Pantograph I - Analysis on Pantographs & Traction Control

Kelvin Lam
Analysis on Pantographs –
Pantograph and Traction Control
Introduction to electric tractions
Summer 2015
19/10/2015 1

Content
1 - Pantograph & Traction Control
1. Introduction
2. Design of Pantograph
3. Working Principle
4. System Interface
2 – Pantograph Safety
5. Safety & Fatigue Evaluation
6. Qualitative Standards of Pantograph
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Analysis on Pantographs –
Pantograph and Traction Control
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Pantographs & Traction Control
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Introduction
i. Introduction to Traction Power System
ii. Introduction to Pantograph
iii. Categorisation of pantographs
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Introduction to Traction Power System
• The advancement in electric systems provided
trains with enhanced performance.
• Pivotal for commuter services: short dwell time,
rapid acceleration, ideal for quasi-high speed use.
• Lack of fuel storage: a method to transport and
deliver energy to electric-powered trains are
needed.
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Rigid rail conductor
• Third Rail System
• Fourth Rail System
• Ground Level Supply
• Guide bar (for rubber
tyres system)
Overhead Lines (OHL)
• Catenary
• Rigid Conductor
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tyres system)
3rd / 4th Rail
• Used in lower speed railways,
frequently in metro services.
• Direct Current (DC) Supplies
only due to skin effect.
• Proximity to passengers:
usually electrified to 750 volts.
• Current obtained from
lower/upper contact shoe.
• Stray voltage with ground:
frequent corrosion due to
chemical electrolysis.
• Skeptical to flooding/snow
(arcing).
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Schematic diagram of Third Rail
and Contact Shoe.
A Contact Shoe.
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A London Underground driver
placing a safety Short Circuit
Device on 4th Rail after shutting
traction current. Source: “Live
or Dead” video.
Third Rail
system.
19/10/2015 11

tyres system)
Ground Level Supply
• “L'alimentation par le sol”
• Used in urban trams to avoid
scenary implications caused by
OHLs.
• Remove fatal hazards of 3rd/4th
Rail.
• Conventionally 10 metres of live
track with 3 metres of “neutral
zones”.
• Trams have 2 shoes, and
electrically energise tracks when
traction current is required.
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tyres system)
Guide Bar
• Typically used in smaller
scales, i.e. People Movers or
Rubber Metros.
• Same as 4th Rail principle –
rubber tyres do not offer an
earthing route.
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• Catenary
• Rigid Conductor
Catenary
• Power line placed above
rolling stocks; current collected
by pantographs.
• Power transmission can be
transmitted either DC/AC
(discussed later).
• Suitable for high speed
application.
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• Catenary
• Rigid Conductor
Catenary
• OHL lines held by mechanical
tension – both pantograph
pushing force and by lineside
mechanical tensioner.
• Clearance problem: higher
infrastructure construction
cost; lack of clearance for
double-decker carriages.
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Schematic diagram of a typical
overhead catenary system.
Zig-zag OHL is
designed to reduce
pantograph carbon strip
wear.
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Mast. The red circles show
ceramic insulators.
Mechanical tensioner
hanged next to a mast.
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• Catenary
• Rigid Conductor
Rigid Conductor
• Resistivity of metal, which is a
physical attribute, cannot be
changed.
• To reduce resistance we increase
diameter of wire.
• Efficient DC transmission, inefficient
for AC due to skin effect.
• Higher structural integrity required
due to increased weight.
• Higher wear for pantograph.
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Direct Current (DC)
• 600V
• 750V
• 1,500V
• 3,000V
Alternating Current (AC)
• 15 kV Single Phase (50Hz)
• 25 kV Single Phase (50Hz)
• 50 kV Single Phase
• 3000V Three Phase (16-50Hz)
(mountain railways only)
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(Conventional)
Choices of traction
current:

