Soft Starter Symbol
Definition: The Soft Starter symbol represents a reduced-voltage motor-starting device drawn as a labeled rectangle with three input terminals (L1, L2, L3) and three output terminals (T1, T2, T3), denoting a power-electronics controller that ramps AC voltage gradually to limit inrush current during motor start-up, as referenced in IEC 60947-4-2 (low-voltage motor starters) and ANSI/NEMA ICS 2.
Also known as: ATS, reduced-voltage soft starter, RVSS, electronic soft starter, motor soft starter.
What the Soft Starter symbol means
The Soft Starter symbol denotes an electronic device inserted in series with a three-phase AC motor supply that controls the voltage applied to the motor windings during starting and, optionally, stopping. By using back-to-back thyristors (SCRs) on each phase, the soft starter ramps the terminal voltage from a low initial value up to full line voltage over a programmable ramp time, preventing the high inrush current and mechanical shock associated with direct-on-line (DOL) starting.
In a schematic, the soft starter symbol bridges the main supply bus and the motor terminals, indicating that this device governs the energy flow during transient conditions while passing full voltage during normal running. Some soft starters also provide built-in overload protection and communication interfaces, making the symbol a shorthand for an intelligent motor management block.
How to identify the Soft Starter symbol
The Soft Starter symbol appears as a rectangular box labeled 'SOFT START' (or 'ATS' or 'SS') with three line connections entering the top edge labeled L1, L2, and L3 (supply side) and three connections leaving the bottom edge labeled T1, T2, and T3 (motor side). A curved ramp waveform inside the box — rising from a low amplitude to a full sine wave — represents the voltage-ramping function. This distinguishes it from a plain contactor rectangle (no ramp curve) and from a star-delta starter (which shows Y and delta notation).
Function in a circuit
The soft starter limits motor inrush current by reducing the RMS voltage applied to the stator windings during acceleration. Internal back-to-back SCR pairs on each phase are phase-angle fired, progressively increasing the conduction angle and therefore the delivered voltage over a set ramp period (typically 2–30 seconds). This limits starting current to 2–4× full-load current, compared with 6–8× for DOL starting, reducing mechanical stress on couplings and driven equipment and avoiding voltage sags on the supply network. During steady-state running, most soft starters bypass the SCRs with a mechanical contactor to eliminate SCR conduction losses.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-4-2 covers electronic soft starters for AC motors. The IEC schematic symbol is a rectangle with the label 'soft starter' or manufacturer abbreviation; IEC 60617 does not assign a unique graphical symbol — designers use a labeled rectangle with terminal identifiers L1–L3 / T1–T3. |
|---|---|
| ANSI/IEEE 315 | ANSI/NEMA ICS 2 and IEEE 315 do not define a single standardised graphical symbol for soft starters; US practice mirrors IEC — a labeled box with L1–L3 input and T1–T3 output terminals is used universally. |
| Key difference | No glyph difference exists between IEC and ANSI representations; both use a labeled rectangle. The terminal labeling convention (L1/L2/L3 in and T1/T2/T3 out) is consistent across both standards. |
Terminals / pins
| Pin | Name |
|---|---|
| l1 | L1 |
| l2 | L2 |
| l3 | L3 |
| t1 | T1 |
| t2 | T2 |
| t3 | T3 |
Typical values
Supply voltage: 200–690 V AC three-phase, 50/60 Hz. Current rating: 6 A to over 1200 A. Starting current limit: typically 200–450% of motor full-load current. Ramp time: 1–30 s (adjustable). Motor power range: 0.4 kW to over 800 kW.
Where the Soft Starter symbol is used
- Large pump and compressor stations where water hammer or pressure surge on DOL start must be avoided
- Conveyor and belt drive systems requiring gentle load acceleration to protect mechanical couplings
- Fan and HVAC air-handling units where slow ramp-up prevents belt slip and bearing shock
- Industrial centrifuges and mixers needing controlled torque during start to protect gearboxes
- Water treatment plants where frequent pump cycling demands current limiting to protect switchgear
- Oil and gas applications with submersible pumps where cable voltage drop during start is critical
Example
In a 55 kW cooling-tower pump circuit, a soft starter is connected between the main 400 V three-phase bus and the pump motor terminals. L1, L2, and L3 are wired from the upstream MCCB; T1, T2, and T3 connect to the motor. A programmable ramp time of 8 seconds limits the starting current to 300% FLA, preventing nuisance trips of the upstream protection device and eliminating the pressure surge that caused pipe joint failures under DOL starting.
