Significance of back EMF in a PMSM motor?
As with all electromagnetic motors, the back-EMF is generated by Faraday's law of induction as the rotor and stator fields rotate relative to each other. The magnitude of the back-EMF \$\mathcal{E}\$ is a function of the rate of change of magnetic flux \$\Phi_B\$:

$$ \mathcal{E} = -{{d\Phi_B} \over dt} \ $$

Any loop of conductor (like the windings in the stator) will experience this EMF whenever the magnetic flux (such as that from the permanent magnets in the rotor) through that loop of conductor changes.

Back-EMF does not, in any sense I can imagine, change the parameters of the motor. It does, however, have a significant effect on the electrical load presented to the rest of the circuit. To change the current in the windings, one must apply a voltage to the windings in excess of the back-EMF. If the back-EMF is higher than the applied voltage, then the winding current will decrease, and you no longer have a motor, but a generator.

The back-EMF is also responsible for the easily observed phenomenon that a fast-spinning motor draws less current than a slowly-spinning motor when driven with the same voltage.

Further reading:


How can I measure back-EMF to infer the speed of a DC motor?
varying load from electric motor