---
title: "Merits and demerits of DC track electrification"
subject: "Utilization of Electrical Energy"
chapter: 4
key: "semester_7/utilization_of_electrical_energy:dc-track-electrification-merits"
source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/dc-track-electrification-merits
site: BCT Engineering
---

# Merits and demerits of DC track electrification

Utilization of Electrical Energy · Chapter 4: Electric Traction

- Asked in: 2073 Chaitra, 6 marks
- Video: [system of track electrification | supply in electric locomotive | dc system | composite system | ac](https://www.youtube.com/watch?v=UXtWUSLnaA0) — Mind of Engineer 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/dc-track-electrification-merits
- Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-4

## Question

What are the merits and demerits of d.c. system of track electrification?

## Answer

In the **dc system of track electrification**, the contact line or third rail carries dc at 600–750 V (tramways, metros) or 1500–3000 V (suburban and main lines). Substations take ac from the grid and convert it to dc with rectifiers; dc series motors (or chopper-controlled motors) drive the vehicle.

#### Merits
1. **Ideal motor characteristic**: the dc series motor gives high starting torque, speed that falls with load, power $\propto \sqrt{T}$, and good load sharing.
2. **Simple, light and cheap locomotive**: no heavy transformer or rectifier on board.
3. **Lower energy per train-km** in frequent-stop urban service, due to good acceleration.
4. **Small clearance and insulation** at low voltage: suits tunnels, underground metros and the third-rail system.
5. **No interference** with telecommunication and signalling lines (no alternating magnetic field).
6. **No single-phase unbalance**: the substation rectifiers draw a balanced three-phase load from the grid.
7. Speed control is simple (series-parallel, chopper), and regenerative braking is possible with chopper control or by separately exciting the fields during braking.

#### Demerits
1. **High line current** at low voltage: heavy, costly overhead conductor or third rail and supports.
2. **Large voltage drop and $I^2R$ loss**; substations must be close together (3–5 km at 750 V, 10–20 km at 3 kV).
3. **Costly substations**: every substation needs rectifier equipment, and many substations are needed, so the capital cost is high.
4. **Stray currents** returning through earth cause electrolytic corrosion of underground pipes and cables.
5. Energy is wasted in starting resistances (with resistance control).
6. Regenerated energy can be used only if another train is drawing power nearby (rectifier substations cannot feed back to the grid without inverters).
7. A third rail at ground level is a safety hazard.

| Aspect | Merit / demerit |
|---|---|
| Motor | DC series motor: best traction characteristic |
| Vehicle cost | Low |
| Line and substation cost | High |
| Losses | High (low voltage, high current) |
| Interference | None with telecom; stray-current corrosion |
| Best use | Metros, trams, suburban service |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the main motor-related merit of the DC system?
   A. Needs no commutator
   B. Constant speed
   C. Power proportional to T
   D. DC series motor gives high starting torque, falling speed with load, power proportional to sqrt(T) and good load sharing
   Answer: D. These are the ideal traction characteristics of the dc series motor. (Key point: Motor merit)

2. Why must DC substations be close together and costly?
   A. Voltage is 25 kV
   B. Rectifiers are not needed
   C. Currents are very small
   D. Low voltage means high current, large drop and I^2R loss, and rectifiers are needed at each substation
   Answer: D. Spacing is 3-5 km at 750 V, with rectifier equipment at every substation. (Key point: Substation demerit)

3. How does DC electrification load the grid?
   A. It unbalances the grid
   B. It injects no harmonics at all
   C. Rectifier substations draw balanced three-phase load, so no single-phase unbalance
   D. It draws a single phase
   Answer: C. Unlike single-phase AC, DC rectifier substations draw balanced three-phase power. (Key point: Grid load)

## Related in Utilization of Electrical Energy

- [Types of electric drives with merits and demerits](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aelectric-drive-types-merits) — Utilization of Electrical Energy, chapter 2
- [Forms of tariff with merits and demerits](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Atariff-forms-merits-demerits) — Utilization of Electrical Energy, chapter 6
- [Comparing systems of traction electrification](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Atraction-electrification-systems-comparison) — Utilization of Electrical Energy, chapter 4

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