Principle of the brushless DC motor

The brushless DC motor is the combination of a permanent excited synchronous motor and a frequency inverter. The inverter has to replace the commutator of a conventional DC motor. Fig. 2 and Fig. 3 show how a brushless DC motor can be derived from a mechanically commutated DC motor with three armature slots. Its armature winding corresponds to a three phase winding in delta connection. The commutator acts like a three phase frequency converter. Stator (excitation) and rotor (armature) change places.

Figure 1: Principle of a permanent magnet excited DC motor: construction

Figure 2: Principle of a permanent magnet excited DC motor: wiring

Figure 3: Principle of a permanent magnet excited DC motor - equivalent circuit

The commutation of a brushless DC motor depends on the position of the rotor. The angle between the magneto-motive forces of stator and rotor is fixed to $90^\circ$ (el.), so the motor produces maximum torque and needs low reactive current - it might be useful to advance commutation by few degrees to compensate the effects of the stray inductance and minimize reactive current.

Speed can only be controlled by the the motor voltage. The motor behaves like a DC motor. Unlike the synchronous motor there are no problems with instability at any speed.

Because of the PWM frequency inverter, variation of the motor voltage can be achieved easily by changing the duty cycle of the pulse width modulation. Suitable PWM techniques allow regenerative breaking, which increases dynamic and efficiency of the drive.

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