Hi, Sum, everyone,
Let's leave the slow speed. How does the system funcion in range of
measureable and motor controllable speed.
How did you get the formula "D=Dold*Tactual/Tdesired"? It's seams as
wrong one. For example our Tactual is changed from 100 to 300 RPM.
Does it mean that D must be changed in 3 times? I do not think so.
Even if all the systems' blocks have transfer function "1" (in real
system it depend on revolution/control signals frequency, any system
is interesting in dynamic analysis, not static) the system is not
going to function as stable.
In linear systems error signal usually is calculated as subtraction:

delta = Tdesired - Tactual, where Tactual = Tout/FeedBack(f);
Tout = delta * S(f);

I hope I am right (but all such calculations should be done with
Laplas transforming).

I still strongly recomend to refer to the Automatic Control Theory
Basis. Please, if someone could give me useful references on the
Enternet.

Thanks.
Best regards,
Oleg