Before you go further and suspect the motor performance it's advisable to carry
out the followings:
-Disconnect the motor connection and test the controller it self to confirm
It’s operation, if the problem still occurring thin used analog signal (current
source 4 to 20 ma) and test controller, if the problem still then replace the
control card which used to firing the Thyristor.
By the way it happens with me 5 years ago and I follow the above and it works