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4.5 BRAKING
Braking is very frequent in electric drives to stop a motor in a reasonably short time.
For example, a plannar must quickly be stopped at the end of its stroke and sometimes
must quickly be stopped at the end of its stroke and sometimes it is necessary to stop the
motor in order to prevent accident. The essential of a good braking system should be
1) Reliable and quick in its action.
2) The braking force must be capable of being controlled.
3) Adequate means be provided for dissipating the stored energy that is kinetic
energy
of the rotating parts.
1) In case of a fault in any part of the braking system the whole system must come to
instantaneous rest or result in the application of the brakes.
There are two types of braking:
1) Mechanical braking:
The motor in this case is stopped due to friction between the moving part of the motor
and the brake shoe that is stored energy is dissipated as heat by a brake shoe or brake
lining which rubs against a brake shoe or brake lining which rubs against a brake drum.
2) Electric braking:
In this method of braking, the kinetic energy of the moving parts that is motor is converted
into electrical energy which is consumed in a resistance as heat or alternatively it is
returned to the supply source. During braking operation, a motor has to function as a
generator. The motor can be held at stand still. In other words, the electric braking cannot
hold the motor at rest. Thus, it becomes essential to provide mechanical brakes in addition
to electric braking. Various types of electrical braking are:
a) Plugging
b) Rheostatic braking
c) Regenerative braking

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PLUGGING
This is a simple method of electric braking and consists in reversing the connections of
the armature of the motor so as to reverse its direction of rotation which will oppose the
original direction of rotation of the motor and will bring it to zero speed when mechanical
brakes can be applied. At the end of the braking period the supply to the motor is
automatically cut off. This method of braking can be applied to the following motors.
1) DC motors
2) Induction motors
3) Synchronous motors
PLUGGING APPLIED TO DC MOTORS
To reverse a DC motors, it is necessary to reverse the connections of the armature while
the connections of the field are kept the same. The direction of mmf remains the same
even during braking periods.
Plugging applied to Series motors:
Total voltage of V+ Eb is available across the armature terminals which causes a current
I to flow around the circuit. When Eb = V then the voltage across the armature is 2V and
at the time of braking twice the normal voltage is applied to the resistance in series with
the armature at this time in order to limit the current. While the motor is being braked,
the current is still being drawn from the supply. This method requires energy from the
supply for its action and not only the kinetic the motor is being wasted, but this energy is
also being dissipated.
Speed and braking torque
Electric braking to torque
𝑇𝐵 ∝ Φ𝐼
𝑇𝐵 = 𝐾Φ𝐼
where K is a constant
𝑉 + 𝐸𝑏
𝐼=
𝑅
𝐸𝑏 = 𝐾1𝑁Φ
where, N is the speed; K1 is a constant

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𝑉+
𝐼= 𝐾1𝑁Φ
𝑅
𝑉 + 𝐾1𝑁Φ ) = 𝐾 Φ + 𝐾Φ 𝑁
𝐾1𝑁Φ 2

𝐾Φ
𝑇 = 𝐾Φ (V ) =(
𝐵 ) + (𝐾Φ 2 3
𝑅 𝑅 𝑅
𝐾𝑉
𝐾2
𝑅
=
𝐾𝐾
1
𝐾3 = 𝑅
Apply the results obtained to the series motor, where, Ф α armature current (Ia)
Then Electric braking in series motor = K4 Ia + K5Ia2
In the case of shunt motor since flux is constant.
Electric braking torque:
Wherever there is a load on the machine the load will also exert braking torque due to it
and then the total braking torque (T)
T = Electric braking torque + Load torque
PLUGGING APPLIED TO INDUCTION MOTORS
In the case of induction motor its speed can be reversed by inter changing any of the two
stator phases which reverses the direction of rotation of motor field. Actually, at the time
of braking when the induction motor is running at near synchronous speed. The point Q
represents the torque at the instant of plugging one can notice that the torque increases
gradually as one approaches the stand still speed. Different values of rotor resistance give
rise to different shapes of speed torque curve in order to give any desired braking effect.
The rotor current I2 can be calculated during the braking period from the following
relation.
I2= sE2/ √ [Re2 + (sX2)2]

