Size motor contactors, overload relays, and feeder cables per IEC 60947-4-1. Compare DOL and Star-Delta starting methods — starting current, voltage dip, torque, and equipment ratings.
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IEC 60947-4-1 is the Low-voltage Switchgear standard for contactors and motor starters. It defines the AC3 utilization category (motor switching with inrush currents up to 6× the rated operational current) and requires contactors to be rated at ≥ 1.15× the starting current Ist.
Direct On-Line (DOL) is the simplest and most reliable method — use it for motors up to 7.5 kW, or for any motor where the voltage dip does not exceed 10% and the load can tolerate 1.5–2× rated starting torque. Star-Delta reduces starting current to 33% of DOL (Ist_sd = Ist_dol / √3) and starting torque to 33% of DOL — use it for motors above 15 kW or where voltage dip is excessive.
Per IEC 60947-4-1 §6.2, the contactor must be able to make and break the starting current. The AC3 rating must be ≥ 1.15 × Ist (the locked-rotor current). Standard IEC contactor sizes are: 9, 12, 18, 25, 32, 40, 50, 65, 80, 95, 115, 150 A and larger.
The overload relay is set to protect the motor against sustained overcurrent. IEC 60947-4-1 §6.4 allows a setting range of 1.0–1.25 × Irated for continuous-duty motors (set at Irated) and 1.15–1.25 × Irated for intermittent duty (set at 1.15 × Irated). The relay is ambient-compensated and class II (max trip at 120% of setting).
When the motor transitions from star to delta connection (~50–100 ms), there is a transient current spike of approximately 2–3× the DOL line current. This occurs because the motor is still spinning near synchronous speed while the induced voltage is collapsing. The spike decays exponentially with a time constant τ ≈ 10 ms. For large motors, this can cause contactor contact wear — specify transition contactors rated accordingly.
Per IEC 60364-4-43 (and NEC 430.22), motor circuit cables must be sized at ≥ 1.25 × Irated to handle the sustained running current plus thermal margin. Then voltage drop is checked: limit to ≤ 3% for lighting circuits and ≤ 6% for motor circuits per IEC 60364-5-52. The cable must also withstand the short-circuit current for the time it takes the upstream protective device to clear (adiabatic equation: A ≥ √(I² × t / k²)).