What is an Electric Locomotive?
An “Electric Locomotive” is a railway vehicle that can move along rails and push or pull a train attached to it using electric power drawn from an external source, usually from overhead cables or a third rail.
Electric Locomotives do not have a conventional “engine” in them as we have seen in a diesel locomotive, but use the electricity collected from the outside source to power traction motors which turn the wheels.
Types of Electric Locomotives
Electric Locomotives are of three types: Those which can work on
- DC (Direct Current),
- AC (Alternating Current) or
- on both (AC/DC – Bi-current).
The process of “modifying” the raw current drawn from outside into “usage” power includes a complicated process of conversion, reconversion, smoothening and transformation of the current to varying values of frequency, Voltage, Current etc.
Construction of AC Electric Locomotive
This involves a bank of many components like transformers, rectifiers, inverters, capacitors, thyristors, compressors and other such paraphernalia, housed within the locomotive body or the “shell”, and there is no central “Engine” or prime mover.
- Modern Electric Locomotive Block Diagram
Power Circuit in AC Electric Locomotive
- Roof equipment,
- Inside equipment and
- Under Frame equipment
1. Roof Equipment:
Circuit Breaker (Air blast circuit breaker, Vacuum circuit breaker)
2. Inside Equipment:
Voltage regulating equipment:
ARNO converter :
3. Under frame equipment
In general, the locomotive is provided with six DC series type traction motors(TMs).
These TMs are mounted in two under frame bogies coupled with wheels through pinion-gear arrangement.
- Air brake system
- Independent brake
- Proportionate brake
- Dynamic (Rheostatic) brake
- Regenerating brake
Working of DC Electric Locomotive
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 & require little maintenance.
- Due to the absence of a commutator, its peripheral speed puts 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. This means that the whole power flow from the Transformer to the motor is chosen at higher operating voltages.
- Against a nominal system of 750 volts, 1000 ampere with DC motor, the three-phase motor works at around 2800 volts, 300 amperes. With the heavy reduction in operating current, power cables & switch gears are much lighter thereby losses are reduced.
- The power to weight ratio of three-phase traction motor is much higher than the DC motor -1500 kW per axle can be packed with these motors.