 Factors in consideration:
 Efficiency & Energy Loss (per kilometre)
 Infrastructure cost:
(Extra clearance; number of transformer substation per
electrified section; neutral zones)
 Electromagnetic Compatibility (EMC)
 Synonymous motor (for terrain railways) uses a 3-phase
supply, but speed control compromised.
 Stray voltage problem present in DC Power Supplies.
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Electric potential is
induced around a
grounded conductor. As
distance increases
moderately potential
reduces, thus a potential
difference is caused,
a.k.a. stray voltage.
A e.m.f. is said to have formed
when a stray voltage is
observed. This catalyse the
electrolysis of the permanent
way, accelerating its corrosion.
“Cathodic corrosion of railways”
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A ‘neutral zone’ lineside
sign. A neutral zone is to
separate power supplies
of different phases; to
avoid harmonic and
unsynchronized
crashes. Only applicable
in AC.
A Section Insulator in a DC Supply
railway. Analogous to neutral zones
of AC railways to prevent harmonic
crashes.
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• Atmospheric clearances should be maintained when OHLE
are used.
• From BS EN 50119 (discussed later in Part II):
• Table outlining clearances is attached below:
• * Maximum value from static and dynamic clearances are
taken for justification reason
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DC 600V 100mm
DC 750V 100mm
DC 1500V 100mm
DC 3000V 150mm
AC 15kV 150mm
AC 25kV 270mm

• In AC traction system, differing potentials caused by phase should
be separated accordingly.
• * Maximum value from static and dynamic clearances are taken for
justification reason
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Nominal
Voltage
Phase
difference
(deg)
Relative
Voltage
Clearance
15 kV 120 26 kV 260
15 kV 180 30 kV 300
25 kV 120 43.3 kV 400
25 kV 180 50 kV 540

Introduction to Pantograph
• A conducting arm extending above the rolling stock, collecting
traction current from OHL system.
• Three broad classifications:
I. Bow collector (now obselete)
II. Trolley Pole
III. Pantograph
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• Trolley Pole:
• An extended arm with revolving conductive wheel, rolls below the
OHL wire.
• Low-speed application.
• Chances of “following the wrong wire” at junction.
• Unidirectional only – requires rotating.
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• Pantograph:
• Folding, mechanical arm pushes against the OHL wire to maintain
un-intermittent traction current supply.
• Expensive and difficult to construct.
• Bi-directional.
• Suitable for higher speed application.
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Categorisation of Pantographs
• Single-arm
Lighter and simpler design.
• Double-arm
Heavier but more forgiving to faults; less aerodynamic for
high-speed usage.
• Wing (T-) shaped
Together to shrouds, designed using F1 technologies to
reduce vortex and associated noise. Very high cost to
manufacture and maintain and are retired.
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A single-arm pantograph from
Odakyu Railway Series 3000.
A double-arm pantograph from
Series 0 Shinkansen trainset.
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Wing-shaped
pantograph from Series
500 Shinkansen trainset.
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Categorisation of Pantographs
• Shrouding
• Given that fluid always have viscous properties, as skin drag
develops.
• Trains travelling faster will increase Reynold’s number; an
appreciable boundary layer is developed. High-speed trains
have higher aerodynamic impacts.
• Boundary layer develops, producing noise and unnecessary
tractive longitudinal resistance.
• With shrouding we create a non-slip boundary condition
around pantograph, keeping unwanted noise to minimum.
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Shrouding
around
pantograph
from N700
tilting
Shinkansen
trainset.
Boundary Layer Theory: as
fluid transverse upon a
surface, the contacting
particles retard, and
subsequent layers slower.
Above the transition point a
different physical layer –
boundary layer is formed. The
flow transited from laminar to
turbulent and drag increases
egregiously.
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Similar ideas in aircrafts – winglets.
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CFD (Computational Fluid Dynamics) and FEM (Finite Element Method) analysis
on pantograph shrouding travelling at 350 km/h. Image extracted from a
Chinese academic publication. (Wang et al.)
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Design of Pantographs
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Design of Pantograph
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1. Pan Head
A Carbon strip that acts as a
conductive, dry lubricant. The
contact surface between rolling
stock and OHL wires.
2. Upper Arm
3. Lower Arm
Pivot support of the pan head.
4. Damper Assembly
Offers improved control for pan
rising/dropping than solely
using compressed air.
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5. Air Cylinder
Acting like a piston-cylinder.
Compressed air pushes the
piston, hence extending the
piston rod to raise pantograph;
vice versa for pan drop.
6. Raising/Counterbalance Spring
Provides pantograph articulation
against vibrations and
harmonics.
10. Base Frame
Base of the pantograph assembly.
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1. Carbon Carrier
Conductive platform
where carbon strip is
housed.
4. 25kV Ceramic
Insulator
A highly insulated
material to isolate car
body from high voltage.
5. Raising Spring
To provide actuator
effort to the lower arm, in
addition to piston power.
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8. Air Feed Insulator
An insulative, solid state
device that allows gas to
be bled into the cylinder.
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3. Foot Insulator
Insulative support that secures
the pantograph assembly on the
roof of the rolling stock.
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• In high speed trains, where
dynamic behaviour could be
dominant:
• Conventional gas power may
not be able to keep pantograph
in contact with OHL wires.
• Rather, aerodynamic effect ∝
velocity2 .
• We exploit aero-behaviour to
maximise contact.
19/10/2015 43
A Brecknell-Wills high speed pantograph on
board a Thai Siemens Class 360 Desiro. The
use of aerofoil generates lift above the chord
line therein propelling pantograph upwards
against the OHL.