Key facts
- A soft starter is a reduced-voltage starting device that uses back-to-back SCR (thyristor) pairs to phase-angle control the supply voltage applied to a three-phase AC motor, as defined in IEC 60947-4-2.
- The schematic symbol is a labeled rectangle with three input pins (L1, L2, L3) on the supply side and three output pins (T1, T2, T3) on the motor side, often containing a rising-ramp waveform icon.
- Soft starters limit motor inrush current to approximately 200–450% of full-load current, compared with 600–800% for direct-on-line (DOL) starting.
- The standard schematic designator for a soft starter is SS or ATS (Adjustable-speed/soft-starter); IEC uses SS and NEMA practice varies by manufacturer.
- Most soft starters include an integral bypass contactor that short-circuits the SCRs after the motor reaches full speed, eliminating SCR conduction losses during steady-state operation.
- Ramp time is a key programmable parameter, typically adjustable from 1 second to 30 seconds, matching the mechanical load inertia.
- Unlike a variable-frequency drive (VFD), a soft starter does not change the supply frequency, so it cannot provide speed control during continuous running — only controlled starting and stopping.
- Soft starters are specified in three-phase configurations for motors from approximately 0.4 kW to over 800 kW at supply voltages of 200–690 V AC.
Diagrams that use this symbol
Frequently asked questions
What does the soft starter symbol look like in a circuit diagram?
The soft starter symbol is a labeled rectangle showing 'SOFT START' or 'SS' with three input terminals at the top (L1, L2, L3) and three output terminals at the bottom (T1, T2, T3). A rising ramp waveform inside the box represents the voltage-ramping function that distinguishes it from a plain contactor or switch symbol.
What does a soft starter do in an electrical circuit?
A soft starter reduces the voltage applied to an AC motor during start-up by phase-angle firing internal SCR pairs, limiting inrush current to 200–450% of full-load current. This prevents mechanical shock on driven equipment and voltage sags on the supply bus. After the motor reaches full speed, most soft starters bypass the SCRs with an internal contactor.
What is the difference between a soft starter and a DOL starter symbol?
A DOL (direct-on-line) starter symbol shows a contactor coil and main contacts with no voltage-control element, indicating that full voltage is applied instantly. The soft starter symbol adds an internal ramp-waveform or 'SOFT START' label inside the rectangle, indicating controlled voltage ramp-up. In wiring diagrams, the soft starter also shows L1–L3 input and T1–T3 output terminal pairs identical to an MCCB but with the ramp notation.
What standard defines the soft starter symbol?
IEC 60947-4-2 defines the performance and testing requirements for electronic soft starters for AC motors. No unique IEC 60617 graphical symbol is assigned; both IEC and ANSI/NEMA ICS 2 practice uses a labeled rectangle with L1–L3 input and T1–T3 output terminal identifiers.
What is the designator letter for a soft starter?
The most common schematic designator for a soft starter is SS or ATS (Adjustable-speed Torque Starter). Some manufacturers and plant standards use the designator U followed by a number (e.g., U1) for variable-speed or reduced-voltage starter blocks. IEC 61346 (reference designations) classifies motor starters under designator type A (assemblies).
What is the difference between a soft starter and a VFD in a schematic?
In a schematic, a VFD (variable-frequency drive) symbol typically shows AC input on one side and a frequency-variable AC output on the other, often labeled with a sine wave of variable frequency, whereas the soft starter symbol shows the same frequency at input and output with only amplitude variation indicated. Functionally, a VFD controls speed continuously by varying both voltage and frequency; a soft starter only controls voltage during start and stop transitions.
How many terminals does a soft starter symbol have?
A three-phase soft starter symbol has six main power terminals: L1, L2, and L3 on the line (supply) side, and T1, T2, and T3 on the load (motor) side. Additional control terminals for enable, fault output, and communications are shown separately on the symbol or in an accompanying control circuit diagram.
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