BLDC motors have a three-phase stator winding and a rotor with surface-mounted permanent magnets. A BLDC motor does not have a commutator and is more reliable than a DC motor. The digital control and power electronics replace the function of the commutator and energize the proper winding. They are used in home appliances (such as refrigerators, washing machines and dishwashers), pumps, fans and other devices that require high reliability and efficiency.
In the BLDC motor, the rotor position must be known to energize the phase pair and control the phase voltage.
If sensors are used to detect rotor position, then sensed information must be transferred to a control unit. This requires additional connections to the motor, which may not be acceptable in some applications. Also, the additional cost of the position sensors and the wiring may be unacceptable. The physical connection problem could be solved by incorporating the driver in the motor body, however, a significant number of applications do require a sensorless solution due to their low-cost nature.
Most BLDC sensorless techniques are based upon extracting position information from the back EMF voltage of the stator windings while the motor is spinning. Those techniques could be used from 5 percent of nominal speed, when back EMF is measurable. BLDC back EMF sensorless techniques can be used without complex control algorithms, due to back EMF voltage sensing in unexcited motor phase.
ADVANTAGES OF BRUSH-LESS DC MOTOR:
• Heat generated in stator is easy to remove.
• High torque per frame size.
• Reliability due to absence of brushes and commutator.
• Highest efficiency.
• Good high-speed performance.
• Precise speed monitoring and regulation possible
DRAWBACKS OF BRUSH-LESS DC MOTOR:
• Rotor position sensing required for commutation.
• Torque ripple.
• Position sensor or sensor-less technique is required for motor operation.
• Difficult to startup the motor for variable load using sensorless technique.