Structural Design of Pantographs
• ‘Pantograph’ is a word
with Greek roots,
meaning ‘every write’.
• Structure with
mechanical linkage so
it works by forming a
parallelogram.
• Modern pantograph
principle is derived
from a linked support
structures called
“Scissors
mechanism”.
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• The height adjustment
is achieved by the
horizontal displacement
motion in the x-axis of
the actuator.
• As force is applied, it is
transferred to the truss.
• Causing elongation to
the crossing pattern.
• May be thought as
‘conversation of area’.
• The diagram
represents a perfect
model for trusses of the
same length.
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• However, if we
consider insulative
gauge of the OHLE
system.
• It is evident that a
scissors structure must
be elongated vertically
to maintain
atmospheric
separation.
• We ‘may be’ able to
achieve it by simply
lengthening one of the
top truss.
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• Every 1cm that A
moves up, W will move
up 3 cm.
• Mechanical Advantage
increases as = 1:3.
• However…
• With increased bending
moments extending
towards the weight, the
bending stress will
inevitably cause beam
deflection.
“Euler-Bernoulli beam
theory”
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• Single-arm pantograph
are used nowadays as
it offer greater
leverage.
• It is important to
understand the speed
requirement of
pantograph concerned.
• Extra speed incur extra
forces, so
reinforcements
required.
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“One-truss” configuration
can save weight but may
not offer appreciation
torsional resistance.

Working Principles
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Working Principle
i. Operator POV
ii. Mechanism POV
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Operator POV
• The train operator (TO) (a.k.a. train driver) raises the pantograph via
the TMS (Train Management System) onboard the driving cab.
• This is done after the ‘sweep’ checks along the exterior and interior
of the train is done.
• With TMS a TO can isolate individual pantograph-at-fault.
• Alternatively, some older trains will contain ‘PAN UP’ and ‘PAN
DOWN’ buttons.
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Operator POV
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“Cockpit” of a
SP1950
trainset. Red
circle denotes
TMS screen.

Driving Cab of a
BREL Class 315.
Red Circle
denotes
‘PANTOGRAPH
UP/RESET’
button.
Operator POV
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Operator POV
• In some dual-voltage system, the TO has to manually set
which traction current the train will run on.
• Failing to do so may cause serious consequence to
trainborne equipment as the correct current cannot be
transformed and achieved.
• These trains are called “multi-system locomotive/electric
multiple unit”.
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Driving Cab of a dual-
voltage BREL Class 313
EMU. The red circle
highlighted the two
switches for 25kV AC
Pantograph & 750V DC
Shoe Gear.
Reseau-Duplex high
speed locomotive are tri-
current, making them
suitable for use in
French, German and
Swiss high-speed rail
network.
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Mechanism POV
• Starting from low, folded position:
1. The compressed air is fed into the operating cylinder.
2. Control link extends, moving a slotted rod in between the
piston link and pivot arm.
3. Spring originally in stretched position moves, offering extra
pivot moment.
4. Spring contracts, leverage causes collector head to rise and
make contact with OHLE.
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System Interface
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System Interface
i. Traction Control
ii. Neutral Zones
iii. Auxiliary Power Supplies
iv. Electromagnetic interferences
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Traction Control
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Evolution of electrical systems
• World’s first electric locomotive
was built by Werner von
Siemens in 1866.
• Powered by 150V DC third
insulated rail.
• Similar trains can be found in
Volk’s Electric Railway, the
oldest electric railway still in
operation.
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Volk’s Electric Railway, Brighton, United Kingdom.
The VER No. 8 train is built in 1901, powered by a Compagnie Electrique Belge
8 hp DC Motor. The railway is electrified in offset 3rd rail 110V.
19/10/2015 62

