and Paulmier D 1996 Friction behavior of a graphite-graphite dynamic
electric contact in the presence of argon Wear 192:151-156
[9] Shunichi K and Koji K 1999 Effect of arc discharge on the wear rate and wear mode transition
IWMCE 2018 - International Workshop on Materials, Chemistry and Engineering
362

of a copper-impregnated metalized carbon contact strip sliding against a copper disk
Tribology International 32:367-378
[10] Wang Y A, Li J X., Yan Y and Qiao L J 2012 Effect of surface film on sliding friction and
wear of copper-impregnated metalized carbon against a Cu-Cr-Zr alloy Applied Surface
Science 258:2362-2367
[11] Rong M Z 1999 Electrical Contacts Fundamentals Xi’an: Xi’an Jiao Tong University 38-45
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