Brushless DC motor (BLDC)

Brushless DC motors are referred to by many aliases: Brushless Permanent Magnet, Permanent Magnet AC Motors, Permanent Magnet Synchronous Motors etc. The confusion arises because a brushless dc motor does not directly operate off a dc voltage source. However, the basic principle of operation is similar to a dc motor.

A brushless dc motor has a rotor with permanent magnets and a stator with windings. It is essentially a dc motor turned inside out. The brushes and commutator have been eliminated and the windings are connected to the control electronics. The control electronics replace the function of the commutator and energize the proper winding.The windings are energized in a pattern which rotates around the stator. The energized stator winding leads the rotor magnet, and switches just as the rotor aligns with the stator.

There are no sparks, which is one advantage of the brushless DC motor. The brushes of a dc motor have several limitations; brush life, brush residue, maximum speed, and electrical noise. BLDC motors are potentially cleaner, faster, more efficient, less noisy and more reliable. However, BLDC motors require electronic control.

Key characteristics of the BLDC Motor:

  • Heat is generated in the stator: Easier to remove and maintain.
  • Rotor has permanent magnets Vs. coils thus lighter less inertia:
    Easier to Start/ StopBrushless DC motor
  • Linear torque/current relationship smooth acceleration or constant torque
  • Higher torque ripple due to lack of information between sectors
  • Low Cost to manufacture
  • Simple, low-cost design for fixed-speed applications
  • Clean, Fast and Efficient
  • Speed proportionate to line frequency (50v/60 Hz)
  • Complex control for variable speed and torque

The Brushless DC motor does not operate directly off a DC voltage source. The Brushless DC motor has a rotor with permanent magnets, a stator with windings and commutation that is performed electronically. Typically three Hall sensors are used to detect the rotor position and commutation is performed based on Hall sensor inputs.

The motor is driven by rectangular or trapezoidal voltage strokes coupled with the given rotor position. The voltage strokes must be properly applied between the phases, so that the angle between the stator flux and the rotor flux is kept close to 90° to get the maximum generated torque. The position sensor required for the commutation can be very simple, since only six pulses per revolution (in a three-phase machine) are required. Typically, the position feedback is comprised using three Hall effect sensors aligned with the back-EMF of the motor. In sensorless control, back EMF zero crossing is used for commutation.



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