ELECTRIC TRACTION

PREM KUMAR.EReg no-1611110018M-TECH(PED)

Content : 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

Introduction

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

Requirements of an Ideal Traction System

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.

COMPONENTS OF AN A.C LOCOMOTIVE

Motors used for the traction purpose

D.C series motors.

Single phase A.C series motors

Three-phase Induction motors

Suitability of D.C series motor for Traction

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.

Suitability of a.c series motor for Traction

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.

Suitability of three phase induction motor for Traction

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.

SEPEX

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

Speed control of dc motor :

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)

Cont..

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)

Speed control by resistors adjustments:

Disadvantage:

The above methods have some demeritsA 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.

Electronic Speed Control Methods for DC Motors :

Compared to the electric and electromechanical systems of speed control, the electronic methods have

higher accuracy

greater reliability

quick response

higher efficiency

Principle : 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

Pulse Width Modulation :• 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

Pictures of the Waveform :

• 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.

Braking in traction motors

Commonly two type braking system used in traction

motors that are

› Dynamic braking

› Regenerative braking

Dynamic Braking 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.

Regenerative Braking

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.

REGENERATIVE BRAKING

Regenerative bridge:

Advantage :

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.

Disadvantages :

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.

THYRISTOR CONTROL

DC Choppers

AC Locomotives with DC Drives

Power supplies used

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

substation

Voltage and current measurements:

RECENT TRENDS IN ELECTRICAL TRACTION

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

Multilevel inverter:

Multilevel inverter:

Any query?