Dez,
When you say when in ran no load it was ok, then when you started it the second time was is still at a no load condition? I assume necessary tests include:
Voltage present and balanced on stator
No grounding of the stator or rotor.
Resistance is balanced on the stator/surge test
Resistance is balanced on the rotor/armature
Brushes, brush shunts, spring tension, slip ring circuit, resistor all check good?
Sounds like a single phase condition or no voltage getting to the rotor/armature, what about the fuses, breaker, wires to motor, junction box on side of motor.
Dave

what is really happening is that the motor will run on no load untill you switch it off but when you want to start it again it will just hum when you rotate the rotor manually and switch it on it runs so its like on certain positions it is locking itself( like stator north-rotor north)we have checked to see if there is any reversed connection on the stator and also checked the polarity on the rotor poles,all is ok but i don't know what is causing the motor to fail to start at certain postions.

Sounds like intermittent electrical connection or a failure to provide balanced voltage to the motor.
http://www.basler.com/downloads/synchmtr.pdf
http://www.kilowattclassroom.c...chive/SyncMotors.pdf
Here are a couple of links that may assist you in basic troubleshooting. Have you contacted the OEM of the motor / manufacturer?
Dave

Thank you Dave for your efforts.The OEM (WEG) has been contacted and we are still waiting for a feedback from them.