Square Root of Negative One

Brushless DC electric motor

Posted in EnS by cheng on December 18, 2009

A brushless DC motor (BLDC) is a synchronous electric motor which is powered by direct-current electricity (DC) and which has an electronically controlled commutation system, instead of a mechanical commutation system based on brushes. In such motors, current and torque, voltage and rpm are linearly related.

In brushed DC motor, magnets are still. Brushes touch with different contacts on rotor to keep current in the same direction. In a BLDC motor, a controller change the direction of current to keep magnets rotate in the same direciton.

BLDC (compared to DC motors)

  • higher efficiency and reliability (absence of electrical and friction losses due to brushes)
  • elimination of ionizing sparks
  • reduced noise
  • more power
  • overall reduction of electromagnetic interference (EMI)
  • no need for cooling air through internal circuit, thus less dust and foreign matter
  • longer lifetime


Brushless motors are given a Kv rating, which is RPM per volt, that lets you determine how fast that motor will rotate with a given voltage supplied to it. A 980Kv motor powered by an 11.1V battery would spin at 980 x 11.1 = 10878 RPM with no load.

The current rating specifies the maximum continuous and/or burst current that the motor is able to handle. When selecting a battery and speed control, choose ones with continuous current ratings equal to or greater that that of the motor.

Inrunner Vs Outrunner:
An inrunner motor has stationary coils which surround the rotating magnet at the center. An outrunner motor has stationary coils at the center, and the rotating magnet on the outside. Outrunner motors generally have lower Kv ratings, meaning they run at a lower speed, but with more torque, which would allow you to direct drive larger props without needing a gearbox. Most RC cars and boats would require an inrunner brushless motor.



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