2.  Introduction
 Requirements of a traction system
 Control of DC motors
 Electronic Speed Control Methods for DC Motors
 Pulse Width Modulation
 Braking operation in DC motors
 Recent Trends in Electric Traction
 Conclusion
 References

3.  Traction motor refers to an electric motor
providing the primary rotational torque of a
machine, usually for conversion into linear
motion (traction).
 Traction motors are used in electrically powered
rail vehicles such as electric multiple units and
electric locomotives

4.  High starting pulling effort in order to have rapid
acceleration.
 Equipment capable of withstanding large temporary
overloads for short periods.
 The locomotive should be self contained and able to run
on any route.
 Capability of withstanding voltage fluctuations and
temporary interruption of supply.
 Parallel running usually more than one motor (two or four
motors ) should be possible.

6.  D.C series motors.
 Single phase A.C series motors
 Three-phase Induction motors

7.  D.C series motor develops high torque at low speeds
& low torque at high speeds. This is the exactly the
requirement of traction units.
 In case of dc series motor , up to the point of
magnetic saturation, torque developed in proportional
to the square of the current. Therefore, dc series
motor requires comparatively less increased power
input with the increase in load torque. Thus the series
motor are capable of withstanding excessive loads.
 Speed of dc series motor can be controlled by various
methods.

8. 1. The construction cost of an ac series motor is much
more easier than of a dc series motor.
2. The starting torque of a.c single phase motor is
lower than that of dc series motor due to poor power
factor at the start.
3. The speed of an ac series motor may be controlled
efficiently by taps on a transformer. Which is not
possible in case of a dc series motor.

9.  It has simple & robust construction.
 Trouble free operation.
 Less maintenance.
 High voltage operation consequently requiring
reduced amount of current.
 Automatic regeneration are the main advantages of 3-
ph. Induction motor for traction.
 But due to their flat speed- torque
characteristics, constant speed operation, developing
low starting torque, drawing high starting
current, complicated speed control systems they are
not suitable for electric traction work.

10.  Short form of SEParate EXcitement of traction
motors where the armature and field coils of
an electric motor are fed with independently
controlled current. This has been made much
more useful since the introduction of
thyristor control where motor control can be
much more precise. SEPEX control also
allows a degree of automatic wheel slip
control during acceleration

11.  The purpose of a motor speed controller is to
take a signal representing the required speed,
and to drive a motor at that speed.
• Motor speed can be controlled by
controlling-
- Armature voltage (Va)
- Armature current (Ia)

12.  It is obvious that the speed can be controlled by
varying
 flux/pole, (Flux Control)
 resistance Ra of armature circuit (Rheostatic Control)
 applied voltage V (Voltage Control)

14.  The above methods have some demerits
A large amount of power is wasted in the
controller resistance. Hence, efficiency is
decreased.
It needs expensive arrangement for dissipation
of heat produced in the controller resistance.
It gives speeds below the normal speed.

15.  Compared to the electric and electromechanical
systems of speed control, the electronic methods
have
higher accuracy
greater reliability
quick response
higher efficiency

16.  The main principle is control of power by varying the duty cycle.
 Here the conduction time to the load is controlled.
 Let
 for a time t1, the input voltage appears across the load ie
ON state.
for t2 time the voltage across the load is zero.
The average voltage at output is given by
Va = 1/T vodt = t1/T Vs = ft1 Vs = kVs
where T is the total time period =t1+t2
k = t1/T is the duty cycle

17. • Pulse width modulation (PWM) is a method for binary signals
generation, which has 2 signal periods (high and low).
• The width (W) of each pulse varies between 0 and the period
(T).
• The duty cycle (D) of a signal is the ratio of pulse width to
period.
• D=(t1+t2) / T

18. • The motor is on for
most of the time and
only off a short while,
so the speed is near
maximum
• The switch is on 50%
and off 50%.
• The motor will only
rotate slowly.

19.  Commonly two type braking system used in traction
motors that are
› Dynamic braking
› Regenerative braking

20.  The motors become generators and feed the resulting
current into an on-board resistance.
 When the driver calls for brake, the power circuit
connections to the motors are changed from their power
configuration to a brake configuration and the resistors
inserted into the motor circuit. As the motor generated
energy is dispersed in the resistors and the train speed
slows, the resistors are switched out in steps, just as they are
during acceleration.

21.  The motors become generators and feed the resulting
current back into the supply system
 a train could use its motors to act as generators and that
this would provide some braking effect if a suitable way
could be found to dispose of the energy.
 Trains were designed therefore, which could return
current, generated during braking, to the supply system
for use by other trains.

24.  PWM duty cycle control techniques enable greater
efficiency of the DC motor .
 PWM switching control methods improve speed control
and reduce the power losses in the system.
 The pulses reach the full supply voltage and will produce
more torque in a motor by being able to overcome the
internal motor resistances more easily.

25.  The main Disadvantages of PWM circuits are the
added complexity and the possibility of generating
radio frequency interference .
 It can give speed below the full speed, not above.
 It cannot be used for fast controlling of speed.

29. 1. Power is purchased from supply authorities who are responsible for the
operation & maintenance of 132/110 kv transmission lines
2. Supply authorities will give only voltage supply of 132/110 kv at the
substation
3. Modern microprocessor technology and the availability of efficient and
compact power components have changed that picture. In 3-phase AC
locos, the input (single-phase AC) from the OHE is rectified and then 3-
phase AC is generated from it, whose voltage, phase, and frequency can
be manipulated widely, without regard to the voltage, phase, frequency
of the input power from the OHE

32.  Multilevel converters:
The main advantage of this kind of topology is that it can
generate almost perfect current or voltage waveforms, because
it is modulated by amplitude instead of pulse-width. That means
that the pulsating torque generated by harmonics can be
eliminated, and power losses into the machine due to harmonic
currents can also be eliminated. Another advantage of this kind
of drive is that the switching frequency and power rating of the
semiconductors is reduced considerably