Soft Starter (3-Phase) Symbol
Definition: The Soft Starter (3-Phase) symbol represents a reduced-voltage motor starting device that uses anti-parallel thyristor (SCR) pairs in each of the three supply phases — with line terminals L1, L2, L3 and output terminals T1, T2, T3 — to gradually ramp supply voltage from a low starting level to full voltage, limiting motor inrush current and mechanical shock during start-up, per IEC 60947-4-2.
Also known as: soft starter, reduced voltage soft starter, RVSS, thyristor motor starter, electronic soft start, solid-state starter.
What the Soft Starter (3-Phase) symbol means
The Soft Starter (3-Phase) symbol in a wiring diagram indicates a device inserted in the three-phase supply to a motor that controls the rate of voltage rise during start-up. By gradually increasing the motor terminal voltage from a reduced starting value (typically 30–70% of supply) to full voltage over a programmable ramp time (1–30 seconds), the soft starter limits the starting current to typically 2–4× full-load current rather than the 5–8× inrush of a direct-on-line (DOL) start.
The soft starter symbol communicates that the motor feeder path passes through a thyristor-controlled voltage reduction stage. The L1, L2, L3 supply-side terminals receive full line voltage; the T1, T2, T3 load-side terminals deliver the ramped voltage to the motor. After the motor reaches full speed, the internal thyristors are fully conducting (or a bypass contactor closes) to deliver full voltage for running.
How to identify the Soft Starter (3-Phase) symbol
The Soft Starter (3-Phase) symbol is drawn as a rectangular block with three line terminals (L1, L2, L3) on the left side and three load terminals (T1, T2, T3) on the right side. The block is labelled 'Soft Starter', 'SS', or 'RVSS'. Some representations show internal thyristor symbols (anti-parallel SCR pairs) within the rectangle to indicate the semiconductor switching mechanism. The six terminals (three in, three out) clearly distinguish the soft starter from a single-pole contactor or VFD symbol.
Function in a circuit
The soft starter uses anti-parallel thyristor (SCR) pairs — one pair per phase — to phase-angle fire, controlling the percentage of each AC half-cycle delivered to the motor. At start, the firing angle is large (low voltage); it reduces progressively over the programmed ramp time until the thyristors conduct for the full cycle (100% voltage). This reduces motor starting current (typically to 2–4× FLA), limiting mechanical stress on the load (couplings, gearboxes), reducing mains voltage dip, and reducing water-hammer in pump systems. Most soft starters also provide soft-stop (ramp-down) capability for pump applications.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-4-2 governs AC semiconductor motor controllers and starters, which includes soft starters. IEC 60947-4-2 defines test conditions, overload classifications, and type-test requirements. The soft starter symbol in IEC 60617 follows general switching-device block conventions with L and T terminal labelling. |
|---|---|
| ANSI/IEEE 315 | ANSI/UL 508 covers motor control equipment including solid-state controllers. NEMA ICS 2 Part 3 covers solid-state reduced-voltage starters. ANSI Y32.2 / IEEE 315-1975 uses a rectangular block for the soft starter with L1-L3 and T1-T3 terminal labels. |
| Key difference | IEC 60947-4-2 and ANSI/UL 508 both represent the soft starter as a rectangular block with three input and three output terminals. IEC symbols use L1/L2/L3 and T1/T2/T3 labelling; ANSI/NEMA drawings may label supply terminals L1/L2/L3 and motor terminals T1/T2/T3 identically. No glyph difference between 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: 7 A–1700 A (wide range of frame sizes); typical starting current: 200–400% FLA (vs. 500–800% for DOL); ramp time: 1–30 s (adjustable); starting voltage: 30–80% of supply (adjustable); protection features: phase loss, phase unbalance, overtemperature, SCR fault.
