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How does a soft starter work?

Put 400 V straight onto a stopped motor and it draws 6–8 times its rated current. A soft starter raises the voltage slowly with thyristors and closes the bypass contactor when the ramp ends. Switch the state at the top and follow the energy, then compare the three methods' peak currents in the simulation.

🔍 Explore🛠 Find the fault

▶ Show me

⏹ Stopped📈 Start ramp✅ Bypass (full speed)📉 Soft stop

⛶ Enlarge

Grid

The 3 phases at the panel input: 400 V between phases, 230 V phase to neutral, 50 Hz. Whatever the motor draws comes from here; with direct-on-line starting the inrush is 6–8 times the rated current, so the supply sags, lights flicker and fuses blow. A soft starter's main job is to soften that sudden load.

Oscilloscope

Multimeter400 V AC (L1–L2), 50 Hz

Tap a block to see what it does and what you'd measure there.

⚙ Compare three starting methods

Start the same 22 kW pump motor (In = 41 A) three different ways. Pick a method, press Start and watch the line current, motor speed and one phase's voltage. Each method's peak current stays in the table below.

Direct on line (DOL)Star-deltaSoft starter

▶ Start↺ Reset

Start ramp time10 sInitial voltage%40

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Rotor slowed down for viewing.

Line current: 0 A (0.0× In)

Motor voltage: %0

L1 phase voltage at the motor

Firing angle α–

Simplified: the motor's inductive load distorts the wave a little more.

Soft starter

Motor stopped. Press Start.

The thyristors ramp the voltage from the initial value to 100%; current and torque rise smoothly. When the ramp ends the bypass contactor closes. Set the initial voltage too low and the motor won't turn for a while (its torque can't beat the load): try it with the slider.

Results (last run)

Direct on line (DOL)not run yet

Star-deltanot run yet

Soft starternot run yet

⚠ On a real panel 400 V is lethal. Make sure the front cover is closed even before touching the setting pots; for maintenance switch off, lock, tag and prove there's no voltage.

🔧 Soft starter panel: what's where?

This drawing isn't a photo; it follows the typical layout of a 22 kW pump panel. At the top the motor protection breaker, in the middle the soft starter with its front cover off (thyristor modules, heatsink, bypass contactor, control board), at the bottom the motor's terminal box. Follow the energy for each mode and tap a region.

Energy path:

📈 Start (thyristors)✅ Bypass (contactor)🎛 Control (24 V)

▶ Show me

⛶ Enlarge

Tap a region to see what it does and how to spot it on the board.

📷 Real-world view

Fotoğraf: ZianMan, CC BY-SA 4.0, Wikimedia CommonsFotoğraf: Mister rf, CC BY-SA 4.0, Wikimedia CommonsFotoğraf: Asurnipal, CC BY-SA 4.0, Wikimedia Commons

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⚠ Safety: The power circuit is 400 V and lethal. Before opening the cover, switch off, lock, tag and prove dead on all three phases. The control circuit may be fed from a separate 230 V source; isolate that too. A shorted thyristor can put voltage on the motor terminals even while the motor is stopped.