Working principle
A permanent magnet synchronous motor has magnets bonded to or buried in the rotor and a distributed three-phase winding in the stator. The stator current creates a rotating magnetic field, and the rotor magnets lock onto it and follow it exactly. Unlike an induction motor there is no slip: the shaft runs precisely at synchronous speed, set only by the supply frequency and the pole count, whatever the load happens to be — right up to the point where the motor loses synchronism.
Load angle and pull-out torque
Torque in a synchronous machine is produced by the angular offset between the stator field and the rotor magnets, known as the load angle. As you add load the rotor falls slightly further behind the field, the load angle grows, and torque rises with it. The maximum occurs at 90 electrical degrees; that value is the pull-out torque. Demand more than the pull-out torque and the rotor drops out of step, torque collapses and the machine stalls instead of gracefully slowing down.
Field weakening above base speed
Below rated frequency a drive holds a constant volts-per-hertz ratio so that the air-gap flux, and therefore the available torque, stay constant. Once the inverter runs out of voltage the flux has to fall as frequency rises. This field-weakening region gives higher speed at reduced torque, which is exactly the constant-power behaviour a traction drive wants. The simulator shows pull-out torque shrinking as you push the frequency above the nameplate value.
PMSM versus BLDC and where it is used
A PMSM and a BLDC motor are physically very similar; the difference is the back-EMF waveform and the drive strategy. A PMSM has a sinusoidal back-EMF and is fed sinusoidal current under field-oriented control, giving very low torque ripple and quiet operation. That makes it the machine of choice for electric-vehicle traction, servo axes, robotics, high-efficiency HVAC compressors and lifts, where efficiency above 95 % and precise control both matter.