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Brushless motors, or more accurately Brushless DC Permanent Magnet Synchronous motors (since there are other types of motors without brushes) contain permanent magnets and electromagnets. The electromagnets are arranged in groups of three and are powered in sequence in order to create a rotating field that drives the permanent magnets. The electromagnets are located on the non-rotating part of the motor, which is normally in the motor casing for traditional motors. In this case, the permanent magnets are around the motor shaft. On hub motors or outrunners motors, such as those found on electric bikes, scooters and electric planes, the electromagnets are on the fixed center part of the motor and the permanent magnets on the rotating outer part.

As the name implies, Brushless motors differ from traditional DC motors in that they do not use brushes for commutating the electromagnets. Instead, it is up to the motor controller to apply, in sequence, current to each of the 3 motor windings in order to cause the rotor to spin. There are fundamentally two methods of generating the rotating magnetic field in the motor’s winding:

  • Trapezoidal Commutation
  • Sinusoidal Commutation

Within each commutation method is then a method for detecting the actual position of the rotor in order to synchronize the generated rotating field. These are:

  • Hall sensors
  • Encoders (Absolute or relative)
  • Sensorless

See this video from Learning Engineering for a visual illustration on how Brushless Motors are constructed and work in trapezoidal mode with Hall sensor feedback.

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