Control of Induction Motor Drives
When comparing the dynamic model of a separately excited DC machine, Eqs. (5.1-5.4), Fig. 5.4, with that of an AC induction machine, Eqs. (10.50- 10.53), Fig. 10.16, it is obvious that the latter represents a much more complex control plant. This is caused by the fact that the main flux and the armature current distribution of a DC machine are fixed in space and can be directly and independently controlled while with an AC machine these quantities are strongly interacting and move with respect to the stator, the rotor as well as each other; they are determined by the instantaneous values of the stator currents, two of which represent independent control variables. An additional complication stems from the fact that the rotor currents cannot be measured with ordinary cage rotors. Hence the AC motor is a nonlinear multi-variable control plant that kept control engineers puzzling for a long time.
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