Resistor Control
• Before the introduction of electrical controls, mechanical
systems were used to control trains.
• Bank of (series of) resistors were switched on/off to alter
current flowing into the motor, hence controlling motor speed.
• In EMUs a ‘cam shaft’, driven by a (pneumatic) motor
operates the shaft, and it then switches respective resistors
on/off to control the traction current – known as Camshaft.
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Simple illustration of a
camshaft controller.
Camshaft controller of a
SEPTA BSS B-IV train.
Source:
http://prr4ever.blogspot.hk/
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Camshaft
• The camshaft makes and
breaks electrical contacts
consecutively in response of
TO’s throttle handle.
• In DC propulsion, motor
velocity = f(voltage)
• Camshaft itself is powered by a
smaller motor in a relay circuit.
• Between electric contacts arc
breakers break any ‘incorrect’
under-loaded electric arcs.
• An analogue of today’s
electronics.
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• After experiments and trials
after WWII, 25kV AC OHLE
became the standard for
future higher speed railways.
• AC traction system was not
previously introduced due to
lack of high-power rectifiers
(mercury-arc).
• Unstable AC motors – it does
not offer the appropriate
tractive characteristics
(speed vs torque relation).
• Hence DC traction motors
remained in use.
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Tractive Effort/Resistance curve
of a locomotive.

Tap Changer
• AC has varying ‘vectors’ –
resistors only hinders the
transmission of a linear current.
Some changes in impedance is
required.
• Transformers operate using
alternating magnetic flux, and
AC coincides with this principle.
19/10/2015 67

• AC Supplied via OHLE to the pantograph.
• An electric locomotive/carriage has two autotransformers, housed in
same casing to offer better coupling.
• Inside casing there is insulation oil present, increasing transformer
efficiency.
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• As TO changes the notch a
different transformer ‘tapping’ is
selected progressively.
• Each tapping represent a pre-
defined displacement along the
secondary transformer coil.
• In transformers…
voltage = f(ratio of coils)
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• In between notches, the on-
load tap changer has links in
between connection to ensure
continuous, uninterrupted
supply.
• However resistor added to
prevent short-circuiting
between taps.
• Tap is changed via a servo
motor, usually pneumatic.
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Chopper / Thyristor
• With advent of semiconductors, more precise and accurate controls of
currents are permitted.
• Removal of mechanical devices – dramatically increases efficiency.
• Analogues previously help us understand how semiconductor
controls work.
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Semiconductor
s
A Gate Turn-off
(GTO) Thyristor

PWM
• To understand chopper we need to appreciate duty cycle and
pulse width modulation (PWM).
• In digital electronics only binary signals are used: ON or OFF.
This is defined by the logic gates.
19/10/2015 72
An illustration of duty
cycles.

PWM
• Duty cycle refers to the occupancy percentage of an active
signal.
• Using a varying duty cycles we are able to transmit information
(a mixture of different data) or also transmit varying currents
(discussed later).
• Not to be confused with frequency modulation (FM), where
signals are arranged using varying wavelengths. In PWM the
frequency remains constant (i.e. the datum pulse time)
19/10/2015 73

Chopper
• Chopper controls traction current in similar manner to PWM.
• Simplified idea:
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Schematic working principle
of a DC Current.
Chopper controlled current.

