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Mathematics of Electronic Motor Control
Multi-phase motor basics
Hall-based commutation
Sinusoidal commutation and FOC
AC induction motors
Summary
 

Multi-phase motor basics

Brushless DC motors are generally, although not always, three phase devices. They are wired in either a Wye or Delta configuration, but in either case there are three connecting wires, and the current input to any two coils must be output through the third. That is,

C = –(A + B) where A, B, and C are the current flowing through each leg of the 3-phase brushless motor.

The three coils generate the magnetic field of the stator (the non-rotating outer portion of the motor), while the rotor magnetic field is created by permanent magnets. This is why this motor is also frequently referred to as a brushless PM motor.

Three-phase brushless DC motor
Three-phase brushless DC motor
he rotor and stator fields interact to create rotational torque, however the timing and relative magnitude of the current through each stator winding must be synchronized with the rotor position to keep the stator electrical field aligned as the rotor rotates. In a DC brush motor, which is a single phase device, generating this is done by a mechanical commutator with brushes. In a brushless DC motor, it must be done by the external controller.

Figure 1 shows two kinds of force that are generated by the magnetic field interactions. They are known as the Q (quadrature) force and the D (direct) force. An ideal controller for a brushless DC motor will maximize Q force generation, which is oriented exactly 90 degrees from the N-S axis of the rotor, and minimize the D force, which is aligned with the rotor’s NS axis, and therefore creates no useful torque.



 
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