To make the rotor rotate, a torque must be applied to the rotor, where torque = force x lever arm. The torque in a permanent magnet synchronous motor has two sources: one is the torque generated by the current between the permanent magnet and the energized wire, and the other is the magnetization effect of the magnetic field generated by the current in the winding on the ferromagnetic material, giving the ferromagnetic material magnetic properties. This magnetic field interacts with the current in the winding. The force exerted by the external magnetic field on the current in the winding wire was discovered by Ampere, so this force is called the Ampere force. The direction of the Ampere force can be determined using the left-hand rule. Position the magnetic field perpendicular to the palm of the left hand, with the other four fingers together in the direction of the current in the energized wire, then the direction of the thumb indicates the direction of the Ampere force.

The figure below shows a current-carrying coil, with current flowing into the wire from the right and out from the left. According to Ampere's left-hand rule, we can determine that the Ampere force acting on the left coil is upward, while the Ampere force acting on the right coil is downward. The distance from the Ampere force to the central axis is equal, creating a torque. Under the action of these two forces, the coil may rotate in a clockwise direction, which is the interaction between the permanent magnet and the energized wire.

Now let's talk about magnetic reluctance torque. Below is a current-carrying coil, with air in the middle. If current is passed in the direction shown in the diagram, a magnetic field will be generated between the core of the coil and the air, directed as indicated by the arrows. If a piece of iron is inserted into the air, we will find that the magnetic field lines will bend, with a significant portion of the magnetic field lines being drawn into the iron. We can imagine the magnetic field lines as elastic strings; if force is applied to both ends, the iron will rotate in the direction of the pulling force, and the torque generated by this pulling force is called magnetic reluctance torque. Eventually, the position of the iron will align parallel to the direction of the current magnetic field, with the N pole and S pole attracting each other, reaching a state of equilibrium. This force is called magnetic reluctance torque, which mainly occurs when iron is inserted into the air, where the resistance experienced by the magnetic field lines varies at different positions in the air, and the resistance from the iron is lower. For a permanent magnet synchronous motor, its torque consists of two parts: permanent magnet torque and magnetic reluctance torque.

It can be seen that whether it is permanent magnet torque or magnetic reluctance torque, if the magnetic field of the coil remains stationary, once the magnetic field of the permanent magnet is parallel to the magnetic field of the coil or the magnetic field induced by the ferromagnetic material is parallel to the magnetic field of the coil, the rotor cannot continue to rotate. To keep the rotor rotating, a rotating magnetic field must be generated. By passing a sine wave current through three symmetrically arranged windings, with a phase difference of 120 degrees between the three windings in space, if the phase difference of the incoming current is also 120 degrees, a rotating magnetic field can be generated in space.