Department of Electrical and Electronics Engineering

Department of Electrical and Electronics Engineering

 Unit – II : ELECTRIC TRACTION:
 General
 By electric traction is meant locomotion in which the driving (or tractive) force
is obtained from electric motors.
 It is used in electric trains, tramcars, trolley buses and diesel-electric vehicles
etc.
 Electric traction has many advantages as compared to other non-electrical
systems of traction including steam traction.

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 ELECTRIC TRACTION: Traction Systems
 Traction Systems
 Broadly speaking, all traction systems may be classified into two categories :
 (a) non-electric traction systems
 They do not involve the use of electrical energy at any stage.
 Examples are : steam engine drive
 used in railways and internal-combustion-engine drive used for road transport.
 (b) electric traction systems
 They involve the use of electric energy at some stage or the other.
 They may be further sub divided into two groups :

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 ELECTRIC TRACTION: Traction Systems
 1.First group consists of self-contained vehicles or locomotives.
 Examples are : battery-electric drive and diesel-electric drive etc.
 2.Second group consists of vehicles which receive electric power from a
distribution network fed at suitable points from either central power stations or
suitably-spaced sub-stations.
 Examples are : railway electric locomotive fed from overhead ac supply and
tramways and trolley buses supplied with dc supply.
 Advantages of Electric Traction
 As compared to steam traction, electric traction has the following advantages :

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 ELECTRIC TRACTION: Traction Systems

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 ELECTRIC TRACTION: Advantages
 1. Cleanliness. Since it does not produce any smoke or corrosive fumes, electric traction is
most suited for underground and tube railways. Also, it causes no damage to the buildings
and other apparatus due to the absence of smoke and flue gases.
 2. Maintenance Cost. The maintenance cost of an electric locomotive is nearly 50% of that
for a steam locomotive. Moreover, the maintenance time is also much less.
 3. Starting Time. An electric locomotive can be started at a moment's notice whereas a
steam locomotive requires about two hours to heat up.
 4. High Starting Torque. The motors used in electric traction have a very high starting
torque. Hence, it is possible to achieve higher accelerations of 1.5 to 2.5 km/h/s as against 0.6
to 0.8 km/h/s in steam traction. As a result, we are able to get the following additional
advantages:
 (i) high schedule speed, (ii) increased traffic handling capacity, (iii) because of (i) and (ii)
above, less terminal space is required — a factor of great importance in urban areas.

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 ELECTRIC TRACTION: Advantages
 5. Braking. It is possible to use regenerative braking in electric traction system. It leads to
the following advantages :
 (i) about 80% of the energy taken from the supply during ascent is returned to it during
descent.
 (ii) goods traffic on gradients becomes safer and speedier.
 (iii) since mechanical brakes are used to a very small extent, maintenance of brake shoes,
wheels, tyres and track rails is considerably reduced because of less wear and tear.
 6. Saving in High Grade Coal. Steam locomotives use costly high-grade coal which is not
so abundant. But electric locomotives can be fed either from hydroelectric stations or pit-
head thermal power stations which use cheap low-grade coal. In this way, high-grade coal
can be saved for metallurgical purposes.
 7. Lower Centre of Gravity. Since height of an electric locomotive is much less than that of
 a steam locomotive, its centre of gravity is comparatively low. This fact enables an electric
locomotive to negotiate curves at higher speeds quite safely.
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 ELECTRIC TRACTION: Advantages
 8. Absence of Unbalanced Forces. Electric traction has higher coefficient of adhesion since
there are no unbalanced forces produced by reciprocating masses as is the case in steam
traction. It not only reduces the weight/kW ratio of an electric locomotive but also improves
its riding quality in addition to reducing the wear and tear of the track rails.