where E2 is the e.m.f. induced in rotor at standstill, R2 is the rotor resistance, X 2 is the
standstill reactance of the rotor and s is the percentage slip
PLUGGING APPLIED TO SYNCHRONOUS MOTORS
Plugging can be applied to the synchronous motors, with the only difference that the field
on the rotor will be rotating in opposite direction to that of the rotating field on the stator
with the synchronous speed and the relative velocity between the two will be twice the
synchronous speed.

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RHEOSTATIC BRAKING
In this method of braking, the motor is disconnected from the supply and run as generator
driven by the remaining kinetic energy of the equipment that is the energy stored in motor
and load which are to be braked.
The following drives can be braked by the rheostatic method:
1) DC motor
2) Induction motor
3) Synchronous motor
DC MOTORS
Shunt motor:
In this type of motor, the armature is simply disconnected from the supply and is
connected to as resistance in series with it, the field; winding remains connect to the
supply.
The braking can be adjusted suitably by varying the resistance in the armature circuit.
In the case of failure of the supply, there is no braking torque because of absence of the
field.
Series motor:
In this case of the connections are made as shown is fig during braking operation.
The motor after disconnection from the supply in made to run as a DC series generator.
Braking torque and speed
Braking current = Eb/ R
= K1ФN/R
Electric braking torque = KK1Ф2N/R = K2Ф2N
where, K2= KK1/R
In the case of a series motor the flux dependent upon the armature current
Electric braking torque for series motor = K3Ia2N
While in the case of shunt motor since flux is constant
Electric braking torque = K4N
INDUCTION MOTOR
In this case the stator is disconnected from the supply and is connected to DC supply
which excites the windings thereby producing a DC field. The rotor is short-circuited

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across through resistance in each phase. When the short circuited rotor moves it outs the
steady flux produced in the air gap due to DC current flowing in the stator produced in
the air gap due to DC current flowing in the stator and an emf is induced in the rotor
conductors Rheostatic braking in the synchronous motors is similar to the rheostatic
braking in induction motors. In this case the stator is shorted across resistance in star or
delta and the machine works like an alternator supplying the current to the resistance,
there by dissipating in kinetic energy in the form of losses in the resistances.
REGENERATIVE BRAKING
In this type of braking, the motor is not disconnected from the supply but remains
connected to it and its feeds back the braking energy or its kinetic energy to the supply
system. This method is better than the first and second methods of braking since no
energy is wasted and rather it is supplied back to the system. This method is applicable
to following motors:
Shunt motor:
In a DC machine where energy will be taken from the supply or delivered to it depends
upon the induced emf, if it in less than the line voltage the machine will operate as motor
and if it is more than the line voltage, the machine will operate as generator. The emf
induced in turn depends upon the speed and excitation that is when the field current or
the speed is increased the induced emf exceeds the line voltage and the energy will be
field into the system. This will quickly decrease the speed of the motor and will bring it
to rest.
Series motor:
In this case, complications arise due to fact that the reversal of the current in the armature
would cause a reversal of polarity of the series field. In the case of induction motors, the
regenerative braking is inherent, since an induction motor act as a generator when running
at speeds above synchronous speeds and it feeds power back to the supply system. No
extra auxiliaries are needed for this purpose. This method is however very seldom used
for braking but its application is very useful to lifts and hoists for holding a bending load
at a speed only slightly above the synchronous speed.

