---
title: "Electric braking in AC and DC traction drives"
subject: "Utilization of Electrical Energy"
chapter: 4
key: "semester_7/utilization_of_electrical_energy:electric-braking-traction-drives"
source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/electric-braking-traction-drives
site: BCT Engineering
---

# Electric braking in AC and DC traction drives

Utilization of Electrical Energy · Chapter 4: Electric Traction

- Asked in: 2072 Kartik
- Video: [Braking of Electric Motors in Traction System](https://www.youtube.com/watch?v=_VpTNb1mrOI) — Nikhil Nigam
- 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/electric-braking-traction-drives
- Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-4

## Question

Explain the common methods of electric braking employed in ac and dc drives for traction.

## Short answer

Electric braking methods are plugging (reverse connections, fast but wasteful), rheostatic/dynamic braking (motor as generator feeding a resistor; dc series self-excites, induction uses dc injection), and regenerative braking (energy returned to supply; needs separate excitation for dc series or VVVF for induction, most efficient).

## Answer

**Electric braking** stops or slows a traction motor by making it act as a generator, so the kinetic energy of the train is turned into electrical energy that is either wasted in resistors or returned to the supply. It reduces wear of brake shoes and gives smooth, controllable braking; mechanical brakes are still needed to hold the train at rest.

#### 1. Plugging (reverse current braking)
- The connections of the armature (dc motor) or two supply phases (induction motor) are reversed, so the motor produces torque opposite to motion.
- A resistor is inserted to limit the very large current.
- Braking is quick but wasteful: energy from both the supply and the train is lost as heat. Used only rarely in traction (for example, emergency stopping).

#### 2. Rheostatic (dynamic) braking
- The motor is disconnected from the supply and its armature is connected across a braking resistor.
- **DC series motor:** the field connections are reversed relative to the armature so the machine self-excites as a generator; two motors may be cross-connected (each excites the other's field) to share load.
- **Induction motor:** the stator is disconnected from ac and fed with dc (dc injection), creating a stationary field; the rotor currents produce braking torque and energy is lost in the rotor resistance.
- Simple and does not depend on the line, but energy is wasted as heat.

#### 3. Regenerative braking
- The motor runs as a generator and returns energy to the supply line. This happens when the back emf exceeds the supply voltage (e.g. going downhill).
- **DC series motors** cannot regenerate directly because their field becomes weak, so they are switched to separate (shunt) excitation during braking. With chopper drives, regeneration is done by the chopper.
- **Induction motor:** regeneration happens naturally when the rotor runs above synchronous speed; with VVVF inverter drives the frequency is lowered so the motor runs super-synchronously.
- Most efficient (about 20–40% energy saving in hilly or frequent-stop routes), but needs a receptive supply or other trains to absorb the energy.

```text
 Supply ===+=== line
           |
       [Motor as generator] --> energy back to line
           |                    (regenerative)
           +--[ R ]--           (rheostatic: heat)
```

| Method | Energy goes to | Efficiency | Use |
|---|---|---|---|
| Plugging | Lost as heat (+ supply) | Poor | Emergency, rarely |
| Rheostatic | Braking resistor | Medium | Common in dc/ac |
| Regenerative | Back to supply | High | Hilly/frequent stops |

## Check yourself

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

1. What characterises plugging?
   A. Energy returned to line
   B. Motor disconnected and shorted
   C. Connections reversed so torque opposes motion; energy lost as heat, rarely used
   D. Needs no resistor
   Answer: C. Plugging is quick but wasteful, used for emergency stops. (Key point: Plugging)

2. In rheostatic braking, where does the energy go?
   A. Back to the supply
   B. Into the track
   C. Dissipated as heat in a braking resistor
   D. Stored in the stator
   Answer: C. The motor acts as a generator across a resistor and energy is wasted as heat. (Key point: Rheostatic braking)

3. Why must a DC series motor be changed for regeneration?
   A. It has no armature
   B. Its field becomes weak, so it is switched to separate excitation
   C. Voltage is too high
   D. It cannot rotate
   Answer: B. Series field weakens at lower current so separate (shunt) excitation is used. (Key point: Regenerative DC series)

4. When does an induction motor regenerate?
   A. When rotor speed exceeds synchronous speed
   B. Below synchronous speed
   C. At standstill
   D. Only with DC injection
   Answer: A. Super-synchronous running returns energy to the supply. (Key point: Induction regeneration)

## Related in Utilization of Electrical Energy

- [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
- [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
- [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
- [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

## 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
- [Short note: ac equivalent of common source MOSFET amplifier](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Amosfet-cs-ac-equivalent-note) — Electronics Devices and Circuit, chapter 3
- [Shock effects for DC and different AC frequencies](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ashock-effects-dc-ac) — High Voltage Engineering, chapter 2
- [Why AC prevailed over DC in early 20th century](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aac-prevailed-over-dc) — Power System Analysis I, chapter 1, asked 2×