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 ELECTRIC TRACTION: Disadvantages
 Disadvantages of Electric Traction
 1. The most vital factor against electric traction is the initial high cost of laying out overhead
electric supply system. Unless the traffic to be handled is heavy, electric traction becomes un
economical.
 2. Power failure for few minutes can cause traffic dislocation for hours.
 3. Communication lines which usually run parallel to the power supply lines suffer from elec-
trical interference. Hence, these communication lines have either to be removed away from
the rail track or else underground cables have to be used for the purpose which makes the
entire system still more expensive.
 4. Electric traction can be used only on those routes which have been electrified. Obviously,
 this restriction does not apply to steam traction.
 5. Provision of a negative booster is essential in the case of electric traction. By avoiding
the flow of return currents through earth, it curtails corrosion of underground pipe work and
interference with telegraph and telephone circuits.
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 Systems of Railway Electrification
 Systems of Railway Electrification
 Presently, following four types of track electrification systems are available :
 1. Direct current system — 600 V, 750 V, 1500 V, 3000 V
 2. Single-phase ac system —15-25 kV, 16 2/3 25 and 50 Hz
 3. Three-phase ac system —3000-3500 V at 16 2/3 Hz
 4. Composite system — involving conversion of single-phase ac into 3-phase ac or dc.

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 Typical Speed/Time Curve
 Typical speed/time curve for electric trains operating on passenger services is shown
in
 Fig. 43.8.
 It may be divided into the following five parts :
 1. Constant Acceleration Period (0 to t1)
 It is also called notching-up or starting period because during this period, starting resistance of
the motors is gradually cut out so that the motor current (and hence, tractive effort) is
maintained nearly constant which produces constant acceleration alternatively called
‘rehostatic acceleration’ or‘ acceleration while notching’.

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 Typical Speed/Time Curve (Contd..)
 2. Acceleration on Speed Curve (t1 to t2)
 This acceleration commences after the starting resistance has been all cut-out at point t1 and
full supply voltage has been applied to the motors. During this period, the motor current and
torque de crease as train speed increases. Hence, acceleration gradually decreases till torque
developed by motors exactly balances that due to resistance to the train motion.
 The shape of the portion AB of the speed/time curve depends primarily on the torque/speed
characteristics of the traction motors.
 3. Free-running Period (t2 to t3)
 The train continues to run at the speed reached at point t2. It is represented by portion BC in
Fig. 43.8 and is a constant-speed period which occurs on level tracks.

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 Typical Speed/Time Curve (Contd..)

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 Typical Speed/Time Curve (Contd..)
 4. Coasting (t3 to t4)
 Power to the motors is cut off at point t3 so that the train runs under its momentum, the speed
gradually falling due to friction, windage etc. (portion CD). During this period, retardation
remains practically constant. Coasting is desirable because it utilizes some of the kinetic
energy of the train which would, otherwise, be wasted during braking. Hence, it helps to
reduce the energy consumption of the train.
 5. Braking (t4 to t5)
 At point t4, brakes are applied and the train is brought to rest at point t5. It may be noted that
coasting and braking are governed by train resistance and allowable retardation respectively.
 Retardation is defined as the rate of decrease of velocity with time. suppose if a car is slowing
down, its velocity is decreasing, so its acceleration is negative. But retardation is positive.
Retardation is negative of acceleration.

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 Average and Schedule Speed
 While considering train movement, the following three speeds are of importance :
 1. Crest Speed. It is the maximum speed (Vm) attained by a train during the run.
 2. Average Speed = distance between stops / actual time of run
 In this case, only running time is considered but not the stop time.
 3. Schedule Speed = distance between stops / actual time of run + stop time
 Obviously, schedule speed can be obtained from average speed by including the duration of
stops.
 For a given distance between stations, higher values of acceleration and retardation will mean
lesser running time and, consequently, higher schedule speed.
 Similarly, for a given distance between stations and for fixed values of acceleration and
retardation, higher crest speed will result in higher schedule speed. For the same value of
average speed, increase in duration of stops decreases the schedule speed.

