Different Types of Soft Starters: A Complete Guide for Engineers
A soft starter is a thyristor-based motor controller that ramps voltage from a reduced level to full line voltage during acceleration, per IEC 60947-4-2. Choosing the correct topology prevents SCR failure, nuisance tripping, and inrush non-compliance.
Why Soft Starter Topology Matters More Than Brand
In our experience, procurement teams often spend weeks comparing ABB PSE versus Schneider ATS480 versus Siemens 3RW55 — then specify the wrong topology entirely. The brand argument is almost irrelevant if you've chosen a two-phase controlled unit for a 250 kW high-inertia centrifuge that needs 30-second ramps every 20 minutes. It will fail. Repeatedly.
Soft starters are not commodities. The internal SCR configuration determines whether your motor sees balanced torque during acceleration, how much heat the device dissipates, and whether you can integrate a bypass contactor. Get this wrong and the consequences show up six months later as cracked thyristors, tripped overloads, and a maintenance ticket queue.
For the fundamentals of how voltage ramping works at the SCR level, our companion article on how soft starters work explained for engineers walks through the firing-angle control logic. This guide focuses on classification.
The international standard governing soft starter design, testing, and AC-53 utilization categories is published by the IEC as IEC 60947-4-2 AC Semiconductor Motor Controllers.
The Four Main Types of Soft Starters by SCR Configuration
The most fundamental classification — and the one that determines 80% of application suitability — is how many phases the soft starter actively controls and how the SCRs are arranged. There are four configurations you will encounter in industrial catalogs.
1. Single-Phase Controlled (One-Phase) Soft Starters
These control a single phase via back-to-back SCRs while the other two phases pass through unrestricted. They reduce starting current modestly — typically to 4–5 times full-load amps versus 6–8 across-the-line — but they do not balance torque. Motor heating during start is uneven. They are essentially obsolete in three-phase industrial applications and are rarely supplied today except as legacy retrofits for fractional horsepower fans below 7.5 kW.
2. Two-Phase Controlled Soft Starters
Two of the three phases are controlled; the third is hardwired through. This is the budget option in many low-cost units. The torque is unbalanced during ramp, which causes audible vibration and accelerated bearing wear on continuous-start applications. Acceptable for pumps with low inertia and infrequent starting (under 6 starts per hour), but engineers should avoid them for compressors, conveyors, or anything with high breakaway torque.
3. Three-Phase Controlled (Full-Wave) Soft Starters
All three phases use back-to-back SCRs (six SCRs total). Torque is balanced, current is symmetric, and the device complies with IEC 60947-4-2 Annex B for type-tested coordinated starting. This is the default specification for any motor above 30 kW and the only acceptable choice for high-inertia loads. The ABB PSTX, Schneider ATS480, and Siemens 3RW55 series all fall into this category at their mid and upper ratings.
4. Inside-Delta (Six-Wire) Soft Starters
The soft starter is wired in series with each motor phase winding inside the delta connection. Because the SCRs only carry phase current (line current divided by √3), the controller can be sized roughly 58% smaller than an equivalent in-line unit. The wiring is more complex and requires access to all six motor terminals, but the cost savings on large motors (above 200 kW) are substantial. Common in pumping stations and large fan applications.
Classification by Control Method
Beyond SCR topology, soft starters differ in how they regulate the ramp itself. This affects starting performance more than most engineers realize.
Open-Loop Voltage Ramp Control
The simplest method. The controller follows a pre-programmed voltage-versus-time curve regardless of motor behavior. Cheap, predictable, and adequate for fixed-load applications like fixed-displacement pumps. The drawback: if load conditions change (a clogged filter, a cold start, a different fluid), the motor may stall or accelerate too quickly. In the field, we still see open-loop ramp on perhaps 60% of installed soft starters because the application is constant.
Closed-Loop Current Limit Control
The controller monitors motor current and modulates SCR firing angle to hold current at a programmed limit (typically 300–400% FLA). This is essential when the upstream supply is constrained — generator sets, weak grids, or installations with utility-imposed inrush limits. The trade-off is longer start times under heavy load.
Torque Control (Closed-Loop)
Premium controllers like the ABB PSTX and Schneider ATS480 calculate motor torque in real time and ramp torque linearly rather than voltage. This produces the smoothest mechanical acceleration and is critical for conveyor systems, where a sudden torque step damages belts and gearboxes. Engineers often overlook this on conveyor procurement, then spend two years replacing couplings.