Chopper
• Using GTO (Gate Turn-off) we
can control traction current
using a smaller, reference
signal.
• In chopper circuits unidirectional
semiconductors are used
(depend upon layering
“charged” structure).
19/10/2015 75

Chopper
• “Chopper” circuit can be sub-
divided into 4 (quadrant) types,
dependent on their
characteristics.
• The ‘quadrants’ are determined
by their task demanded and flow
of voltage & current.}
• However principle remain the
same.
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Chopper
• Some explanation on chopper
circuit:
 CH: Chopper
 D: freewheel diode; to absorb
surges from excessive
inductance
 L: inductance; to smoothen
traction current when chopper
control = OFF to maintain
tractive characteristics
 R: source of resistance; a
load: i.e. motor
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Type A – forward
motoring Type B – forward braking
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Chopper

Thyristor
• Begin with a conventional thyristor:
• As pre-requisite we need to appreciate how semiconductor and
transistor work.
• A transistor works by inserting a signal voltage to gate, therein
putting electrons into ‘base’ so that the ‘emitter’ would have enough
delocalised electrons in the impurified silicon to conduct current.
19/10/2015 79

Thyristor
• A transistor is biased; it is unidirectional.
• Different layering configuration would mean different operating
directions.
• If transistors are placed in a structured manner we can exploit their
logic characteristics to achieve new performances.
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Thyristor
• When opposite current is
applied, in forward-biased
mode electrons are dragged
towards the terminal, leaving no
delocalised electrons midway.
• Or, the thyristor behave like a
conventional semi-conductor
and no current passes through
when forward current is applied
in correct direction, without a
gate signal. This is said to be
reverse-biased.
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Thyristor
• Only when the correct signal voltage is applied, and current is at
correct direction then thyristor works.
• Owing to unidirectional properties, regenerative braking feature is
not available for a thyristor.
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Thyristor
• The third state of a thyristor is
the forward conducting mode.
• If we consider a thyristor using a
circuit diagram…
• Once the signal voltage ‘fired’
the thyristor it works eternally:
this is because a closed loop is
formed.
• It does not switch off when
signal voltage no longer applies.
• Only controllable by entirely
switching the power source off.
• Known as ‘latches on’.
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Gate Turn-Off Thyristor
• Because of the ‘latching’ limitations of normal thyristors, we need a
safer design in order to protect traction characteristics.
• General Electric invented a new high-powered thyristor that can be
gate-controlled.
• In retardation, some of the traction current is ‘stolen’ to form a
negative voltage. This reverse current is fed into the GATE.
• However some residual current is left within the GTO so the reverse
current has to be applied longer to induce forward current to fall and
eventually disappear.
• Long application of reverse negative signal = long switching time.
This means inefficiency (but still much efficient than mechanical
controls previously). Limitations to 1 kHz.
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Phase-fired Thyristor
• Given AC’s repetitive
characteristics, we cannot
simply ‘chop’ and imagine if a
pulse as an average current.
• A newer way to ‘chop’ current is
needed.
• Any sinusoidal wave has
‘phase’.
We imagine a single-phase AC
supply will complete a 360
degree circle.
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Phase-fired Thyristor
• If the thyristor is fired at a
calculated, pre-defined time
that matches the ‘phase angle’
of the AC current, we are able
to capture a desired output
voltage.
• This is known as ‘phase
cutting’.
• Analogous to current ‘chopping’
using phasor feature.
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“Snubbing”
• GTO make use of a reversed signal voltage to gradually
reduce the forward current, eventually powering off.
• However, introducing a gigantic reverse current rapidly would
be excess current within the GTO, exceeding the safe
operating area.
• Hence GTO requires a snubber as safeguard in voltage
transient stages.
19/10/2015 87
Snubbers.
Source: Wikimedia

IGCT
• An advancement from GTO technology
• IGCT = Integrated Gate Commutator Transistor
• A fully controllable device: switched purely by gate signals
instantly, where GTO can only be switched off gradually.
• However based upon obsolete technology: it suffers from high
power loss.
• Not commonly seen in railway vehicles.
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• Older EMUs utilises DC
traction motors and controls.
With introduction of AC drives
in late 1980s these controls
become obsolete.
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Inverter
• With advancements in AC motors, it is apparent that DC
motors do not offer the best traction characteristics (i.e. does
not exactly obey tractive curve/Davis formula).
• A new set of controls are needed.
• Understanding how current are converted from one to
another; and strong appreciation in single- and three-phase
AC is a pre-requisite.
19/10/2015 90