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 Factors Affecting The Schedule Speed

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 Factors Affecting The Schedule Speed (Contd..)

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 Factors Affecting The Schedule Speed (Contd..)

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 Numerical and Solution
 Find the schedule speed of an electric train for a run of 1.5 km if the ratio of its maximum to
average speed is 1.25. It has a braking retardation of 3.6 km/h/s, acceleration of 1.8 km/h/s
and stop time of 21 second. Assume trapezoidal speed/time curve.

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 Numerical and Solution-Analysis of Trapezoidal Speed-Time Curve

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 Numerical and Solution-Analysis of Trapezoidal Speed-Time Curve (Contd..)

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 Numerical and Solution-Analysis of Trapezoidal Speed-Time Curve (Contd..)

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 Numerical and Solution-Analysis of Trapezoidal Speed-Time Curve (Contd..)

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 Numerical and Solution-Analysis of Trapezoidal Speed-Time Curve (Contd..)

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 Numerical and Solution-Analysis of Trapezoidal Speed-Time Curve (Contd..)

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 Numerical and Solution - Trapezoidal Speed-Time Curve

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 Numerical and Solution - Trapezoidal Speed-Time Curve (Contd..)

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 Numerical and Solution - Trapezoidal Speed-Time Curve (Contd..)

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 Tractive Effort for Propulsion of a Train
 The tractive effort (Ft) is the force developed by the traction unit at the rim of the driving
wheels for moving the unit itself and its train (trailing load).
 The tractive effort required for train propulsion on a level track is
 Ft = Fa + Fr
 If gradients are involved, the above expression becomes
 Ft = Fa + Fg + Fr — for ascending gradient
= Fa − Fg + Fr — for descending gradient
 where Fa = force required for giving linear acceleration to the train
 Fg = force required to overcome the effect of gravity
 Fr = force required to overcome resistance to train motion.

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 Tractive Effort for Propulsion of a Train (contd..)
 (a) Value of Fa
 If M is the dead (or stationary) mass of the train and a its linear acceleration, then
 Fa = M*a
 Since a train has rotating parts like wheels, axles, motor armatures and gearing etc., its
effective (or accelerating) mass Me is more (about 8 − 15%) than its stationary mass. These
parts have to be given angular acceleration at the same time as the whole train is accelerated
in the linear direction.
 Hence, Fe = Me*a
 (i) If Me is in kg and α in m/s2, then Fa = Me*a Newton
 (ii) If Me is in tone and α in km/h/s, then converting them into absolute units, we have
 Fa = (1000 Me) × (1000/3600) a = 277.8 Me*a Newton

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 Tractive Effort for Propulsion of a Train (Contd..)

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 Tractive Effort for Propulsion of a Train (Contd..)

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 Tractive Effort for Propulsion of a Train (Contd..)

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 Numerical and Solution
 A 250-tonne motor coach driven by four motors takes 20 seconds to attain a speed of 42 km/h,
starting from rest on an ascending gradient of 1 in 80. The gear ratio is 3.5, gear efficiency
92%, wheel diameter 92 cm train resistance 40 N/t and rotational inertia 10 percent of the
dead weight. Find the torque developed by each motor.

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 Numerical and Solution

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 Numerical and Solution (Contd..)

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 Adhesive Weight and Coefficient of Adhesion
 Adhesive Weight
 It is given by the total weight carried on the driving wheels. Its value is Wa = x W, where W
is
dead weight and x is a fraction varying from 0.6 to 0.8.
 Coefficient of Adhesion
 Adhesion between two bodies is due to interlocking of the irregularities of their surfaces in
contact.
 The adhesive weight o f a train is equal to the total weight to be carried on the driving wheels.
 It is less than the dead weight by about 20 to 40%.

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 Adhesive Weight and Coefficient of Adhesion (Contd..)

