Brushless DC Motors

Structurally, its an inside-out brushed DC-motor

• Coils make the stator while the magnet makes the rotor
• This is very good in terms of thermal performance
• A key idea is to do the field-switching electrically rather than mechanically with brushes
• The induced force is generated by Lorentz’s force found using the left hand rule
• The induced magnetic flux is found by the right hand rule
• You essentially commute one channel at a time and drag the other along
• No brushes reduces friction
• Technically AC motors running on DC power

Note

These are technically AC motors running on a DC power

• Stators are very similar to 3 phase AC induction motors but coils do not overlap in windings
• Rotors are synchronized with the stator field rotation by some kind of position sensor (halls effect sensor for example) to know where the rotors position is to tell the electornics which stator coil to energize
• Sensorless operation is possible (RC motors)
• Can provide constant speed for any load torque and the speed can be controlled by a variable frequency drive

2-pole 3 Slot BLDC Operation

• Two windings and energized at a time using transistors to switch power to the stator phases
• Different hall sensors detect the rotors position by sensing the passing of the rotors magnetic field
• By energizing the stator windings in the right sequence with the right polarity, we can pull or push the motor magnets forward creating a rotating magnetic field that the rotor follows
• The difference in different positions is the pair of phases that are energized, the polarity of those phases, and the rotors orientation relative tot he stator
• There are 6 positions

4-pole 6 Slot BLDC Operation

This follows the same principles as the simpler version just with more poles and stator slots

• These windings are connecting in a wye (star) three phase configuration.
• Here, $n=\frac{120f}{\#poles}$ in rpm
• More poles can provide higher torque at lower speed and more stators can help reduce torque ripple

Torque-Speed Characteristics

• The torque-speed characteristics of BLDC are very similar to PMDC