Thanks for answering the question. If we use servo motor with 1000 ppr then the lowest angular speed that controller can measure is fs/4000. Fs is velocity control loop update frequency and let say fs=4000 Hz. Then the lowest detected speed is 1 Hz or 60 rpm and it's not infinitely low speed. Speed estimation using some interpolation or observer is only remaining possibility that I know. Does VSD-E use speed estimation?