A current limiting technique for V/f controlled induction machines

This paper describes a current limiting technique for V/f controlled induction machines. In V/f control method stator current is not directly controlled so the implementation of current limiter is a challenging task. The current limiter proposed in this paper is composed of a programmable ramp and two proportional-integral (PI) controllers which keep the stator current below the limit. The programmable ramp provides limiting of the stator current during the induction machine acceleration and deceleration whereas the PI controllers, if necessary, ensure a further decrease of the stator current during the transients. Moreover, the PI controllers also keep the stator current under the limit during the steady state. The proposed method is implemented in digital control system and verified on the laboratory model. Experimental setup consists of a squirrel-cage induction machine (560 kW) connected to the grid via back-to-back converter and permanent magnet machine (375 kW) connected to the grid via industrial frequency converter. Experimental results show the satisfactory performance of the proposed current limiting technique.