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Tramways:
The tramway is perhaps the cheapest type of transport available in very dense traffic. It
receives power through a bow collector or a grooved wheel from an overhead conductor
at about 600 V D.C., the running rail forming the return conductor. It is provided with at
least two driving axles in order to secure necessary adhesion, start it from either end and
use two motors with series- parallel control. Two drum-type controllers, one at each end
used for controlling the tramcar. Though these controllers are connected in parallel, they
have suitable interlocking arrangement to prevent their being used simultaneously. The
main frame of the car body is made from high tensile steel. Aluminium is extensively
used for bodywork. The under frame is of rolled steel sections. Seats are either in
transverse direction or a combination of transverse and longitudinal arrangement is used.
The equipment is similar to that used in railways but the output is considerably smaller
and does not exceed 60 to 75 H.P. For normal service rheostatic and mechanical braking
are employed. For mechanical braking, electro-mechanical drum brakes are used. Also,
magnetic tracks brakes are used for giving better retardation.
Trolley-Bus:
Serious drawback of tramway is the lack of maneuverability in congested areas and noise;
this is overcome by the trolley-bus drive. It is an electrically- operated pneumatic-tyred
vehicle which needs no track in the roadway. It receives its power at 600 V D.C. from
two overhead contact wires. A D.C compound motor of output of 50 to 100 kW is
normally used. Speed control is obtained by field weakening method. Foot operated
master controllers are used so that drive may have his hands free to steer the vehicle and
apply hand brake. One pedal controls the starting, speed control and regenerative braking,
if any and second pedal control rheostatic and compressed air brakes. Regenerative
braking is usually not employed in trolley-bus drive because of difficulty of ensuring that
supply system is always in a position to absorb the energy regenerated. The lighting
system in the car is low-voltage D.C supplied from a motor- generator set connected in
parallel with a battery. The vehicles are usually provided with secondary batteries so that
the vehicles can be maneuvered in case of emergency. Since the body of the car is
insulated from earth on account of the rubber-tyred wheels, it must be properly checked
for adequate insulation resistance lest it leaks and causes electric shocks to the passengers

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while boarding and alighting from the bus. The insulation resistance is checked at the end
of the day. Trolley – buses have more passenger carrying capacity, higher acceleration
and braking retardation than oil-engine buses. These are, therefore, used for medium
traffic density as obtained in inner suburbs. Oil engine buses, on the other hand, are used
for outer suburbs and country side where there is low traffic density.

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4.3 MECHANICS OF TRAIN MOVEMENT

TRAIN MOVEMENT
Speed-Time Curve
The curve drawn between speed and time is called the speed-time-curve and is given in
figure 4.3.1. The speed-time curve gives complete information of the motion of the train.
The curve gives the speed at various instants after the start of run directly. Slop of the
curve at any point gives the acceleration at the corresponding instant or speed. The area
covered by the curve, the time axis and the ordinates through the instants between which
the time is taken, represents the distance covered in the corresponding time interval.

Figure 4.3.1 Driving mechanism of electric locomotives

[Source: “A Textbook of Electrical Energy” by B.L.Theraja, Page: 1711]

Speed-time curve mainly consists of

1) Initial acceleration
(a) Constant acceleration or acceleration while notching up and
(b)Speed curve running or acceleration on the speed curve
1. Acceleration period or Notching up period (0 to t2):
From starting to the stage when locomotive attains maximum speed, the period is known
as acceleration period, as the vehicle is constantly accelerated. This is represented by OA
portion of the curve and time duration is t1. During this period of run (0 to t1), starting
resistance is gradually cut so that the motor current is limited to a certain value and the