Inverter
• Numerous attempts to use AC
controls were made without
semiconductors.
• AC-DC-AC motors were used.
An AC motor runs directly from
AC OHLE supply drives a DC
dynamo.
• This is very inefficient – a lot of
energy is wasted via friction.
This also increases
maintenance cost.
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Inverter
• An inverter oscillates, which convert a DC to an AC current at pre-
defined frequency.
• AC motor works by applying current to stators at different angle at
different times. To control speed and torque we need different
frequency output.
• VFD = Variable Frequency Drive is introduced. Also known as VVVF
(Variable Voltage Variable Frequency).
19/10/2015 92

Inverter
• AC Motors on EMUs are 3-
phase motors, so three
switches are needed in the
matrix inverter.
• GTO is not fast enough for high
frequency switching, so newer
semiconductor is needed.
• Born of IGBT – Insulated Gate
Bipolar Transistor.
• Advantage: low signal current
needed, also allowing higher
forward traction current.
19/10/2015 93
Application of inverter in a water
pump.

Inverter
• IGBT is a MOSFET (Metal
Oxide Semiconductor Field
Effect Transistor) device.
• To understand a MOSFET we
need to first understand what
exactly is a FET (field effect
transistor):
19/10/2015 94
Illustration of a FET.
IGBT

Inverter
• Consider a normal n-type transistor. Because the ‘source’ and
‘drain’ are all insulated by the material.
• However if we apply a small current, then free electrons are
‘induced’, by Field Effect, so charged particles exists to carry the
forward current.
19/10/2015 95
Illustration of a FET.

Inverter
• Inverter control itself does not produce traction sound.
• The oscillator oscillates at varying frequency in which
electromagnetic interferences produced are absorbed by the
car body.
• “Photons” are absorbed by car-body material and are
converted to kinetic energy in bond vibrations.
• Sudden noises propagate as train acceleration due to
material strong resonance at harmonic frequencies.
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SiC
• In DC power supply, the MOSFET modules oscillates to
‘pretend’ that there is an AC current output.
• Impurities and ‘noises’ in AC current can cause disturbance
and deteriorate AC traction characteristics.
• New methods that utilise low ‘gate signal’ consumption and
higher switch frequency is preferable.
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SiC
• SiC = Silicon Carbide
• Developed by Mitsubishi Corporation.
• Compared to IGBT and IGCT (an
advancement from GTO) it has 40%
higher switch frequency.
This allow traction characteristics to be
achieved using lower energy, which is
previously impossible.
• Achieved by improved reverse
resistance and better breakdown
performance.
• Smaller mass and dimension: coincide
with ‘weight reduction’ trend.
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• Modern trains employs
inverter control which offer
higher efficiency and better
traction characteristics.
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Associated Systems
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Neutral Section
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Neutral Section
• In AC railways traction current
are supplied from different
power sources.
• Electric sources are
unsynchronised and has
different phases.
• To prevent harmonic crashes
they are separated by a section
of insulated wire without any
current.
• Trains through neutral sections
have to move through it with
residual momentum due to
lack of OHLE power supply.
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UK Signage System: a
trackside board reminding TO
of an approaching neutral
section.

Neutral Section
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Neutral Section
• Configuration of a typical neutral section
• Track Magnet: activates on-board bogie magnetic sensor,
hence trips/resets VCB in pantograph cars.
• Run Off: section of energised wire before/after the neutral
zone to permit control to decrease traction current and
(de)activates circuit breaker.
• Ceramic Insulator: section of wire made of ceramic that
deters electric potential due to differing phases.
• Dead Wire: A non-energised wire connected to current
return path, acting as earth.
• Skid and ceramic insulator makes up an section insulator.
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Track
Magnet
300mm
Run-off
zone
5 ft
Ceramic
Insulato
r
5ft Dead
Wire
5ft
Ceramic
Insulato
r
40 ft
Run-off
Zone
Track
Magnet
Total:
155
feets

