Q4Utilization Of Electrical Power

Requirements of an Ideal Traction System

• High starting tractive effort per ampere of motor current drawn, so that heavy trains can be accelerated rapidly from rest without drawing excessively large starting currents from the supply.
• Motors must be capable of withstanding large, frequent overloads during acceleration, and must have inherently overload-safe characteristics, since traction duty involves frequent, rapid, and large variations in torque demand.
• Speed control must be simple, smooth, and efficient over a wide range, to allow the vehicle to be run economically at both low speeds (starting, negotiating curves and gradients) and higher cruising speeds.
• The traction equipment must be capable of regenerative braking, returning kinetic energy to the supply system during deceleration and descending gradients, both for energy economy and for reducing mechanical brake wear.
• Equipment must be light in weight per unit power output (high power-to-weight ratio), to minimize the dead weight that must itself be accelerated and to maximize the payload-carrying capacity of the vehicle for a given total permissible axle loading.
• High overall efficiency, reliability, and low maintenance requirements are essential given the continuous, arduous duty cycle and the high cost of traction system downtime on a busy transportation network.
• The system should not cause interference with neighbouring communication and signalling circuits, and should have minimal adverse environmental impact (noise, emissions).

Classification of Traction Systems

Traction systems are classified into two broad groups: self-contained (non-electric) traction systems, in which the tractive power source is carried on board the vehicle itself, and electric traction systems, in which power is supplied continuously from an external, fixed electrical distribution network via an overhead catenary or third rail. Self-contained systems include steam traction (largely obsolete, using a steam engine directly coupled to the driving wheels), diesel-mechanical traction (a diesel engine coupled through a mechanical gearbox and clutch), and diesel-electric traction (a diesel engine driving a generator, whose electrical output in turn powers electric traction motors, combining the fuel-carrying independence of a diesel prime mover with the superior speed-torque characteristics and control flexibility of electric traction motors). Electric traction systems, in which the diesel or steam prime mover is entirely eliminated and power is drawn continuously from an external supply, are further subdivided into DC traction systems (typically 600V-3000V DC supplied via third rail or overhead catenary) and AC traction systems (typically single-phase AC at 25kV, 50Hz supplied via overhead catenary, with onboard transformers and rectifiers converting to the DC or variable-frequency AC actually required by the traction motors).

Comparison of Traction Systems

Diesel-electric traction remains valuable for non-electrified or partially electrified routes and for freight operations where the capital cost of full-route electrification is not justified by traffic density, offering full route independence at the cost of lower overall thermal efficiency and more limited regenerative braking capability. DC electric traction, historically the earliest form of electric railway traction, benefits from the direct compatibility of DC supply with simple, robust DC series traction motors having ideal traction speed-torque characteristics, but the relatively low DC supply voltage necessitates heavy conductors and closely spaced substations to limit voltage drop and I^2R losses over the route. AC electric traction at 25kV allows a much higher transmission voltage (and correspondingly lower current for the same power), permitting lighter overhead conductors, more widely spaced substations, and lower distribution losses, with an on-board step-down transformer reducing the high catenary voltage to the level required by the traction motors or modern power-electronic converters, making AC electrification the preferred choice for new main-line electrification projects worldwide, particularly for longer routes carrying dense high-power traffic.

Practical selection between these traction system options for a given railway route depends on traffic density (which determines whether the capital cost of electrification can be economically justified), route length and terrain (affecting substation spacing economics), and strategic considerations of energy security and long-term operating cost, which is why most heavily trafficked main lines worldwide have been progressively electrified (predominantly using AC systems for new works) while lower-density branch lines and freight-only routes frequently continue to rely on diesel-electric traction.

A further requirement of an ideal traction system, closely related to those listed at the outset, is the ability to negotiate steep gradients and sharp curves without excessive loss of speed or tractive capability, since urban and hilly-terrain railway alignments frequently cannot avoid such features; this requirement particularly favours traction technologies offering high, sustained tractive effort at low speed (electric traction with series-characteristic motors, as discussed further in relation to the DC series motor question elsewhere in this examination) over technologies whose torque-speed characteristic is comparatively flat or which lose significant efficiency at reduced speed and high torque, reinforcing electric traction's general suitability for demanding route profiles in addition to its energy-efficiency and control-flexibility advantages already discussed.

This closes the requested statement of the main requirements for an ideal traction system, the classification of different traction systems, and their comparison.