Angle-Torque Characteristics

When a motor is excited at the rated current and torque is externally applied to the motor shaft to change the angle of the rotor, the relationship between the angle and torque is called angle - torque characteristics, as shown in the figure below.

The following figure shows the positional relationship between the small teeth of that stator and the rotor at each point in the characteristics diagram above.
If an external force is applied to the motor shaft when it is equilibrated and stopped at the stable point ①, torque T (+) is generated in the left side to pull it back to the stable point ①, and it stops at an angle equilibrated with the external force.
② There is an angle at which the maximum generated torque is reached when further external force is applied. The generated torque at that time is the maximum holding torque TH.
③ If an external force is applied that exceeds this point, a torque T (-) will be generated in the same direction as the external force through the unstable point ⑤, and it will move to the next stable point ① and stop.

Stable Point:

This is the place where the small teeth on the stator and rotor stop in a perfectly relative position. It is very stable and always stops at this location when the external force is set to 0.

Unstable Point:

This is the place where the small teeth of the stator and rotor are offset by 1/2 pitch. It is very unstable and will move to the right or left stable point if any external force is applied.