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voltage across the motor is gradually increased and the traction motor accelerates from
rest. To cut the starting resistance, the starter handle has to be moved from one notch to
another. Hence this period is called notching up period. The acceleration is almost
uniform during this period. Now the torque decreases and speed increases according to
the speed torque characteristics of the motor. Now the acceleration gradually decreases
with the increase in speed and finally reaches the required torque for the movement of
the train (at time t2).
2. Free running period (t2 to t3):
During this period i.e. t2 to t3 the power supplied to the motor is at full voltage and speed
of this period is constant, also during this period. Power drawn from the supply is
constant. During this period the motor develops enough torque to overcome the friction
and wind resistance and hence the locomotive runs at constant speed. This is shown by
the portion AB of the curve.
3. Coasting (t3 to t4):
At the end of free running period supply to the motor is cut off and the train is allowed to
run under its own kinetic energy. Due to train resistance speed of the train gradually
decreases. The rate of decreasing of speed during this period is known as “coasting
retardation”. When the locomotive is running at certain speed, if the motor is switch off,
due to inertia the vehicle will continue to run, of course with little deceleration due to
friction and windage.
4. Braking (t4 to t5):
At the end of coasting period the brakes are applied to bring the train to stop. During this
period speed decreases rapidly and finally reduces to zero. The locomotive is retarded to
stop it within short distance and at a particular spot. The shape of the curve will change
depending upon the distance between consecutive stations. It is the curve drawn between
speed of train in km/hour along y-axis and time in seconds along x-axis. The speed time
curve gives complete information of the motion of the train. This curve gives the speed
at various times after the start and run directly. The distance travelled by the train during
a given interval of time can be obtained by determining the area between the curve and
the time axis corresponding to this interval.

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TYPES OF SERVICES
There are three types of electric traction services.
1. Main line service
2. Sub-urban service
3. Urban service
Speed – Time curve for suburban service
In this type of services, the distance between two successive stations is in the range of
1.5 km to 8 km. Figure represents speed-time curve for sub-urban service. Acceleration
and braking retardation required are high. Free running period is not possible and
coasting period will be comparatively longer than urban service.
Speed – Time curve for urban or city service
In city service the distance between the two stations is very short i.e., between 0.75 to 1
km. The time required for this run between the adjacent and retardation should be
sufficient high. Fig shows the speed-time curve for urban or city service. It will be seen
that there will be no free running period. The coasting period is also small.
MECHANICS OF TRAIN MOVEMENT
Essential driving mechanism of an electric locomotive is shown in figure 4.3.2.

Figure 4.3.2 Driving mechanism of electric locomotives

[Source: “Generation and Utilization of Electrical Energy” by Sivanagaraju, Balasubba Reddy, Srilatha, Page: 499]

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The electric locomotive consists of pinion and gear wheel meshed with the traction motor
and the wheel of the locomotive. Here, the gear wheel transfers the tractive effort at the
edge of the pinion to the driving wheel. The armature of the driving motor has a pinion
diameter ‘d’ attached to it. The tractive effort at the edge of the pinion is transferred to
the driving wheel by means of a gear wheel.
Torque developed by the motor,
𝑑1
𝑇 = 𝐹𝑝 × , (𝑁𝑚)
2
Tractive effort at the edge of the pinion,
2𝑇
𝐹𝑝 = , (𝑁)
𝑑1
Tractive effort at the edge of the pinion transferred to the wheel of locomotive,
𝑑2
𝐹𝑡 = 𝜂𝐹𝑝 × , (𝑁)
𝐷
2𝜂𝑇 𝑑2 2𝜂𝑇𝑟
𝐹= ( ) , (𝑁)
𝐷 = 𝐷
𝑑1
where, T is the torque exerted by the driving motor in Nm, d is the diameter of gear wheel
in metres, D is the diameter of driving wheel in metres, η is the transmission efficiency
and r is the gear ratio which is equal to d2/d1. For obtaining train motion without slipping
tractive effort F should be less than or at the most equal to μW where μ the coefficient of
adhesion between the wheel is and the track and W is the weight of the train on the driving
axles (called the adhesive weight).
TRACTIVE EFFORT
The effective efforts required to run a train on track are:
➔ Tractive effort needed to provide acceleration, (Fa)
➔ Tractive effort needed to overcome the train resistance (Fr)
➔ Tractive effort needed to overcome gradients (Fg)
Tractive effort required for acceleration (Fa):
𝐹𝑜𝑟𝑐𝑒 = 𝑚𝑎𝑠𝑠 × 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛
𝐹 = 𝑚𝑎
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑡𝑟𝑎𝑖𝑛, 𝑚 = 1000 𝑊𝑘𝑔

