Thanks Melanie for your information, I think my idea will not be suitable for my project, so when I try the dimmer switch with its analog potentiometer with my DC motor, then there are aloudy buzzing sound from the transformer when I decrease the speed or voltage.

But I want to ask :
is't important to add the flyback diode across motor terminals beacuse I add many diodes and everytime they damaged and damegd my mosfets. ?