Permanent magnet synchronous motor
Curvo Studio

Current model·Synchronous machines

Permanent magnet synchronous motor

See why a synchronous rotor locks exactly to the rotating field — and what happens when the load pulls it out of step.
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ClockwiseDrive-end view
↖ Drag to rotate
U phaseV phaseW phase
Rotor speed0 rpm
Synchronous speed1,500 rpm
Load angle0 %
Phase current0 A
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Operating experiment

Speed and current curves

Observe the motor transition from startup to steady state over time.

00375375051,12581,500100 s4 s8 s11 s15 sMeasurements will appear here when you start the experiment

What is a permanent magnet synchronous motor (PMSM)?

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.

Frequently asked questions

What is the load angle of a permanent magnet synchronous motor?
The load angle is the angle between the rotor's magnetic axis and the rotating stator field. It is near zero at no load and grows as load torque increases; torque is roughly proportional to its sine. The peak at about 90° is the pull-out torque, and beyond it the rotor slips out of synchronism. The simulator shows the load angle as you add load.
Why does a PMSM run exactly at synchronous speed?
The rotor magnets lock onto the stator's rotating field, so the rotor turns at the field's speed, ns = 120 × f / P rpm, with no slip. Load changes the load angle, not the speed. To change speed you change the supply frequency, which is why PMSMs run from an inverter or drive.
What is the difference between a PMSM and an induction motor?
An induction motor creates its rotor field by induction, so it needs slip and has rotor copper losses. A PMSM's field comes from permanent magnets, so it runs synchronously with higher efficiency and power density. The trade-offs are magnet cost, a drive needed to start and control it, and a limited field-weakening range.
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