As a matter of fact, it is rather difficult for people to analyze the operation characteristics of three-phase brushless motor accurately. Motor air gap flux density distribution along with the air gap by sinusoidal. When the winding is energized, magnetic flux generated by the current impact of the air gap is negligible. In the condition of the switch operation, the power transistor is constant pressure drop. Each winding is symmetry and the corresponding circuit is completely consistent. The relevant electric time constant can be neglected. The power of sensored brushless motor and others that control circuit are also negligible. Three-phase brushless DC motors are also synchronous motors. It is very important to protect the three-phase brushless dc motor drivers. Power transistor would be burnt down when it is overload or use improperly, because it may cause high current.

A, Brushless motor works through the the alternating electromotive force of armature coil,by the commutator with brushes for the role, so that leads from the brush end into the DC when the force of the principle. EMF direction is determined by the right-hand (magnetic induction line to the palm of the hand, thumb pointing to the direction of conductor movement, the other four fingers pointing is the direction of the electromotive force induced in the conductor.) Instant shown in Figure 1.1, the conductor ab, cd induction force direction by a and b point to point by d c. A positive polarity when the brush, brush B was negative.

Figure 1.1 brushless motor theory model

When the coil is rotated counterclockwise 180 °, then the conductor cd in the N pole, the conductor ab in the S pole, the force of the conductor are changed direction, respectively.

Figure 1.2 brushless dc motor theory model

Seen from the figure, and the brush contact with the conductor A is always located at the N pole, the same, and brush contact with the conductor B is always lower in the S pole. Therefore, A is always positive polarity brush, brush B always negative, so the brush end can lead to changes in the direction of change but the size of the pulsating force. If the increase in the number of coils on the armature, and in accordance with certain rules to connect them together, can reduce the level of pulsating, direct current electromotive force can be obtained. This is the brushless motor works.

Second, the working principle of DC motor force of the conductor with the left hand rule to determine direction. This form of electromagnetic force acting on the armature a moment, this moment is called the rotating motor, electromagnetic torque, the torque direction is counterclockwise in an attempt to make the armature move counterclockwise. If the electromagnetic torque to overcome the torque on the armature resistance (eg resistance caused by the friction torque, and other load torque), the armature will be able to counter-clockwise spin.

Figure 1.3 DC motor theory model

When the armature turn 180 °, the conductor under the cd to N pole, S pole to the next conductor ab, due to the current direction of the DC power supplies unchanged, still flows from the brush A, the conductor cd, ab, the B from the brush out. At this time the direction of conductor cd into force from right to left, conductor ab direction from left to right by force, the electromagnetic torque is still counter-clockwise direction.

Figure 1.4 DC motor theory model

Therefore, the armature upon rotation, the commutator with the brush commutation of the current role of DC current alternately flows from the conductor ab and cd, so long as the coil in the N pole side, the one by the electric current direction is always A brush into the direction and under the S pole, always out in the direction B from the brush. This ensures that each side in the very next coil current is always in one direction, the direction to form a constant torque, the motor can be continuously rotated. This is the DC motor works.