Electric Locomotives /Electric Trains

What is an Electric Locomotive?

An “Electric Locomotive” is a train vehicle that move along with rails and push and pull a train attached to it using electric power drawn from overhead cables or a third rail.

Electric Locomotives use the electricity collected from the outside source to power traction motors which turn the wheels.

What isTypes of Electric Locomotives

Electric Locomotives are of three types: which work on
AC or on both (AC/DC – Bi-current).

Electric Locomotives, though high on electrical engineering, work on the single principle of drawing current from external sources and then after sufficiently “modifying” it, feed it to the traction motors.

The process of "modifying" the raw current drawn from outside into "use" power, which would be a process of conversion, re-conversion, smoothing and transformation of current to different values of frequency, voltage, current, etc. is loaded.

AC Electric Locomotive Construction

With this system a bank of many components like transformers, rectifiers, inverters, capacitors, thyristors, compressors and other su paraphernalia, housed in the locomotive body or the “shell”, and there is no central “Engine” or prime mover.
All this has to be done for top performance of the traction motors under different situations and loads. Bi-Current locomotives work according on same principles, only they have more equipment packed inside them to enable them to work under both type of currents.

Power Circuit of AC Electric Locomotive

The electric locomotive draws current from the over head line (OHE) with the help of Pantograph and converts this electrical energy to mechanical energy, in requirement basis through Traction Motors which drive the axles.

To enable the locomotive for this task, it is equipped with genious equipments, which enable loco pilot to control the speed of the train requirement basis by controlling voltage to traction

motors.

In conventional locomotives, 25KV, Single phase, AC supply is collected by a roof-mounted pantograph from the OHE and is stepped down by a transformer inside the locomotive.
Later then supply is t converted to DC supply by a full wave silicon rectifier associated with smoothing filter before feeding to the traction motors.

Speed control is activated by variation of the AC input voltage to the rectifier via an on-load tap changing arrangement on the primary winding of the transformer.

The equipments on electric locomotive, can be classified in three different categories .

Roof equipment,
Inside equipment and
Under Frame equipment

Both the roof and the equipment under the frame have to withstand a lot of dust and are therefore designed to withstand these severe conditions.

1. Roof Equipment:

Pantograph

The High Tension current (25 kV) for supply to locomotive is taken from current collecting device called pantograph. Each loco is given with two similar pantographs on roof.

When the Circuit breaker closed through remote control, OHE supply collected by pantograph is made available to the main transformer in side the locomotive.
It disconnected automatically in case of over current or earth fault in the loco occurs.
Nowadays, air blast circuit breakers are replaced by vacuum circuit breaker (VCB) because of its superior qualities with less maintenance.

2. Inside Equipment:

Voltage regulating equipment:

The high OHE voltage is stepped to low voltage by the main transformer comprising of an Auto transformer with 32 taps and a stepped down transformer with two separate secondary windings.

The low voltage can be controlled through on load tap changer which is like a fan regulator used to control the fan speed.

Silicon Rectifiers

Traction motors are DC motors, alternating current supplied by secondary windings of main transformer is converted to direct current by two silicon rectifiers (RSI), one each feeding to set of three traction motors.

ARNO converter :

ARNO converter converts the single-phase 380 Volt input from transformer auxiliary winding to 3 phase 380 Volt output.
The output of ARNO converter is supplied to various auxiliary motors provided for supplying compressed air, creating a vacuum in train and cooling of electrical equipment like Traction motors,main transformer smoothing reactor, etc.

3. Under frame equipment

Traction Motorslocomotive is added with six DC series type traction motors(TMs).
These TMs are mounted in two under frame bogies coupled with wheels through pinion-gear arrangement.

Smoothing reactor

As output of rectifier is of undulating (Pulsating) nature, it is passed through an inductive choke called smoothing reactor (SL) to reduce the undulation of the current and to make current smoother.

4. Brakes

Loco is provided with following brakes:

Air brake system
Independent brake
Proportionate brake
Dynamic (Rheostatic) brake
Regenerating brake

Working of DC Electric Locomotive

In DC locomotives, the speed of DC traction motors is controlled by connecting all the traction motors in series during starting period.

These starting resistances are gradually cut out from the circuit and the voltage across the traction motors are gradually increased.

This is further changing motor combinations from series to series parallel and finally all the motors are connected in parallel so that loco can control the train at its permissible speed.

Advantages of Three Phase Induction Motors over DC Motors

Nowadays DC motors are largely replaced by Three Phase Induction Motors due to their advantages over DC motors. Some of them are pointed below.

Three Phase Induction Motors are robust and require less maintenance.
Due to absence of a commutator, its peripheral speed have no limit on the speed of the motor. AC traction motors can easily operate at 4000 RPM in contrast to DC motors which normally operate at speeds of 2400 RPM.
The limit imposed due to bar to bar voltage for DC commutator is eliminated in induction motors. It means that whole power flow from the Transformer to the motor is choosed at high operating voltages.
Against a nominal system of 750 volts, 1000 ampere with DC motor, the three-phase motor works at approx 2800 volts, 300 amperes. With the heavy reduction in operating current, power cables & switch gears are much lighter hence losses are reduced.
The power to weight ratio of three-phase traction motor is much higher than the DC motor -1500 kW per axle may be packed with these motors.

Working Of Three-Phase Locomotive

Three%2Bphase%2Blocomotive

25 kV overhead AC supply is stepped down by main transformer and fed to a front end (line) twin 4-quadrant line converter there AC is converted to DC through Pulse Width Modulation( PWM) and achieve unity power factor.

This supply is linked to input side converter through DC link which is a reservoir of energy.

Drive converter (VVVF Converter) converts DC supply into 3 phase wand then fed to 3 phase traction motors.

Gate turn off (GTO), thyristors are used in converter/inverter.
The output of Drive converter (inverter) is Variable Voltage Variable Frequency (VVVF) supply which helps in controlling the starting and running torques of three-phase traction motors as per the traffic requirements