--- title: "Traction motors: applications and characteristics" subject: "Utilization of Electrical Energy" chapter: 4 key: "semester_7/utilization_of_electrical_energy:traction-motors-applications-characteristics" source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/traction-motors-applications-characteristics site: BCT Engineering --- # Traction motors: applications and characteristics Utilization of Electrical Energy · Chapter 4: Electric Traction - Asked in: 2071 Shrawan - Video: [Features of Traction Motors |Electric Traction & Control|](https://www.youtube.com/watch?v=7tV1Xvok6C4) — Anish Vora - 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/traction-motors-applications-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-4 ## Question Discuss the applications of different types of motor used in electric traction with their characteristics. ## Short answer DC series motor provides very high starting torque and self-regulating speed, used in dc railways and diesel-electrics. DC compound has a definite no-load speed, used in trolley buses. AC series commutator served older ac lines. Modern traction uses three-phase induction motors with VVVF for high torque and regenerative braking, and PMSM for high efficiency in e-buses. ## Answer A traction motor must give **high starting torque, series-type speed-torque characteristic (speed falls as load rises), simple speed control, easy braking, rugged construction and good load sharing** when several motors work in parallel. The main types used are listed below. #### 1. DC series motor - **Characteristics:** torque $\propto I^2$ before saturation, so starting torque is very high; speed falls sharply with load (self-regulating); power demand on the supply varies roughly as $\sqrt{T}$, so heavy loads do not overload the line; motors share load well in parallel. - **Speed control:** series-parallel control, field weakening, chopper control. - **Applications:** the classic traction motor for dc railways, tramways, metro and suburban trains, and in diesel-electric locomotives. - **Drawback:** commutator and brushes need maintenance; regenerative braking needs change to separate excitation. #### 2. DC compound motor - **Characteristics:** cumulative compound gives high starting torque with a definite no-load speed, so it does not race on light load; regenerative braking is easier than with series motors. - **Applications:** trolley buses and some hilly-route dc vehicles where regeneration is useful. #### 3. AC single-phase series (commutator) motor - **Characteristics:** series-type speed-torque curve similar to dc series motor; works on low-frequency ac (16⅔ or 25 Hz) to limit commutation trouble; speed controlled by a tap-changing transformer. - **Applications:** older single-phase low-frequency ac main-line railways (Europe). #### 4. Three-phase induction motor - **Characteristics:** nearly constant speed on fixed frequency, simple and rugged, automatic regenerative braking above synchronous speed. With **VVVF inverters** it gets high starting torque and smooth speed control. - **Applications:** modern electric locomotives, metro trains, EMUs and electric buses; earlier used on 3-phase hill railways. #### 5. Synchronous and other modern motors - Linear induction motors (some metros, maglev), permanent-magnet synchronous motors (high efficiency in new trains and electric vehicles), and brushless dc motors in e-rickshaws and electric buses. | Motor | Starting torque | Speed control | Typical use | |---|---|---|---| | DC series | Very high | Series-parallel, chopper | Tram, metro, dc rail | | DC compound | High | Rheostat/chopper | Trolley bus | | AC series | High | Tap changer | Old ac main line | | 3-ph induction | High (with VVVF) | VVVF inverter | Modern locos, metro | | PMSM/BLDC | High | Inverter | E-buses, new trains | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which set is needed of a traction motor? A. Constant speed B. Low starting torque C. Flat characteristic D. High starting torque, speed falling with load, easy control and good load sharing Answer: D. A series-type characteristic suits traction. (Key point: Required characteristics) 2. Where is the DC series motor used? A. Only 3-phase hill railways B. E-rickshaws only C. DC railways, trams, metro and suburban trains D. Maglev Answer: C. It is the classic traction motor with commutator maintenance as drawback. (Key point: DC series) 3. What makes the 3-phase induction motor suitable now? A. It has a commutator B. VVVF inverters give high starting torque and smooth control; regenerates above synchronous speed C. It uses tap changer D. Constant speed only Answer: B. Inverter drive removes its fixed-speed limitation. (Key point: Induction motor) ## Related in Utilization of Electrical Energy - [Electric traction; current collection arrangements](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aelectric-traction-current-collection) — Utilization of Electrical Energy, chapter 4 - [Justify: series motor power; shunt motor for traction](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Ajustify-series-motor-power-shunt-traction) — Utilization of Electrical Energy, chapter 4 - [Advantages of electric traction; trains, trolley, tramways](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aadvantages-traction-trains-trolley-tramways) — Utilization of Electrical Energy, chapter 4 - [Electric traction in Nepal; self-contained vehicles](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aelectric-traction-nepal-self-contained-vehicles) — Utilization of Electrical Energy, chapter 4 - [Key components of electric traction system](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Acomponents-electric-traction-system) — Utilization of Electrical Energy, chapter 4 - [DC vs AC electric traction systems](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-vs-ac-traction-systems) — Utilization of Electrical Energy, chapter 4 ## Related in other subjects - [Electric vehicle architecture and traction motors](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aev-architecture-and-motors) — Automotive Technology, chapter 12 - [Suitability of DC motors for applications](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-motor-suitability-applications) — Electrical Machines, chapter 4 - [Stepper motor construction, principle and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Astepper-construction-applications) — Electric Machines II, chapter 3 - [Torque and speed characteristics of DC motors](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ashunt-series-motor-characteristics) — Electrical Machine, chapter 4, asked 2×