Estimate motor inrush current and the voltage dip it causes on starting.
Direct-on-line starting draws six to eight times full-load current, which causes a voltage dip proportional to how weak the supply is relative to the motor. Softer starting methods trade starting torque for reduced inrush. Voltage dips above about 5 per cent cause visible light flicker and can drop contactors elsewhere in the installation.
Motor Starting Current
Starting current = full-load current × method multiple; dip ≈ starting kVA ÷ (fault level + starting kVA)
Starting current = full-load current × method multiple; dip ≈ starting kVA ÷ (fault level + starting kVA) Direct-on-line starting draws six to eight times full-load current, which causes a voltage dip proportional to how weak the supply is relative to the motor. Softer starting methods trade starting torque for reduced inrush.
Voltage dips above about 5 per cent cause visible light flicker and can drop contactors elsewhere in the installation.
This calculator takes 4 inputs: Full-load current, Starting method, Supply fault level, Motor apparent power. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.