Static limiting characteristics of induction motor drive with frequency vector control

The paper describes the research considered relevant due to the wide-scale usage of frequency controlled asynchronous electric drive based on squirrel-cage induction motor (IM) in the most popular regulated electric drive industrial mechanism systems. The research aims to analyze estimated limiting static characteristics, which correspond either to IM voltage nominal value or to the maximum value of invertor output voltage of frequency controller (FC) on set supply voltage value for various control systems of three-phase invertor. These characteristics allow estimating the possibility to generate the required velocities on set static load torque in frequency controlled asynchronous electric drive with vector control. The research novelty involves using the limiting static characteristic formed by functional converter of rotor flux setting in the velocity open loop FC - IM system with maximum inverter voltage to select the velocity of IM flux weakening depending on its electromagnetic torque for reaching electric drive operating mode. The research methods, used in the paper, are based on employing the electric drive theory, electric machines theory, mathematical and simulation modeling methods with modern software. The research outcomes establish that with the insufficient output FC inverter voltage in an asynchronous electric drive with frequency vector control, the generation of the required IM flux and torque values is due to the reduction of its angular velocity by reducing the output frequency of the FC. Moreover, the initial rate of IM flux weakening should be selected depending on the actual value of the IM electromagnetic torque, using the limiting static characteristic of the open loop FC-IM system. The paper proves that the vector frequency control systems of an asynchronous electric drive with the velocity selection of the IM flux weakening depending on the IM electromagnetic torque feature a possibility to maintain the optimum IM flux value in steady-state mode, allowing for the higher value of IM torque at the IM rated current. To reduce the current used by IM and to exclude its overheating at high loads, the nominal value of the flux linkage should be set, and for lower loads, the value of the flux linkage specification should be reduced.