Calculate the large starting current (inrush current) that flows the instant a three-phase induction motor is connected to the supply. Change the rated power, voltage, efficiency and power factor, and compare how direct-on-line, star-delta and soft-starter methods change the starting current and torque in real time.
Parameters
Rated power P
kW
Rated voltage V
V
Efficiency η
%
Power factor cosφ
Starting-current ratio (full voltage) k_LR
Locked-rotor current as a multiple of rated current at full voltage
Starting method
Reduced-voltage starting lowers the torque when it lowers the current
Results
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Rated current (A)
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Full-voltage start current DOL (A)
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Starting current of method (A)
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Multiple of rated current (×)
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Starting-torque fraction (%)
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Starting-current verdict
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Starting current vs time — start-sequence animation
A tall inrush-current spike rises at switch-on and decays to the steady running current as the motor accelerates. The spike height changes with the selected method. The bar at the right compares the peak currents of the three methods.
Rated (full-load) line current I_rated and the direct-on-line starting current I_start,DOL. P: rated power, V: rated voltage, η: efficiency, cosφ: power factor, k_LR: locked-rotor ratio.
Star-delta starting applies 1/√3 of the line voltage to each winding, so the starting current and torque both fall to one third. With reduced voltage, current scales with voltage while torque scales with the square of the voltage.
What is Motor Starting Current?
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Sometimes the factory lights dim for a moment the instant a big motor is switched on. What is actually happening there?
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That is exactly the effect of the "starting current". The instant a three-phase induction motor is switched on, the rotor is still stationary. With the rotor at rest, the rotating field sweeps past the rotor bars at full speed, so the slip of the rotor circuit is 100%. At that moment the motor is electrically almost the same as a transformer with its secondary short-circuited, and the current is set only by the winding impedance. So a huge current — the locked-rotor or starting current — flows, typically five to eight times the rated current.
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Five to eight times! If that current kept flowing it would be a disaster, wouldn't it?
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It would, but the surge only lasts a few seconds. As the motor accelerates up to speed the slip drops and the current falls back to its running value. But those few seconds cause trouble. The large current flowing through the supply impedance produces a sharp voltage dip across the whole local network. Lights flicker, contactors drop out, electronics on the same supply misbehave. It also stresses the windings and switchgear thermally, and on a weak supply the motor may not even start.
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So big motors somehow reduce that starting current before they run?
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Right — large motors are very often started with a reduced-voltage method. The classic one is the star-delta starter. During starting the windings are connected in star (Y), which puts only 1/√3 of the line voltage across each winding; once it is up to speed it switches to delta. That cuts both the starting current and the starting torque to one third of their direct-on-line values. Try switching "Starting method" on the left to star-delta — you will see the starting current value drop sharply.
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There is also a soft starter option. How is that different?
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A soft starter uses thyristors to ramp the applied voltage up smoothly. Current scales with voltage, but torque scales with the square of the voltage. So reducing the voltage to 50% gives 0.5× the current but only 0.25× the torque — torque drops more steeply than current. This is the key point: every reduced-voltage method carries the same trade-off — you cannot lower the starting current without also lowering the starting torque.
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Lowering the current is nice, but losing too much torque sounds bad. How do I choose?
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It comes down to whether enough starting torque is left to accelerate that particular load. For light-starting loads like pumps and fans, star-delta or a soft starter can limit the voltage dip nicely. For heavy loads like conveyors and compressors that need high torque from a standstill, a reduced-voltage method may not provide enough, so DOL is used instead. Look at both the starting current and the starting-torque fraction in this tool to pick the method that suits the load.
Frequently Asked Questions
At the instant a three-phase induction motor is switched on, the rotor is still stationary. With the rotor at rest, the rotating stator field sweeps past the rotor bars at full speed and the slip of the rotor circuit is 100%. The motor then behaves electrically almost exactly like a transformer with its secondary short-circuited, so the current is limited only by the winding impedance and a locked-rotor current of five to eight times the rated current flows. This is the starting (inrush) current. This tool derives it from the locked-rotor ratio k_LR and the rated current.
When a large starting current flows through the supply impedance, it produces a sharp voltage dip across the whole local network. This can dim lights, drop out contactors and disturb sensitive electronics on the same supply. It also stresses the motor windings and switchgear thermally, and on a weak supply it may even prevent the motor from starting at all. For these reasons large motors are very often started with a reduced-voltage method.
Connecting the windings in star (Y) during starting places only 1/√3 of the line voltage across each winding. Because current is proportional to voltage, each winding current also drops to 1/√3, and the line starting current falls to one third of the direct-on-line value. Torque, however, is proportional to the square of the voltage, so the starting torque also drops to one third. Cutting both the starting current and the starting torque to one third is the defining feature of star-delta starting.