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𝑘𝑚 1000 𝑚 𝑚
𝐴𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 = 𝛼, ( ℎ𝑟 ) = 𝛼 × ,(
𝑠𝑒𝑐 3600 𝑠 2) = 0.2778𝛼, ( 2)
𝑠
Tractive effort required for linear acceleration,
𝐹𝑎 = 1000𝑊 × 0.2778𝛼 = 277.8𝑊𝛼, (𝑁)
When a train is accelerated in a linear direction, its rotating parts like the wheels and
armature of motors have to be accelerated in an angular direction. Therefore, the
accelerating mass of the train is greater than the dead mass of the train. The accelerating
weight (We) is much higher (about 8-15%) than the dead weight (W) of the train.
Tractive effort required for linear and angular acceleration,
𝐹𝑎 = 277.8𝑊𝑒 𝛼, (𝑁)
Tractive effort required to overcome the train resistance (Fr):
While moving, the train has to overcome the opposing force due to the surface friction
and wind resistance. The train resistance depends upon various factors such as shape,
size, condition of track etc.
Tractive effort required to overcome the train resistance,
𝐹𝑟 = 𝑊 × 𝑟, (𝑁)
where W – dead weight of the train in ton, r is the specific train resistance in N/ton of the
dead weight.
Tractive effort required to overcome gradients (Fg):
Consider that an electric train is moving upwards on a slope. The dead mass of the train
along the slope will tend to bring it downward. To overcome this effect of gravity, tractive
effort is required in opposite direction.
Tractive effort to overcome the effect of gravity,
𝐹𝑔= ±𝑀𝑔 sin 𝜃 = ± 1000𝑊𝑔 sin 𝜃, (𝑁)
where, g is the acceleration due to gravity = 9.81 m/sec2, θ is the angle of slope.
𝐹𝑔= ±1000𝑊 × 9.81 sin 𝜃, (𝑁)

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Figure 4.3.3 Train moving on up-gradient

[Source: “Generation and Utilization of Electrical Energy” by Sivanagaraju, Balasubba Reddy, Srilatha, Page: 501]

In railway practice, the gradient is expressed in terms of rise or fall in every 100 m of
track and it is denoted by G %. From figure 4.3.3, we get,
𝐵𝐶 𝐸𝑙𝑒𝑣𝑎𝑡𝑖𝑜𝑛
𝐺𝑟𝑎𝑑𝑖𝑒𝑛𝑡, 𝐺 = sin 𝜃 = =
𝐴𝐶 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑎𝑙𝑜𝑛𝑔 𝑡ℎ𝑒 𝑡𝑟𝑎𝑐𝑘
%𝐺 = 100 sin 𝜃
𝐺
𝐹𝑔 = ±1000𝑊 × 9.81 × = ±98.1𝑊𝐺, (𝑁)
100
Positive sign is to be used for up-gradient and negative sign for down-gradient.
The total tractive effort required for the propulsion of train,
𝐹𝑡 = 𝐹𝑎 + 𝐹𝑟 ± 𝐹𝑔
𝐹𝑡 = 277.8𝑊𝑒 𝛼 + 𝑊𝑟 ± 98.1𝑊𝐺, (𝑁)

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4.1 MERITS OF ELECTRIC TRACTION

