An autotransformer starter reduces motor terminal voltage during acceleration. The network sees less starting current, but the load also receives less motor torque.
Calculate the tap effect
As a screening relationship:
Starting torque at tap ≈ full-voltage starting torque × tap voltage²
A 0.8 per-unit voltage tap therefore produces roughly 0.8² = 0.64 per-unit of the full-voltage starting torque. Final approval must use the actual motor curve and starter connection.
| Check | Required input | Pass condition |
|---|---|---|
| Bus voltage | Source impedance and other loads | Above project minimum |
| Acceleration | Motor/load torque-speed curves and inertia | Positive margin through run-up |
| Thermal duty | Acceleration time and starts/hour | Within motor and starter limits |
| Transition | Circuit sequence and timing | Acceptable current/torque disturbance |
Model the worst credible supply and loaded-start condition. Protection settings must allow legitimate acceleration without leaving a stalled motor energized beyond its withstand time.
Commission by trending bus voltage, motor current and acceleration time. Compare results with the study and investigate a slower-than-predicted start before increasing trip settings.
Related reading: motor starting methods compared and motor code letter and starting current.