Explain the construction and working of a brushless DC (BLDC) motor. List its advantages and applications.

Answer: A BLDC motor has permanent magnets on the rotor and a three-phase winding on the stator; Hall sensors detect the rotor position and an electronic controller switches the stator phases so that the stator field always pulls the rotor along. This replaces the brushes and commutator.

Construction:

• Stator: laminated steel core carrying a (usually three-phase, star-connected) winding.
• Rotor: permanent magnets (ferrite or rare-earth NdFeB) with alternate N and S poles; can be inner rotor or outer rotor (as in ceiling fans).
• Position sensors: typically three Hall-effect sensors placed 120° (electrical) apart sense the rotor pole position.
• Electronic driver: a three-phase inverter (six MOSFETs/IGBTs) controlled by a microcontroller, fed from a DC supply.

Working: the controller reads the Hall sensors and energises two of the three stator phases at a time. The energised phases create a magnetic field that attracts the nearest rotor pole. As the rotor moves, the sensor outputs change and the controller switches the next pair of phases (six steps per electrical cycle). The stator field therefore keeps stepping ahead of the rotor, producing continuous torque. Speed is controlled by varying the average voltage (PWM), and direction by reversing the switching sequence. In a brushed DC motor this switching is done mechanically by the commutator; in a BLDC motor it is done electronically.

Advantages: no brushes so no sparking, wear or brush maintenance; higher efficiency; high torque-to-weight ratio; quiet, long life; precise speed control.

Disadvantages: needs an electronic controller; higher initial cost.

Applications: energy-efficient ceiling fans, computer cooling fans and hard drives, electric vehicles and e-bikes, drones, washing machines, medical equipment.