In one line
A brushless DC (BLDC) motor has permanent magnets on the rotor and windings on the stator, and an electronic controller switches the stator currents in sequence using rotor-position feedback, doing the commutator's job without brushes.
Explained simply
In an ordinary DC motor, carbon brushes rub on a commutator to switch current in the spinning coils, which causes wear, sparks and noise. A BLDC motor turns this inside out: the magnets spin and the coils stay still. Since the coils do not move, they can be switched by transistors instead of brushes, guided by sensors that tell the controller where the rotor is.
Technically, the stator carries a (usually three-phase) winding; the rotor carries permanent magnets (often rare-earth such as neodymium). Hall-effect sensors (or sensorless back-EMF detection) give rotor position; an inverter of six electronic switches energises two phases at a time so the stator field always stays ahead of the rotor magnets.
How it works
- Hall sensors detect which rotor pole is near which winding.
- The controller turns on the correct pair of switches so the energised stator poles attract the rotor magnets forward.
- As the rotor moves, sensor signals change and the controller switches to the next pair: six steps per electrical cycle (trapezoidal commutation).
- Speed is controlled by varying the average voltage with PWM; direction by reversing the switching sequence.
- Construction types: inrunner (rotor inside stator, e.g. power tools) and outrunner (rotor outside, e.g. drones, ceiling fans).
| Brushed DC | BLDC |
|---|---|
| mechanical commutator and brushes | electronic commutation |
| brush wear, sparking, noise | long life, no sparking, quiet |
| simple to drive | needs controller |
| heat produced in rotor | heat in stator, easier to cool |
Quick example
BLDC ceiling fans in India are rated around 28–35 W against roughly 70–75 W for conventional induction fans, because there are no rotor copper losses and the controller runs the motor efficiently at each speed setting.
Common mistakes
- Saying BLDC motors run directly from a DC battery without electronics; they need a controller.
- Saying the windings are on the rotor.
- Calling it an induction motor; the rotor has permanent magnets and runs in synchronism with the switched field.
How MU asks it
Numericals are not expected in this module (as per the MU syllabus), so questions are theory. Typical: "Explain the construction and working of a BLDC motor" (5 marks), "Compare BLDC and conventional DC motors", or "State applications of BLDC motors". For full marks draw the stator with three windings, the PM rotor, Hall sensors and the inverter block, explain electronic commutation step by step, and give a comparison table and applications.