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At the instant of starting, an induction motor is like a short-circuited transformer — with the rotor stationary it draws a locked-rotor (inrush) current of typically 6–8 times the full-load current. This can cause voltage dips and nuisance tripping, so reduced-voltage starters are used on larger motors. Because both current and torque depend on the applied voltage, reducing the starting voltage cuts the inrush — but the starting torque falls with the square of the voltage, so there is always a trade-off.
| Starter | Starting current | Starting torque |
|---|---|---|
| Direct-on-line (DOL) | FLA × LRC (100%) | 100% |
| Star-delta | ÷ 3 (33%) | 33% |
| Autotransformer (tap t) | × t² | × t² |
| Soft starter (limit I) | set current limit | ∝ (I/IDOL)² |
Star-delta starting connects the windings in star to start (each winding sees 1/√3 of line voltage), giving one-third of the DOL current and torque, then switches to delta to run. An autotransformer at tap t reduces the line starting current to t² of DOL. A soft starter ramps the voltage to hold the current to a chosen limit. Reduced-voltage methods suit loads that start unloaded (pumps, fans).
It is the high current a motor draws at the instant of starting, when the rotor is stationary. Also called inrush or locked-rotor current, it is typically 6–8 times the full-load current for a direct-on-line start.
It is the ratio of starting current to full-load current, usually 6–8 for standard induction motors. NEMA code letters (A–V) or the IEC starting-current ratio on the nameplate specify it.
Starting in star connects each winding to 1/√3 of the line voltage, which reduces both the starting current and the starting torque to one-third of the direct-on-line values. The motor then switches to delta to run.
It applies a reduced voltage at a chosen tap (for example 65%) during starting. The line starting current falls to the square of the tap fraction (0.65² ≈ 0.42) of the DOL value, as does the starting torque.
Motor torque is proportional to the square of the applied voltage, and current is proportional to voltage. So any voltage reduction that cuts current also cuts torque by the square — reducing inrush always costs starting torque.
A soft starter uses thyristors to ramp the voltage up smoothly, holding the current to a set limit (often 2–4 times FLA). This gives a controlled, gentle start with less mechanical and electrical stress than DOL.
DOL is simplest and cheapest and is fine for small motors or where the supply can handle the inrush and full starting torque is needed. Larger motors on weaker supplies usually need reduced-voltage starting.
The inrush causes a temporary voltage dip proportional to the current and the supply impedance. Too large a dip can dim lights, trip protection or stall other equipment, which is why inrush must be managed.
Only during acceleration, typically a fraction of a second to a few seconds depending on the load inertia. Once the motor reaches running speed, the current settles to the full-load value.
Yes, dramatically. A variable-frequency drive starts the motor at low frequency and voltage, so the starting current can be kept near the full-load value while still providing full torque — the gentlest starting method.
They are often used interchangeably. Strictly, inrush is the very first transient peak (including a magnetising surge), while locked-rotor current is the sustained RMS current with the rotor held still; both are several times FLA.
Motor protection must allow the inrush to pass without tripping (using time-delayed or motor-rated breakers) while still protecting the cable. Short-circuit protection is set above the locked-rotor current but below the fault level.
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