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 Adhesive Weight and Coefficient of Adhesion (Contd..)
 It has been found that tractive effort can be increased by increasing the motor torque but only
up to a certain point.
 Beyond this point, any increase in motor torque does not increase the tractive effort but
merely causes the driving wheels to slip.
 It is seen from the above relation that for increasing Ft, it is not enough to increase the kW
rating of the traction motors alone but the weight on the driving wheels has also to be
increased.
 Adhesion also plays an important role in braking.
 If braking effort exceeds the adhesive weight of the vehicle, skidding takes place.

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 General Features of Traction Motor

 Electric Features
 - High starting torque
 - Series Speed - Torque characteristic
 - Simple speed control
 - Possibility of dynamic/ regenerative braking
 - Good commutation under rapid fluctuations of supply voltage.
 Mechanical Features
 - Robustness and 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 D.C. series for D.C. systems and A.C. series
for A.C. systems. While using A.C., three phase motors are used. With the advent of Power
Electronics it is very easy to convert single phase A.C. supply drawn from pantograph to three
phase A.C.
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 General Features of Traction Motor (Contd..)

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 General Features of Traction Motor (Contd..)

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 General Features of Traction Motor (Contd..)

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 Speed Control of Traction Motors

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 Speed Control of Traction Motors (Contd..)

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 Speed Control of Traction Motors (Contd..)

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 Series- Parallel Control

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 Series - Parallel Control (Contd..)

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 Series - Parallel Control (Contd..)

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 AC SERIES MOTOR

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 AC SERIES MOTOR (Contd..)

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 AC SERIES MOTOR (Contd..)

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 AC SERIES MOTOR (Contd..)

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 Braking in Traction
 Braking in Traction
 Both electrical and mechanical braking is used.
 Mechanical braking provides holding torque.
 Electric Braking reduces wear on mechanical brakes, provides higher retardation, thus
bringing a vehicle quickly to rest.
 Regenerative type of braking is one of the electrical braking used in traction is discussed
below.

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 Regenerative Braking with D.C. Motors
 In order to achieve the regenerative braking, it is essential that (i) the voltage generated by the
machine should exceed the supply voltage and (ii) the voltage should be kept at this value,
irrespective of machine speed.
 Fig. 43.34 (a) shows the case of 4 series motors connected in parallel during normal running
i.e. motoring.
 One method of connection during regenerative barking, is to arrange the machines as shunt
machines, with series fields of 3 machines connected across the supply in series with suitable
resistance.
 One of the field winding is still kept in series across the 4 parallel armatures as shown in
figure 43.34 (b).

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 Regenerative Braking with D.C. Motors (Contd..)
 The machine acts as a compound generator. (with slight differential compounding)
 Such an arrangement is quiet stable; any change in line voltage produces a change in
excitation which produces corresponding change in e.m.f. of motors, so that inherent
compensation is provided
 e.g. let the line voltage tends to increase beyond the e.m.f. of generators. The increased
voltage across the shunt circuit increases the excitation thereby increasing the generated
voltage.
 Vice-versa is also true.
 The arrangement is therefore self compensating.
 D.C. series motor can’t be used for regenerative braking without modification for obvious
reasons.
 During regeneration current through armature reverses; and excitation has to be maintained.
 Hence field connection must be reversed.
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 Regenerative Braking with D.C. Motors (Contd..)

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 Regenerative Braking with D.C. Motors (Contd..)

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 LINEAR INDUCTION MOTOR

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 LINEAR INDUCTION MOTOR (Contd..)

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 LINEAR INDUCTION MOTOR (Contd..)

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 LINEAR INDUCTION MOTOR (Contd..)

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 LINEAR INDUCTION MOTOR (Contd..)

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 LINEAR INDUCTION MOTOR (Contd..)

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 Factors Affecting Specific Energy Consumption

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 Factors Affecting Specific Energy Consumption (Contd..)

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 Factors Affecting Specific Energy Consumption At a given Schedule

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 Factors Affecting Specific Energy Consumption At a given Schedule (Contd..)

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 Thank You

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