Skids
Ceramic Insulator
Dead Wire
19/10/2015 105

Neutral Section
• If an electric train/locomotive did not disconnect its power
supply as it moves through a neutral section it could cause
damage to trainborne electrical system (from harmonics,
phase crash, surges etc.)
• Trackside signage are placed to remind drivers to ‘coast’
trains before entering neutral section.
• Human errors can still happen!
Safeguard system(s) introduced to deter preventable errors.
19/10/2015 106

Neutral Section
• Trackside magnets can be
installed, which activates a
bogie-mounted sensor.
• This deactivates traction
current using a circuit breaker.
• Common type of circuit breaker
is VCB – vacuum circuit
breaker.
19/10/2015 107

Neutral Section
• A VCB is a type of circuit
breakers that exploit vacuum’s
excellent dielectric properties to
insulate two terminals and
quench any arcing.
• Vacuum has 8x higher
dielectric ability than air; SF6
(sulphur hexafluoride) has 4x.
• This is because there are no
gas atoms present; with no
(un)-stable electrons in sub-
shells there are absence of
charge carrier, making it a
perfect dielectric material.
19/10/2015 108

Neutral Section
• Gas sealage can be
apparent; glass insulation are
needed to prevent
condensation trapped in
moving contacts.
• Thermal problem: Using
perfect gas law, expansion
inside & outside of gas are
drastically different, and
egregious movements is
undesirable.
The VCB is therein housed
within an insulation vessel.
19/10/2015 109

Neutral Section
• Within a VCB, there is one fixed
terminal and one moveable
terminal.
• Upon receiving command, the
moving terminal retracts and
arcing happens.
• Arcing is caused by ionisation of
metallic atoms.
• In vacuum vessel arcing is
quickly suppressed by insulating
vessel, retracting contacts and
in minor extent, metallic vapours
re-condensed onto the cooled
moving terminals.
19/10/2015 110

Neutral Section
• There is no need for a long
neutral section for DC
traction supply.
• A section insulator is
installed in between two
energised sections fed by
separate DC source. This is
to avoid power supply
interferences.
19/10/2015 111

Auxiliary Power Supplies
19/10/2015 112
Dark room…?
Imagine what trains would look
like without an APS.

• Trains traversing in between section insulator/neutral sections
do not have power supplies.
• Loss of traction current would stop power supplies to power-
thirst equipment, i.e. traction motor, air conditioning… etc.
• APS are installed on board to ensure limited lightings,
emergency ventilation and communications are not
jeopardised for safety reasons.
19/10/2015 113

An APS battery
pack of an electric
train.
19/10/2015 114

• Lightings and ventilation need low voltage (LV) supplies.
Normal traction current is excessive for its use.
• Methods to obtain LV currents are needed.
• Previously wiring for step-down devices are bulky and space-
consuming.
• DC is also not preferable for use: there are excessive power
loss. At the time solid state rectifiers were not available and
current conversion are very inefficient.
• (Motor alternator: motor-alternator-motor)
19/10/2015 115

• A motor generator is placed on each car.
• Part of the DC traction current drives a DC motor, which is
permanently coupled to an AC generator.
• Voltage regulator included to act as a buffer: safeguard from
surges/dips in 3rd rail gaps/section insulator.
19/10/2015 116

19/10/2015 117

• With introduction of solid states, conversion from DC/single-
phase AC to three-phase AC 380V (industrial & railway
wagon standard) became easier.
• If supply is DC, part of the DC supplied into the train will
charge up battery and remainder is fed into a static inverter
(SIV) and to be converted to a 3-phase current.
• If supply is single-phase AC it has to be rectified first.
19/10/2015 118

• Contrary to motor controls, SIV only outputs a constant voltage
and frequency.
19/10/2015 119

• Some older locomotives only
provide traction to trailing
coach; no electricity is fed into
hauled coaches.
• Power supplies from loco are
known as head-end power or
electric train supply (ETS).
• These trains require a
generator van: coach that
contains a diesel generator to
provide 380AC 3-phase.
19/10/2015 120

Pantograph I - Analysis on Pantographs & Traction Control

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Pantograph I - Analysis on Pantographs & Traction Control