The locomotive in which the driving or tractive force is obtained from electric
motors is called Electric traction. Electric traction has many advantages as compared to
other non-electrical systems of traction including steam traction.
Electric traction is used in:
i) Electric trains
ii) Trolley buses
iii) Tram cars
iv) Diesel-electric vehicles etc.
Traction systems
All traction systems, broadly speaking, can be classified as follows:
1. Non-electric traction systems:
These systems do not use electrical energy at some stage or the other. Examples: Steam
engine drive used in railways at some stage or the other. These are further sub-divided
into the following two groups:
a) Self-contained vehicles or locomotives
Examples:
i) Battery-electric drive
ii) Diesel-electric drive
b) Vehicles which receive electric power from a distribution network or suitably placed
sub-stations.
Examples:
i) Railway electric locomotive fed from overhead AC supply
ii) Tramways and trolley buses supplied with DC supply
Here power is applied to the vehicle from an overhead wire suspended above the track.
2. Electric traction systems
Electric traction systems may be broadly categorized as those operating on:
1. Alternating current supply
2. Direct current supply

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In general, the following electric traction systems exist:

a. AC 3 phase 3.7 kV system
b. AC single phase 15/16 kV - 161/25 Hz
c. AC single phase 20/25 kV - 50/60 Hz
d. DC 600 V
e. DC 1200 V
f. DC 1.5 kV
g. DC 3 kV
Advantages:
Electrical transmission is applied to high power units and has following advantages:
i. It has smooth starting without shocks.
ii. Full driving torque is available from standstill.
iii. Engine can be run at its most suitable speed range. This given higher
efficiency range.
iv. Characteristics of traction motor and generator are so chosen that the speed of
the traction unit automatically adjusts according to the load and gradient so as
to maintain constant output and not to overload the diesel engine.
v. Electrical transmission docs not only work as torque converter but also works
as reversion gear.

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REQUIREMENTS OF ELECTRIC TRACTION SYSTEM

The following are some of the important requirements of the driving equipment used for
traction purposes:
a. High adhesion coefficient, so that high tractive effort at the start is possible to have
rapid acceleration.
b. The locomotive or train unit should be self-contained so that it can run on any route.
c. Minimum wear on the track.
d. It should be possible to overload the equipment for short periods.
e. The equipment required should be minimum, of high efficiency and low initial and
maintenance cost.
f. It should be pollution free.
g. Speed control should be easy.
h. Braking should be such that minimum wear is caused on the brake shoes, and if
possible, the energy should be regenerated and returned to the supply during braking
period.

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4.6 RECENT TRENDS IN ELECTRIC TRACTION

Magnetic levitation, maglev or magnetic suspension is a method by which an object is
suspended with no support other than magnetic fields. Magnetic pressure is used to
counteract the effects of the gravitational and any other accelerations. Earnshaw's
theorem proves that using only static ferromagnetism it is impossible to stably levitate
against gravity, but servomechanisms, the use of diamagnetic materials, super
conduction, or systems involving eddy currents permit this to occur. In some cases, the
lifting force is provided by magnetic support bearing little load that provides stability.
This is termed levitation, but there is a mechanical pseudo-levitation. Magnetic
levitation is used for maglev trains, magnetic bearings and for product display purposes.
Mechanical constraint (pseudo-levitation):
With a small amount of mechanical constraint for stability, pseudo-levitation is relatively
straightforwardly achieved. If two magnets are mechanically constrained along a single
vertical axis, for example, and arranged to repel each other strongly, this will act to
levitate one of the magnets above the other. Another geometry is where the magnets are
attracted, but constrained from touching by a tensile member, such as a string or cable.
Another example is the Zippe-type centrifuge where a cylinder is suspended under an
attractive magnet, and stabilized by a needle bearing from below.
Diamagnetism:
Diamagnetism is the property of an object which causes it to create a magnetic field in
opposition to an externally applied magnetic field, thus causing a repulsive effect.
Specifically, an external magnetic field alters the orbital velocity of electrons around their
nuclei, thus changing the magnetic dipole moment. According to Lenz's law, this opposes
the external field. Diamagnets are materials with a magnetic permeability less than μ0 (a
relative permeability less than 1). Consequently, diamagnetism is a form of magnetism
that is only exhibited by a substance in the presence of an externally applied magnetic
field. It is generally quite a weak effect in most materials, although superconductors
exhibit a strong effect. Diamagnetic materials cause lines of magnetic flux to curve away
from the material, and superconductors can exclude them completely (except for a very
thin layer at the surface).

