Method for enabling permanent magnet synchronous motor to start in reverse direction and associated motor device

A motor device, comprising:

• a permanent magnet synchronous motor (PMSM);
• a load torque estimation circuit, arranged to estimate a load torque according to a torque and an actual rotational speed of the PMSM, to generate an estimated load torque;
• and
• a torque control circuit, comprising:
• a torque controller, arranged to:
• control the torque according to the estimated load torque to decelerate the PMSM until the actual rotational speed of the PMSM is reduced to a predetermined target rotational speed;
• in response to the actual rotational speed of the PMSM being reduced to the predetermined target rotational speed, determine whether an energy of the PMSM is not greater than a threshold value, wherein the energy of the PMSM comprises a potential energy;
• and
• in response to the energy of the PMSM not being greater than the threshold value, start to control the torque for making the PMSM rotate in a forward direction.