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

Traction motor refers to an electric motorproviding the primary rotational torque of amachine, usually for conversion into linearmotion (traction).

Traction motors are used in electrically poweredrail vehicles such as electric multiple units andelectric locomotives

High starting pulling effort in order to have rapidacceleration.

Equipment capable of withstanding large temporaryoverloads for short periods.

The locomotive should be self contained and able to runon any route.

Capability of withstanding voltage fluctuations andtemporary interruption of supply.

Parallel running usually more than one motor (two or fourmotors ) should be possible.

D.C series motors.

Single phase A.C series motors

Three-phase Induction motors

D.C series motor develops high torque at low speeds& low torque at high speeds. This is the exactly therequirement of traction units.

In case of dc series motor , up to the point ofmagnetic saturation, torque developed in proportionalto the square of the current. Therefore, dc seriesmotor requires comparatively less increased powerinput with the increase in load torque. Thus the seriesmotor are capable of withstanding excessive loads.

Speed of dc series motor can be controlled by variousmethods.

1. The construction cost of an ac series motor is muchmore easier than of a dc series motor.

2. The starting torque of a.c single phase motor islower than that of dc series motor due to poor powerfactor at the start.

3. The speed of an ac series motor may be controlledefficiently by taps on a transformer. Which is notpossible in case of a dc series motor.

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 startingtorque, drawing high starting current, complicatedspeed control systems they are not suitable for electrictraction work.

Short form of SEParate EXcitement of tractionmotors where the armature and field coils ofan electric motor are fed with independentlycontrolled current. This has been made muchmore useful since the introduction ofthyristor control where motor control can bemuch more precise. SEPEX control alsoallows a degree of automatic wheel slipcontrol during acceleration

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)

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)

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.

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

higher accuracy

greater reliability

quick response

higher efficiency

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 (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

• The motor is on formost of the time andonly off a short while,so the speed is nearmaximum

• The switch is on 50%and off 50%.

• The motor will onlyrotate slowly.

Commonly two type braking system used in traction

motors that are

› Dynamic braking

› Regenerative 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.

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.

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.

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.

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

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