as you Know, The PM Motor either should rotate same as stator field speed or not rotate anymore, however there are a little vibration in application with PM motors, whats the reason of vibration?

unbalance

Exactly! vibration can result from Unbalance. but in PM motor why it happend? as you know Pm Motor has no slip.

The rotor is unbalance so the motor has vibration when it run.

Hi Davoud

1. The assembly could be wrong.
2. The magnets always try to align them as per the magnetic fields even when there is no current given to the motor.
3. Are you talking of the bower vibrating ? If yes that may be mainly because of shaft selection.

You can choose to give more details of your problem also .

I talked about vibration below 1% of nominal speed.
all type PM motors that i used have such vibration.
im sure that these motors are intact and there are no problem about shaft and balancing.
i know when the load torque change, a little vibration occurs in the motor speed.
also we Know PM motor should be running either with the preset speed(Command speed) or stop. but always there are a little vibration in motor speed.

Does the vibration frequency changes with motor speed? You have to identify the relationship between the vibration frequency and motor speed. Many reasons could cause motor vibrate.