A VFD operating without feedback from the motor is described as which control type?

• A. Closed-loop
• B. Open-loop
• C. Sensorless
• D. Hybrid

Answer: (B)

When a VFD operates without any signal reflecting how fast the motor is turning, it’s using open-loop control. In this setup, the drive just applies a voltage-to-frequency relationship based on the user command, with no feedback path to compare actual motor speed or torque to the desired value. That means there’s no automatic correction if the load changes or if disturbances occur, so speed can drift.

If there were feedback from the motor, such as an encoder or tachometer providing real speed data to the drive, the system would continuously compare actual speed with the target and adjust output to minimize error—this is closed-loop control. Sensorless control eliminates a physical speed sensor but still relies on internal estimates and measurements to regulate performance, so it isn’t truly without motor feedback. Hybrid combines elements of different approaches and would include feedback as part of the control.