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Direct diamagnetic levitation:

A substance that is diamagnetic repels a magnetic field. All materials have diamagnetic
properties, but the effect is very weak, and is usually overcome by the object's
paramagnetic or ferromagnetic properties, which act in the opposite manner. Any
material in which the diamagnetic component is strongest will be repelled by a magnet.
Earnshaw’s theorem does not apply to diamagnets. These behave in the opposite manner
to normal magnets owing to their relative permeability of μr < 1 (i.e. negative magnetic
susceptibility). Diamagnetic levitation can be used to levitate very light pieces of
pyrolytic graphite or bismuth above a moderately strong permanent magnet. As water is
predominantly diamagnetic, this technique has been used to levitate water droplets and
even live animals, such as a grasshopper, frog and a mouse. However, the magnetic fields
required for this are very high, typically in the range of 16 teslas, and therefore create
significant problems if ferromagnetic materials are nearby. The modern trend is towards
the use of d.c motors (both separately excited and d.c series motors) equipped with
thyristor control. The operating voltages are 600V or 1,000V. Braking employed are
mechanical, rheostatic and regenerative, Thyristorised converters provide accurate
control and fast response. Main advantages of thyristor control are the absence of bulky
on load tap changer and electromagnetic devices, saving of energy, notch less control,
increase in pulling ability of the motive power, and minimum wear and tear because of
absence of conventional moving parts in the motor control circuits. In electric traction, it
is desirable that the train accelerates and decelerates at a constant rate for the comfort of
the passengers. Using thyristors, this objective can be met in the following way: When
the speed goes down during breaking the generator voltage decreases. For a particular
braking torque, a particular armature current is required. This is achieved by increasing
the field excitation to a relatively high value. If, however, the generator voltage exceeds
the supply voltage, in dynamic braking, this increase is permissible as the armature is not
connected to the supply and the energy of the generator can be dissipated in the braking
resistors, external resistances in series with the armature are connected in case of
regenerative braking to absorb the voltage difference between the armature voltage and
supply voltage. With this, of course, part of the generated power is wasted in the external
resistors and the efficiency of the overall system is decreased. Various methods of speed

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control and electric braking employing thyristors have already been studied in power
electronic subjects. In addition to ordinary phase control methods, cycle selection
methods of control of SCR for varying the voltage applied to the traction motors are also
employed. In this method the required average voltage is obtained by accepting or
rejecting a certain numbers of complete half cycles. In practice, at the start only one half
out of eight is accepted and as the speed builds up, it is gradually raised to 2/8, 3/8 and
finally 8/18 for full power operation. This method is advantageous due to low frequency
harmonics, low rate of rise of current, better power factor etc. In chopper control of
traction motors, at start, the ‘on’ period of pulse is kept very short which lengthens during
the period of controlled acceleration. Thus, the average voltage applied across the traction
motors is gradually increased keeping the mean value of the input current close to the
desired value. Figure shows a typical thyristorised dc traction system supplying a group
of four separately excited motors. The armatures are supplied from half controlled bridge
converters. However, it is desirable to feed the field windings through fully controlled
bridge converters so as to reduce the ripple in the field current. Low ripple in the field
current ensures low iron losses in the machines. However, if regenerative braking is
required then the armature should be supplied from fully controlled bridges.
Freewheeling diodes are connected as illustrated to ensure good waveform of armature
current. The armatures are connected in series – parallel arrangement to ensure good
starting and running characteristics. It is seen that armatures are supplied by three bridges
connected in series. For starting first only bridge A is triggered and firing angle is
advanced as speed builds up. When bridge A is fully conducting (i.e. when α=0), bridge
B is triggered and then bridge C is triggered. During starting field currents are set to
maximum to provide high starting torque. The use of three bridges ensures better power
factor than would be possible with a single bridge.

