The analysis of the fundamental equation of the speed of electric motors of direct current shows https://www.mrosupply.com/popular_products/15-hp-56h-motors/ that speed can be controlled by the change of two electric quantities, the terminal voltage of armature Vta and the flux of the field inductor φf. The variation of the inductor flow is achieved by moving the current If or equivalently from the variation of the voltage Vf. The variables by which speed control is achieved – Vta, Vf and If – are called actuation variables, while velocity is the controlled or output variable.
The armature voltage and the field current are defined in electrically decoupled circuits, and make the control of DC motors simple to implement, in addition to being in direct current. We know that in alternating current the flow is a function of the current value as well as the electric frequency. Instead of direct control of the Vta and Vf voltages, we can use variable resistors in series with the armature and field circuits, which indirectly guarantees the control of the effective voltages on these circuits.