Where the Soft Starter (3-Phase) symbol is used
- Centrifugal pump systems: soft starters eliminate water hammer and pressure surge by ramping pump motor speed gradually at start and stop
- Conveyor and belt drive systems: soft starters reduce mechanical shock and belt wear during conveyor motor start-up under load
- Compressor starts in HVAC systems: soft starters limit mains voltage dip caused by compressor motor starting current in buildings with weak supply
- Fan and blower systems: soft starters reduce starting torque shock on fan impellers and drives in air handling units
- Crushers and mixers in process industry: soft starters allow high-inertia loads to accelerate smoothly without mechanical shock
- Lift and escalator motor starters: soft starters provide smooth acceleration and deceleration for passenger comfort
Example
In a water utility pump station single-line diagram, the Soft Starter symbol (L1, L2, L3 connected to the 400 V three-phase bus via a main circuit breaker; T1, T2, T3 connected to the 75 kW centrifugal pump motor) is set with a 10-second voltage ramp and a starting voltage of 40% nominal. On start command, the soft starter ramps the motor voltage from 160 V to 400 V over 10 seconds, limiting starting current to 280% FLA and preventing the pressure surge that would occur with a DOL start.
Key facts
- The Soft Starter (3-Phase) symbol represents a thyristor-based reduced-voltage motor starting device with L1, L2, L3 supply terminals and T1, T2, T3 motor output terminals, per IEC 60947-4-2.
- Soft starters limit starting current to typically 200–400% of motor FLA, compared to 500–800% for direct-on-line (DOL) starting.
- The soft starter uses anti-parallel SCR (thyristor) pairs in each phase, controlled by phase-angle firing to ramp the motor terminal voltage.
- Ramp time (the duration to increase from starting voltage to full voltage) is typically adjustable from 1 to 30 seconds to suit the load inertia.
- After the motor reaches running speed, most soft starters either close an internal bypass contactor or conduct thyristors at 100% to eliminate power dissipation in the SCRs during normal running.
- Soft starters are governed by IEC 60947-4-2; they are used where DOL starting causes excessive mains voltage dip, mechanical shock, or water hammer.
- Unlike a VFD (variable frequency drive), a soft starter does not control motor speed during running — it only controls voltage during the start (and optionally stop) transient.
Frequently asked questions
What does the soft starter symbol mean in a wiring diagram?
The soft starter symbol means the motor circuit includes a reduced-voltage starting device that gradually ramps supply voltage from a low starting level to full voltage. It limits motor inrush current and mechanical shock during start-up compared to a direct-on-line (DOL) start.
What does the soft starter symbol look like?
The soft starter symbol is a rectangle with three supply terminals (L1, L2, L3) on the left side and three motor output terminals (T1, T2, T3) on the right side. It may be labelled 'Soft Starter', 'SS', or 'RVSS'. Some representations show internal thyristor (SCR) symbols within the block.
What is the difference between a soft starter and a VFD (variable frequency drive)?
A soft starter controls motor voltage only during start-up and stop; once running, the motor operates at full supply frequency and voltage. A VFD controls both voltage and frequency continuously, allowing variable speed throughout the entire run cycle. Soft starters are less expensive; VFDs provide full speed control.
What is the difference between a soft starter and a star-delta starter?
A star-delta starter reduces starting current by switching the motor windings from star to delta (reducing voltage to 58% at start) in two discrete steps; the transition creates a second current transient. A soft starter provides a continuous smooth ramp with no switching transient, giving better inrush limiting and smoother acceleration.
What are the terminal labels on a soft starter?
Per IEC 60947-4-2 and standard industry practice, soft starter supply terminals are labelled L1, L2, L3 (line side) and motor output terminals are labelled T1, T2, T3. These correspond to the three phases of the three-phase AC supply and motor connection.
What IEC standard governs soft starters?
Soft starters are governed by IEC 60947-4-2 (AC semiconductor motor controllers and starters including soft starters). This standard defines performance ratings, overload classification, and type-test requirements for electronic reduced-voltage starters.
Does a soft starter reduce motor speed?
No — a soft starter does not reduce motor running speed. It only controls the rate of voltage rise during motor start-up (and ramp-down during soft-stop). Once the motor reaches full speed, the soft starter delivers full line voltage and the motor runs at its rated synchronous or near-synchronous speed.
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