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4.2 SUPPLY SYSTEMS

Mainly, there are three supply systems for Electric traction:
➔ The direct current system -
➔ The single-phase AC system operating at low frequency 162/3 Hz or 25 Hz and
at normal frequency 50 Hz
➔ The three phase AC system
Direct current system:
The transformation and high voltage generation of dc is very inconvenient to the dc
supply used is at normally 600 V and this voltage is almost universal for use in urban and
suburban railways. For direct current equipment, the series motor is universally employed
as its speed-torque characteristics are best suited to traction requirements. Generally, two
or more motors are used in single equipment and these are coupled in series or in parallel
to give the different running speeds required. The motors are initially connected in series
with starting rheostats across the contact line and rails, the rheostats are then cut out in
steps, keeping roughly constant current until the motors are running in full series. After
this the motors are rearranged in parallel, again with rheostats, the rheostats are cut out
in steps, leaving the motors in full parallel. The power input remains approximately
constant during the series notching, then jumps to twice this value during the parallel
notching. Thus a 4-motor unit will have three economical speeds when the motors are
running in series, series-parallel connections. The rheostats are operated electro
magnetically or electro-pneumatically.
AC single phase system:
In this supply is taken from a single overhead conductor with the running rails. A
pantograph collector is used for this purpose. The supply is transferred to primary of the
transformer through on oil circuit breaker. The secondary of the transformer is connected
to the motor through switchgear connected to suitable tapping on the secondary winding
of the transformer. The switching equipment may be mechanically operated tapping
switch or remote-controlled contractor of group switches. The switching connections are
arranged in two groups usually connected to the ends of a double choke coil which lies
between the collections to adjacent tapping points on the transformer. Thus, the coil acts

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as a preventive coil to enable tapping change to be made without short circuiting sections
of the transformer winding and without the necessity of opening the main circuit.
AC three phase system:
In case of 3-phase system, energy can be drawn directly from the existing 3-phase electric
network or by using transformer substation in case the network is operating at higher
voltage. This system, therefore, has high efficiency as no converting equipment is
involved. Here, two trolly wires per track are required and are connected between two
phases of the supply. The induction motor is used as the drive which is robust in
construction and cheap in first cost. It has high efficiency and it acts as an induction
generator when runs at a speed more than its synchronous speed, thus during regenerative
braking by changing the number of poles (increase) the synchronous speed can be
reduced and hence power can be pumped back into the supply system. Since, it is a
constant speed motor, which can be used to limit the speed of the train to a definite value.
Three phase main line railways operate at a voltage between 3300 and 3600 and a
frequency of 162/3 Hz. Low starting torque, high starting current and constant speed
characteristics of induction motors are some reasons why 3-phase systems could not
become popular for traction purposes.

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4.4 TRACTION MOTORS AND CONTROL

TRACTION MOTORS
The motor for electric traction has to operate under different conditions which are arduous
than those in most industrial applications and a special type of motor known as traction
motor has been developed which incorporates the following features:
Electric features:
High starting torque
Series speed-torque characteristics
Simple speed control
Possibility of dynamic/regenerative braking
Good commutation under rapid fluctuations of supply voltage
Mechanical features:
Robustness
Ability to withstand continuous vibrations
Minimum weight and overall dimensions
Protection against dirt and dust
No type of motor completely fulfills all these requirements. Motors, which have been
found satisfactory are DC series motor for DC systems and AC series motor for AC
systems. Also, AC three phase motors can be used.
TRACTION MOTOR CONTROL
The control of traction motors for starting and for smooth acceleration is very
much essential to avoid damage to the motors. The control equipment is provided for
manual and automatic operation. Usually a master controller is used for the purpose.
1) D.C series motor control or plain rheostat control
2) Series –Parallel control
i. Open circuit transition
ii. Shunt transition control
iii. Bridge transition control
3) Metadyne control
4) Multiple Unit control

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