Chapter 5

Motor and Torque Generation

Hover over the animation to see how the poles interact with each other. The arrows represent the relative magnetic forces and the arrow thickness indicates field strength. The two identical poles repel each other, making the rotor turn counterclockwise. At the same time, the opposite poles attract each other, adding torque in the same direction.

Once the rotor completes 60 degrees of rotation, the next commutation occurs.

Superimposing the stator magnetic field discussed earlier on the animation, it becomes clear that commutation occurs in such a way that the rotor never aligns with the stator magnetic field but is always chasing it.

There are two reasons for switching phases in BLDC motors in exactly this way. First, if the rotor and stator magnetic fields were allowed to align perfectly the motor would create zero torque, which is unfavorable. Second, maximum torque occurs when the fields are at 90 degrees to each other. The goal therefore is to bring this angle close to 90 degrees.

However, in BLDC motors, it is impossible to constantly achieve 90 degrees using six-step commutation and the angle fluctuates between 60 and 120 degrees as seen in the following animation. This is owed to the comparably simple nature of trapezoidal control. More advanced techniques such as field-oriented control, commonly used to control PMSMs as discussed before, allows generation of larger torques by achieving 90 degrees between the stator and rotor magnetic fields.