Control variables are classified into input variables (input voltage, input frequency), output variables (speed, angular displacement, torque), and internal variables (armature current, magnetic flux). The mathematical model of an a.c. motor is nonlinear, which for small variations of the input voltage, input frequency, and output speed can be linearized.

*Scalar control methods* are based on changing only the amplitudes of controlled variables. A typical example is to maintain constant torque (magnetic flux) of a.c. motors by keeping constant the *V/f* ratio. The scalar control can be implemented both in the *open loop* (most of industrial applications) and *closed loop* control systems with speed feedback. ...

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