Motor Restart / Undervoltage Trip

Set an undervoltage (27) / contactor undervoltage-release trip and assess motor re-acceleration after a supply interruption: residual motor voltage decay, phase-angle drift and the reclose-inrush against the DOL starting current (IEC 60947-4-1 / IEC 60034).

Inputs

Motor rated voltage Uₙ V

UV trip setting (% Uₙ) %

UV re-attract setting (% Uₙ) %

Sustained / recovered voltage V

Motor residual V at release V

Residual-voltage time constant τ s

Time since release s

Slip frequency Δf Hz

Combined reactance X Ω

DOL starting current I_st A

Results

Note — The undervoltage release should drop out below the sustained fault voltage but re-attract above the recovered voltage; the drop-out ratio U_reset/U_trip should be coherent (typically 0.6–0.95 for a contactor UV release). A motor may be re-accelerated only when the supply has recovered above ~0.85·Uₙ, the motor residual voltage has decayed below ~0.25·Uₙ and the phase drift is within ~30°. Reclose inrush can exceed the DOL starting current when the two voltages are nearly in phase. This is a steady-state approximation; confirm against the relay / contactor data and a certified engineer.