Write short note on any one: (i) Single phase induction motor (ii) BLDC motor.

(Answering option (ii); a note on (i) is also given briefly.)

Brushless DC (BLDC) motor: a DC-supplied motor in which mechanical commutator and brushes are replaced by electronic commutation.

Construction:

• Stator: laminated steel core with a (usually 3-phase, star-connected) armature winding in slots, like an AC machine stator.
• Rotor: permanent magnets (ferrite or rare-earth NdFeB) mounted on the shaft surface or inside the rotor (inner-rotor or outer-rotor types).
• Position sensors: three Hall-effect sensors (120° apart) or sensorless back-EMF detection, to find rotor position.
• Electronic controller: a 3-phase inverter (6 MOSFETs/IGBTs) fed from the DC supply, switched by a control circuit.

 DC supply --> [Controller + 6-switch inverter] --> stator phases A,B,C
                    ^                                   |
                    |     Hall sensors (rotor position) |
                    +------------- rotor (PM: N S) <----+

Working: the Hall sensors signal the rotor position to the controller. The controller energises two stator phases at a time (six-step / trapezoidal commutation) so that the stator field is always about 90° ahead of the permanent-magnet rotor field. The attraction/repulsion between the stator field and rotor magnets produces torque and the rotor turns. As it moves, the sensors change state and the controller switches the next pair of phases, so the stator field "rotates" in steps and the rotor follows it. Speed is controlled by varying the applied voltage (PWM duty cycle); direction by changing the switching sequence.

Advantages: no brush wear or sparking, high efficiency, high power/weight ratio, low noise, long life, precise speed control. Applications: ceiling fans (BLDC fans), computer fans, hard-disk/CD drives, electric vehicles and e-bikes, drones, washing machines, pumps, robotics.

(i) Single-phase induction motor (brief): stator has a single-phase main winding, rotor is squirrel cage. A single-phase winding produces a pulsating (not rotating) field, which by double-revolving-field theory equals two equal fields rotating in opposite directions, giving zero net starting torque; so the motor is not self-starting. An auxiliary (starting) winding displaced 90° in space, with current phase-shifted by a capacitor or higher resistance, creates a rotating field for starting; a centrifugal switch disconnects it at about 75% speed. Types: split-phase, capacitor-start, capacitor-start capacitor-run, permanent-split capacitor, shaded-pole. Used in fans, pumps, washing machines, refrigerators.