Soft Starter Sizing Per IEC 60947-4-2 AC-53 Utilization
Sizing is where soft starter procurement most often goes wrong. The nameplate current is meaningless without the AC-53 duty class.
Formula: AC-53a Duty Class Designation — Source: IEC 60947-4-2 §4.4
AC-53a: Ie × X-Tx-Sy
| Symbol | Description | Unit |
|---|---|---|
| Ie | Rated operational current | A |
| X | Multiple of Ie during start (e.g., 4 = 400% FLA) | — |
| Tx | Start duration | s |
| Sy | Starts per hour | 1/h |
A designation like "AC-53a: 105 A × 4-30-10" means the device handles 105 A continuous, 420 A starting current for 30 seconds, 10 starts per hour. If your application demands 15 starts per hour, that unit will overheat — regardless of brand reputation.
For applications without bypass contactors, the AC-53a class applies. With an integrated or external bypass contactor, the AC-53b class applies, which gives much higher continuous current capacity because the SCRs only carry current during the ramp itself.
In-Line vs Inside-Delta vs Bypass Configurations
The wiring topology affects cost, footprint, and serviceability. We typically see three configurations in industrial MCCs.
| Criteria | In-Line (3-Wire) | Inside-Delta (6-Wire) | In-Line with Bypass |
|---|---|---|---|
| Wiring complexity | Low | High | Medium |
| Controller current rating | 100% FLA | 58% FLA | 100% FLA (start), bypassed at run |
| Heat dissipation at run | ~3 W per A | ~3 W per A | Negligible |
| Required terminals | 3 motor leads | 6 motor leads | 3 motor leads |
| Best application | General purpose | Large motors >200 kW | High-cycling, hot ambient |
| Typical cost (relative) | 1.0× | 0.6× | 1.15× |
Bypass configurations deserve special attention. A common mistake is assuming any 200 A contactor will work as a bypass. It will not — the bypass contactor must be rated AC-1 (resistive equivalent) at the motor's full-load current with appropriate utilization category. For a 75 kW motor at 400 V, an ABB 1SFL447101R1311 AF140-40-11-13 200 A AC-1 contactor is the correctly specified bypass element. Browse the full range of contactors and relays at Stoklink for matched bypass solutions.
Coordination with Upstream Protection Devices
A soft starter is one component in a coordinated motor branch circuit. IEC 60947-4-2 §8.2.5 specifies type-tested combinations for short-circuit coordination, and the responsibility for verifying coordination falls on the panel builder or specifying engineer.
For motors below 16 A, a manual motor protection device such as the ABB 1SAM360000R1011 MO132-16 motor protection circuit breaker provides the short-circuit element ahead of the soft starter. For very small motors below 6.3 A, the ABB 1SAM360000R1009 MO132-6.3 works similarly. Below 0.25 A — typically dosing pumps or instrumentation drives — the ABB 1SAM360000R1002 MO132-0.25 covers the lower end.
For larger feeders above 800 A, you need a moulded case circuit breaker with electronic trip such as the ABB 1SDA101711R1 XT7H 1000 A 100 kA MCCB with Ekip Dip LS/I trip unit. The Ekip trip's adjustable instantaneous magnetic setting is essential — set it too low and the breaker will trip on the soft starter's own SCR turn-on transient.
For the broader feeder protection picture, the air circuit breakers collection covers higher ratings, while MCBs and RCDs handle smaller branch circuits.
Soft Starter vs Other Motor Starting Methods
Engineers often ask whether a soft starter or a star-delta starter is the right call. The honest answer: it depends on duty cycle, supply impedance, and how much you care about start quality. We cover the full comparison in our soft starter versus star-delta starter analysis, but the short version is this — star-delta gives you two fixed torque steps and 33% inrush reduction, while a soft starter gives you a continuous adjustable ramp and current limiting. Star-delta is cheaper for occasional starts; soft starters win on anything cycling more than 6 starts per hour or feeding high-inertia loads.
For applications requiring full speed control rather than just controlled starting, a variable frequency drive (VFD) is the answer — but VFDs cost roughly 3–4× more than equivalent soft starters and introduce harmonic distortion that may require filtering. If you only need to start gently and run at 50/60 Hz, paying for a VFD is overspecification.
