---
title: "BLDC motor construction and applications"
subject: "Electric Machines II"
chapter: 3
key: "semester_5/electric_machines_ii:bldc-construction-applications"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/bldc-construction-applications
site: BCT Engineering
---

# BLDC motor construction and applications

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2082 Bhadra (new course), 3 marks
- Video: [bldc motor | brushless dc motor | bldc motor working | bldc motor used in electric vehicle | bldc EV](https://www.youtube.com/watch?v=lcvDixHYA10) — Mind of Engineer 
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/bldc-construction-applications
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

What are the constructional features of brushless DC motor? How is it different from induction motor? State two applications of BLDC motor.

## Short answer

BLDC motor has a3-phase stator winding, permanent-magnet rotor, and position sensors (Hall) feeding an electronic inverter that produces trapezoidal back EMF for brushless commutation. Unlike an induction motor, the rotor has permanent magnets (no slip), needs an electronic drive, has no rotor copper loss, and offers higher efficiency. Applications include computer fans, hard drives, electric vehicles, and drones.

## Answer

A **brushless DC (BLDC) motor** is a permanent-magnet synchronous motor fed through an electronic inverter that switches the stator phases according to rotor position, replacing the mechanical commutator and brushes of a DC motor.

#### Constructional features
- **Stator:** laminated core with a 3-phase (usually star) winding, like an AC motor.
- **Rotor:** permanent magnets (ferrite or NdFeB) on the surface or inside the rotor; inner-rotor or outer-rotor types.
- **Position sensors:** Hall-effect sensors (or sensorless back-emf detection).
- **Electronic commutator:** 3-phase inverter controlled from the sensor signals; back emf is trapezoidal.

```text
 DC --> [Inverter] --> stator A,B,C --> PM rotor
            ^                             |
            +------ Hall sensors <--------+
```

#### Difference from induction motor

| Point | BLDC motor | Induction motor |
|---|---|---|
| Rotor | Permanent magnets | Cage/wound, induced current |
| Speed | Synchronous, no slip | Below synchronous (slip) |
| Supply | DC via electronic drive | Direct AC supply |
| Rotor loss | Almost none | Rotor copper loss |
| Efficiency | Higher | Lower |

#### Applications
Computer fans and hard disk drives; electric vehicles, e-bikes and drones (also inverter fans, washing machines).

## Check yourself

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

1. What replaces the mechanical commutator and brushes in a BLDC motor?
   A. Slip rings
   B. An electronic inverter driven by rotor-position signals
   C. A shading ring
   D. A centrifugal switch
   Answer: B. Rotor-position sensors (e.g. Hall sensors) control an inverter that switches the stator phases. (Key point: Commutation)

2. How does the BLDC rotor differ from an induction motor rotor?
   A. It has permanent magnets instead of induced-current cage windings
   B. It has a wound field winding with slip rings
   C. It has no iron
   D. It is the same
   Answer: A. BLDC uses permanent magnets, so almost no rotor copper loss, while an induction rotor relies on induced current. (Key point: Rotor of BLDC vs induction)

3. Which statement is correct?
   A. BLDC runs below synchronous speed with slip
   B. BLDC runs synchronously with no slip and has higher efficiency
   C. Induction motor needs a DC electronic drive
   D. BLDC has higher rotor loss
   Answer: B. BLDC is synchronous with no slip and minimal rotor loss, so its efficiency is higher. (Key point: Speed and efficiency)

4. Which pair is a typical BLDC application?
   A. Computer fans and electric vehicles
   B. Railway traction with series motor only
   C. Large cement kilns
   D. Textile mills with Schrage drives
   Answer: A. BLDC motors are used in computer fans, hard drives, EVs, e-bikes and drones. (Key point: Applications)

## Related in Electric Machines II

- [Synchronous motor features, applications and excitation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-features-applications-excitation) — Electric Machines II, chapter 2
- [BLDC motor torque equation and characteristics](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Abldc-torque-equation) — Electric Machines II, chapter 3
- [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
- [Schrage motor construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aschrage-motor-construction) — Electric Machines II, chapter 3
- [Synchronous motor features and lagging/leading operation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-features-lagging-leading) — Electric Machines II, chapter 2
- [Short note: DC servo motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashort-note-dc-servo-motor) — Electric Machines II, chapter 3

## Related in other subjects

- [Construction and working of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-construction-working) — Electrical Machines I, chapter 5
- [DC servo motor and applications](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-servo-motor-principle-applications) — Electrical Machine, chapter 7, asked 2×
- [Constructional features of belted cable](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Abelted-cable-construction) — High Voltage Engineering, chapter 1
- [Induction motor equivalent circuit from two-wattmeter tests](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-400v-noload-blocked-two-wattmeter) — Electrical Machines I, chapter 5