Open-Loop Control

In open-loop V/F control, or Volts-per-Hertz control, a constant ratio is maintained between the applied stator voltage and the supply frequency to keep the motor's air-gap flux at its rated level. By keeping the flux constant, the motor can produce its rated torque across a wide range of operating speeds without saturating the magnetic core or losing efficiency. In practice, the controller adjusts the output frequency of the inverter to set the motor speed, while simultaneously scaling the voltage magnitude according to a predefined linear relationship. For operation at very low frequencies, a voltage boost is typically applied to compensate for the resistive voltage drop in the stator windings and ensure sufficient starting torque.

I-F control provides good control over torque during motor startup. Unlike open-loop control, I-F control starts the motor without any current overshoot as well as provides smooth I-F control to closed-loop control transition.