Every reduced-voltage method carries the trade-off that lowering the starting current also lowers the torque. Direct-on-line (DOL) gives the highest starting torque but also the highest current. Star-delta cuts both current and torque to one third, while a soft starter at, say, 50% voltage gives 0.5x current and 0.25x torque (torque scales with voltage squared). For light loads with torque margin, use star-delta or a soft starter to limit the voltage dip; for heavy loads needing high starting torque, choose DOL. Check both the starting current and the starting-torque fraction in this tool to decide.
Real-World Applications
Pumps, fans and blowers: HVAC blowers, water-supply pumps and cooling-tower fans are classic light-starting loads. They need little torque to accelerate from rest, so star-delta starting or a soft starter can limit the starting current without trouble. When the motor is large relative to the installed supply capacity, reduced-voltage starting to suppress the voltage dip is the standard choice.
Compressors, conveyors and crushers: Reciprocating compressors, belt conveyors and crushers are heavy loads that need high torque from the moment they start. Cutting the torque to one third or one quarter with a reduced-voltage method may leave too little to accelerate the load, causing a "failed start". Such applications either use direct-on-line starting or require careful selection of the starting method and motor size to keep enough torque.
Power-system design and protection coordination: The magnitude and duration of the starting current are key inputs when sizing substation equipment and cables and setting protective relays. The instantaneous element must be set above the starting current so that it does not trip spuriously during starting. An estimate of the starting current like this tool gives is useful for the first sizing of cables and breakers.
Motor starting on captive or weak supplies: On supplies with limited capacity, such as standby diesel generators or island grids, the voltage drop from a large motor's starting current becomes severe. If the generator capacity is inadequate for the motor's starting current, the voltage collapses, the motor cannot accelerate, and the disturbance can drag down other loads. Designs that limit the peak current with reduced-voltage or inverter starting are essential here.
Common Misconceptions and Pitfalls
The most common misconception is "the starting-current ratio is always about 6". The locked-rotor ratio k_LR depends on the motor design (the IEC code letter or the NEMA design class) and spans roughly four to nine times the rated current. High-efficiency motors (IE3, IE4) actually tend to have a higher starting current, and the same power rating can exceed eight times. In design, check the real k_LR on the motor nameplate or the manufacturer's data sheet, and never just assume a flat factor of six.
Next, "star-delta only reduces the current, the starting torque stays the same". Star-delta starting does not only reduce the starting current — the starting torque also drops to the same one third. A starting torque of twice the rated torque on DOL becomes only 0.67× on star-delta. If the load torque exceeds that, the motor cannot accelerate, and a large inrush current can flow again at the changeover from star to delta. When choosing a starting method, always compare it against the load's starting-torque curve.
Finally, thinking that "the lower the starting current, the better the design". Reduced-voltage starting that suppresses the starting current always comes at the cost of lower starting torque. In this tool, a soft starter at 50% voltage gives 0.5× current but only 0.25× torque. Lowering the starting current too far brings other problems — the load cannot be accelerated, or the start time lengthens and the windings overheat. Look at the starting current, the starting torque and the start time together, and choose a method that reliably brings the load up to speed.
How to Use
Enter motor nameplate power (kW) and supply voltage (V) for your three-phase induction motor
Input motor efficiency (%) and power factor from the motor rating label
Select starting method: Direct-On-Line (DOL) shows peak inrush at 5–7× rated current; Star-Delta reduces it to 1.7–2.3× by stepping down voltage; soft starters limit inrush to 2–4× with electronic ramps
The simulator calculates rated current from P = √3 × V × I × cosφ × η, then applies method-specific multipliers to estimate starting current and torque fraction
Worked Example
A 15 kW, 400 V three-phase motor with η = 88% and PF = 0.85 has rated current I = 15000 / (√3 × 400 × 0.88 × 0.85) ≈ 31.2 A. Direct-On-Line starting produces inrush ≈ 186 A (5.95×), which stresses switchgear and causes voltage dips. Star-Delta reduces this to ≈ 62 A (1.98×) with starting torque ≈ 65% of full torque. A solid-state soft starter limits inrush to ≈ 78 A (2.5×) while delivering 80% starting torque, protecting 10 m cable runs from I²R losses and eliminating mechanical shock to driven equipment.
Practical Notes
DOL is cost-effective for small motors (≤5.5 kW) on stiff grids; avoid on long distribution lines where voltage sag exceeds 3% rated voltage
Star-Delta requires three-phase motor windings rated for dual voltage and three-phase contactors; starting time ≈ 5–10 s before switching to delta
Soft starters prevent nuisance tripping of upstream protection and reduce mechanical fatigue on gearboxes, pumps, and fans by ramping current linearly over 10–30 seconds
Starting current verdict: "Acceptable" ≤ 3.5× for grid-connected motors, "Review" 3.5–5.5×, "Reduce" >5.5× to avoid coordination with cable and transformer protection