Motor Inrush Current Principles
When an AC induction motor is energized, it draws a significant surge of current, known as starting current or inrush current, before settling down to its normal run current (Full Load Amps).
The multiplier depends on the starting method:
- Direct On Line (DOL): 500% to 800% of FLA
- Star-Delta: Approx. 33% of DOL current (~200% FLA)
- Soft Starter: 200% to 400% of FLA
- VFD: 100% to 200% of FLA
Frequently Asked Questions
For a Direct-On-Line (DOL) starter, the starting current is typically 6 times (or 600% of) the motor's Full Load Amps (FLA).
Motor starting current (inrush or locked-rotor current) is the high current drawn the moment a motor starts, when the rotor is stationary. It typically equals 6–8× the motor's full-load current (FLC) for direct-on-line (DOL) starting. This surge typically lasts 2–10 seconds while the rotor accelerates to rated speed, then current drops to the normal running level.
At startup, the stationary rotor generates zero back-EMF (counter-electromotive force). Without back-EMF to oppose the supply voltage, the stator windings act as near-short-circuit load. As the rotor accelerates, back-EMF builds up and limits current to the normal operating value. The starting current is limited primarily by stator impedance, which is low at standstill.
Methods to reduce motor starting current: Star-Delta starter — reduces starting voltage to 1/√3, limiting starting current to ~33% of DOL (but starting torque also reduces to 33%); Autotransformer starter — selectable 50/65/80% voltage taps; Soft starter — electronic gradual voltage ramp, 2–4× FLC starting current; VFD (Variable Frequency Drive) — smoothest start, typically 1.5× FLC, highest cost and most flexible.
When a motor starts, the inrush current (typically 600–800% FLC) causes a voltage dip at the transformer secondary proportional to: ΔV% ≈ (Istart × %Ztransformer) / Irated_transformer × 100. A large motor starting on a small transformer can cause a 10–20% voltage dip, tripping sensitive equipment. Rule of thumb: motor kW should not exceed 25–33% of transformer kVA for DOL starting.
NEMA assigns code letters (A–V) to motors indicating locked-rotor kVA per horsepower. Common codes: Code F — 5.0–5.6 kVA/hp; Code G — 5.6–6.3 kVA/hp; Code H — 6.3–7.1 kVA/hp; Code J — 7.1–8.0 kVA/hp; Code K — 8.0–9.0 kVA/hp. Higher letters mean higher starting current. Most standard NEMA squirrel-cage motors are Code G or H. The code letter is on the motor nameplate.