Traction Motors

1. TRACTION MOTORS
• Series and Compound motors are employed in d.c traction systems
• Whereas single phase series and three phase induction motors are
used in a.c systems.
Features of traction motors:
Mechanical features:
 Light in weight and small space requirement
 Robust and should be able to withstand vibrations.
 Totally enclosed especially when mounted beneath the locomotive, to
provide protection against ingress of dirt, dust, water, mud etc.
Electrical Features:
 High starting torque
 Simple speed control
 Possibility of dynamic or regenerative braking
 Capability of withstanding voltage fluctuations
 series speed-Torque characteristic
 Better commutation

2. Motors employed in Traction
1.D.C series motor
Advantages:
 High starting torque
 Simple speed control
 Better commutation up to twice full load
 Simple and robust in construction
 Less susceptible to variations in supply voltage
 Capable of withstanding excessive loads.
Disadvantages:
 It has number of limitations over speed, voltage and current.
 There is a risk of flash-over and the brush gear requires
more maintenance
These motors are more particularly suitable for suburban
and urban services where high rate of acceleration is
essential.

3. A.C Series motor
 Many single phase a.c motors have been
developed for traction purposes but only
compensated series type commutator motor is best
suited for traction.
 Single phase induction motors are not capable of
developing high starting torque hence it is not used.
Advantages:
 Higher efficiency
 Improved commutation
 The weight per KW output is greater for the higher
frequency because of larger dimensions of the
motor.
 Efficient speed control of motor by providing taps on
a transformer which is not possible in d.c series
motor.

4. A.C Series motor...
Disadvantages:
Low power factor at starting and on
overloads due to high inductive nature of the
series field and armature circuits.
Low starting torque due to low p.f at starting
Construction cost is more than that of a d.c
series motor.
These motors are used for main line
services. These are not suitable for urban
and suburban services because of low
starting torque and poor p.f at start.

5. Three phase induction motor
• Though 3-phase IM have the advantages of simple
and robust construction, high voltage operation, less
maintenance etc, they are not used for traction work
due to their flat speed-torque characteristics,
constant speed operation, low starting torque and
complicated overhead feeding system.
• In traction system employing 3Φ induction motors it
become necessary to couple all the driving axles
through a connecting rod so that there is no
possibility of a difference in speeds.
• With the development of thyristorised inverter
circuits, it is now possible to invert the supply and
obtain a variable frequency supply and smooth
speed control can be obtained in 3Φ induction
motors.

6. Traction motor control
Need for traction motor control:
Starting without drawing excessive current
from the supply
Smooth acceleration without causing
sudden shock so as to avoid damage to
the couplings and inconvenience to the
passengers.
Speed control as per requirements.
Providing rheostatic or regenerative
braking.

7. Multiple control
• The coaches which are fitted with traction motors
are called motor coaches.
• The motor coaches are usually employed in city and
suburban service.
• The length of the train depends upon the No. of
motor and trailer coached employed which of
course is determined by the No. of passengers to
be carried.
• The No. of motor coaches depends upon the traffic
requirements.
• Thus, during heavy traffic period 2 or more than 2
motor coaches and a number of trailer coaches may
be required whereas during off-peak periods one
motor coach and one trailer may be good enough.

8. Multiple control….
• Each motor coach may have two or four motors and must
be capable of control from a single point and this control
system is known as multiple unit control.
• The equipments provided in motor coaches are Master
controller, Series Parallel controller, and Multi-core cable for
control circuits, Accelerating relays, Reverser, Starting
resistance and collector gear.
• The controllers and reverser are controlled by a master
controller in the cab of any motor coach.
• The equipments required on the trailer coach are Master
controller (if required), Multi-core cable with couple sockets
for the control circuits.
• The motor coach is also provided with certain devices for
the protection of motors against overloads. These consists
of over current relay with Driver’s control switch, isolating
switch for master controller, switches with fuses for isolating
the control circuit of the Motor controller from the control bus
line.

9. Multiple control….
• With all types of controllers, a Dead Man’s handle
device is provided in order to stop the train
automatically in case the driver fails and/or not in a
position to control the operation.
• This is in the form of a contact attached to the knob
of the controller Handle.
• If the driver is not able to operate the handle
properly,the knob rises which causes the contact to
close and operate the main circuit breaker and
apply the brakes.

10. Braking
• In electric traction, both electrical and mechanical
braking is employed for bringing the vehicle to rest.
• In electric braking, the braking energy is converted
into electrical energy and either dissipated in the
resistances mounted on the vehicle or returned to
the supply system.
• It greatly reduces the wear of the brake shoes,
provides higher retardation, thus bringing a vehicle
quickly to a halt thereby the running time is
shortened to a considerable extent.
• In regenerative braking,a part of braking energy is
returned to the supply system thereby affecting 15
to 20% saving in energy consumption.

11. Basic requirements of a braking systems
• It should be simple, robust, quick and reliable in
action
• Easy to use for driver to operate
• Maintenance should be minimum
• The braking system should be inexhaustible
• In case of emergency braking, safety consideration
is taken into account.
• Kinetic energy of the train must be storable during
braking which could be used subsequently during
acceleration of the train.

12. Advantages of electric braking:
Less maintenance
Smooth braking
No dust is produced
Replacement and adjustment of brake shoes
is eliminated
Heat energy is fed back to the supply.
Disadvantages of Electrical braking:
The rating of braking equipment required is
higher than the rating required for motor
alone.
Additional equipment is required in some
cases
Economical consideration.

13. Braking methods
• There are three methods of applying
electric braking are
(1) Plugging or Reverse current braking
(2) Rheostatic braking
(3) Regenerative braking