Q4Utilization Of Electrical Power

Merits and Demerits of DC System Track Electrification

• Merit: DC series traction motors have an inherently ideal traction speed-torque characteristic (high torque at low/zero speed, automatically reducing torque as speed rises) without requiring additional power-electronic conversion equipment on board the vehicle, giving simple, robust, well-proven traction drive equipment.
• Merit: DC systems permit straightforward, well-understood regenerative and rheostatic braking control using simple field/armature reconfiguration of the same series motors used for traction.
• Merit: at the relatively low voltages typically used (600V-3000V), electrical clearance and insulation requirements are less onerous than for high-voltage AC systems, simplifying tunnel, bridge, and structure clearances, particularly valuable for third-rail metro and underground systems.
• Demerit: the relatively low operating voltage means a much larger current must be carried by the conductor (OHE or third rail) for a given power delivered, requiring heavier, more expensive conductors to keep voltage drop and I^2R losses within acceptable limits.
• Demerit: substations must be spaced much more closely along the route than for an equivalent AC system, since voltage drop accumulates rapidly with distance at the heavier currents involved, substantially increasing the number of substations (and hence capital cost) needed per unit route length.
• Demerit: because third-rail conductor rail systems (commonly used with DC electrification) involve an exposed, ground-level live conductor, additional safety measures (conductor rail covers, warning signage, strict trackside access control) are required compared to overhead systems.

Suitability of the DC Series Motor for Traction Duty

In a DC series motor, the field winding is connected in series with the armature, so the same current that flows through the armature (and hence determines developed torque, since torque is proportional to the product of flux and armature current) also produces the field flux; since flux is directly proportional to this same current (up to magnetic saturation), developed torque in the unsaturated region is approximately proportional to the square of the armature current, giving the series motor a very high starting torque per ampere of starting current drawn, precisely the characteristic most valuable for accelerating a heavy train from rest with the lowest possible starting current demand on the supply system.

Correspondingly, since flux is proportional to armature current in the unsaturated region, motor speed (which is inversely proportional to flux for a given back-EMF) is approximately inversely proportional to armature current, and hence inversely related to torque demand - giving the series motor its characteristic property of automatically running at high speed under light load (low torque demand, low current) and automatically reducing speed as load (and hence torque demand and current) increases, exactly the self-regulating, load-adaptive behaviour required of a traction motor that must accelerate a heavy train from standstill (very high torque, low/zero speed) and then run efficiently at speed once the train is up to running velocity with only the (much lower) torque needed to overcome running resistance and any residual gradient.

• High starting torque per ampere: ideal for rapidly accelerating a heavy train from rest with minimum starting current, reducing the peak demand placed on the traction supply system during the frequent starts characteristic of railway operation.
• Automatically falling speed with increasing load: the series motor naturally slows down as load torque increases (such as when climbing a gradient) without requiring active external speed control intervention, an inherently safe, self-protecting characteristic.
• Good overload capacity: series motors can tolerate the large, frequent torque and current overloads inherent in traction duty (rapid acceleration, gradient climbing) better than shunt or separately excited motors of comparable rating.
• Simple, robust speed control: series traction motors permit straightforward speed and tractive effort control via simple methods such as rheostatic (resistance) control and series-parallel motor grouping, well suited to the harsh, high-vibration railway operating environment.
• Suitability for multiple-unit and multiple-motor operation: series motors connected in series-parallel combinations across the traction supply, as used in classical DC traction control schemes, share load naturally and predictably owing to their characteristic current-torque relationship.

These combined properties, very high starting torque per ampere, automatically falling speed with rising torque demand, and robust overload tolerance, together explain why the DC series motor was, for the great majority of the twentieth century, the traction motor of choice for virtually all DC-electrified railway and tramway systems worldwide, and why its fundamental speed-torque behaviour remains the reference characteristic that modern power-electronically controlled induction and permanent-magnet traction drives are still designed to emulate for optimal traction performance.

The series motor's speed control is also conveniently achieved by the classical methods of series-parallel motor grouping combined with rheostatic (resistance) control, as used throughout DC traction practice: at low speed, motors are grouped in series across the supply and starting resistance is progressively cut out, after which the motors are switched to parallel connection across the full supply voltage for higher-speed running, giving two efficient (all-resistance-cut-out) running speed points, as discussed in more detail in relation to the series-parallel starting question elsewhere in this examination; field weakening (reducing the field flux by shunting part of the field winding with a diverter resistance) provides a further, continuously variable means of extending the motor's speed range upward beyond the full-field parallel-connection speed, without requiring any additional armature voltage, again exploiting the direct, simple relationship between field current and developed torque/speed characteristic of the series-wound configuration.

It is also worth contrasting the series-wound traction motor with the shunt-wound and separately-excited configurations used in other applications: a shunt motor's field flux depends on a supply-voltage-derived field current largely independent of armature (load) current, so its speed remains nearly constant across a wide range of torque, precisely the opposite of the strongly load-dependent, self-adjusting speed behaviour that makes the series motor so well suited to traction duty; this comparison underlines why the series connection is deliberately chosen for traction motors specifically because of its speed-torque characteristic, and not merely as an incidental or historical wiring convention.

This closes the requested discussion of the merits and demerits of DC system track electrification, together with the description of the suitability of the DC series motor for its application in electric locomotives for traction duty.