Dear Hamid, Thank you and congratulations for your blog and your expertise ! I would like to say that the speed increase of a DC motor through field working weakening is also limited by the armature reaction flux that becomes very significant versus the inductor decreased flux , distorting it and leading to speed and current instability at high velocity (even on compensated machines). I faced this phenomenon on hoisting machines working at constant power between 2.5~3.5 times their base speed .