Starting Methods Comparison of Medium Voltage Three-Phase AC Motors

With full-voltage starts on induction motors, locked rotor current and asymmetric inrush current occur. Using reduced-voltage solid-state soft-starters the AC phase current is limited to a value less than the locked rotor value. This extends the start time in favor of lessening the impact on the grid and mechanical stress of the motor. In this paper, the torque per Ampere versus speed are shown for several starting methods. These fixed frequency starting methods are compared to the adjustable frequency drive approach. A presentation of torque per ampere versus angular velocity along with a traditional RMS phase current versus time plot is provided for all methods.