Novel Voltage Observer for Reduced Voltage Soft-Starter Torque Ramp: Description and Analysis Results

Reduced-voltage soft starters (RVSSs) are typically phase-angle fired with an open-loop ramping voltage with an alpha stop at the current limit. The advanced controls for soft starters for pump control, which in most cases will be a closed-loop real power ramping reference, can be configured for a softer shaft speed pull-in resembling a convex S-curve. Another advanced control similar to pump control is a torque ramp. Authors of previous works have shown a stator flux-oriented torque RVSS control using an output motor stator voltage feedback approach. This article presents a novel voltage observer using a voltage-sensorless technique based upon the measured motor phase current and knowledge of the plant. The novel control method and results are shown, including an IEEE 399 Brown Book motor-starting analysis.