Some installations also use auxiliary control elements such as the ABB 1SFA611410R1056 MT-105B potentiometer for analog setpoint adjustment on the soft starter's ramp parameters from a panel-mounted HMI.
Selecting the Right Type for Your Application
After 20 years of specifying these things, here's the decision framework that holds up.
Constant-load pumps under 75 kW, infrequent starts: A two-phase or three-phase open-loop voltage ramp is fine. ABB PSE or Siemens 3RW40 class.
Conveyors, mixers, mills, anything belt-driven: Three-phase torque control, full stop. The ABB PSTX or Schneider ATS480 with torque ramp eliminates mechanical shock entirely.
Compressors and high-inertia fans above 75 kW: Three-phase closed-loop current limit, with bypass contactor. The continuous heat dissipation savings from bypassing alone justify the additional contactor.
Large pumps above 200 kW where panel space is tight: Inside-delta connection with three-phase controller. The 42% size reduction matters when you're squeezing into an existing MCC.
Generator-fed installations: Always closed-loop current limit, regardless of motor size. The genset cannot tolerate uncontrolled inrush, and a voltage-ramp open-loop unit will sag the bus and trip the generator AVR.
Common Field Failures and What They Tell You About Type Selection
In our experience, the failure modes of soft starters are surprisingly consistent and almost always trace back to the wrong type being chosen.
SCR thermal failure within 12 months: Almost always undersized AC-53 duty class. The customer specified 6 starts per hour but the application actually does 20.
Nuisance overload trips during start: Open-loop voltage ramp on a load with variable breakaway torque. Replace with current-limit control.
Audible vibration during ramp: Two-phase controlled unit on a torque-sensitive load. Upgrade to three-phase controlled.
Bypass contactor welding shut: Bypass contactor undersized or wrong utilization category. The bypass must be sized AC-1 at full motor current with adequate make/break rating.
None of these are brand problems. They are topology problems.
Related Reading
- What Is a Soft Starter? How It Works Explained for Engineers
- Soft Starter vs Star Delta Starter: Key Differences Explained
- What Is a Soft Starter and How Does It Work? Complete Engineering Guide
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Frequently Asked Questions
What are the main types of soft starters?
The four main types classified by SCR configuration are single-phase controlled, two-phase controlled, three-phase controlled (full-wave), and inside-delta. Three-phase controlled is the default for industrial applications above 30 kW because it produces balanced torque during acceleration. Inside-delta is used for large motors where the controller can be sized 42% smaller. For full background on operation, see our explanation of how soft starters work.
What is the difference between AC-53a and AC-53b duty classes?
AC-53a applies to soft starters operating without a bypass contactor, where the SCRs carry continuous current. AC-53b applies when an integrated or external bypass contactor short-circuits the SCRs after the ramp completes, allowing significantly higher continuous current ratings. The duty designation includes start current multiple, start duration, and starts per hour per IEC 60947-4-2 §4.4.
When should I use an inside-delta soft starter instead of in-line?
Inside-delta is appropriate for motors above approximately 200 kW where the 42% reduction in controller size delivers meaningful cost and footprint savings. It requires access to all six motor terminals (both ends of each phase winding) and slightly more complex wiring. Avoid inside-delta on motors with only three terminal connections or in retrofit applications where rewiring the motor is impractical.
Do I need a bypass contactor with my soft starter?
For motors that run continuously after starting, a bypass contactor reduces SCR heat dissipation to zero during running, extending semiconductor life and reducing panel cooling requirements. For applications with frequent starts and short run times, the bypass adds little value. Most modern controllers above 100 A include integrated bypass internally, so external bypass is mainly a concern for older or budget units.
How do I size the upstream circuit breaker for a soft starter?
The upstream circuit breaker must be coordinated per IEC 60947-4-2 type-test tables provided by the manufacturer. The instantaneous magnetic trip should be set to at least 10× the soft starter's rated current to avoid nuisance tripping on SCR turn-on transients. For larger feeders, a moulded case breaker with electronic trip such as the ABB XT7H 1000 A MCCB provides the necessary adjustability.
Can a soft starter replace a variable frequency drive?
No, not if you need speed control. A soft starter only controls voltage during acceleration and deceleration; once at full speed, it operates at line frequency. A VFD provides continuous speed control across the entire operating range. Use a soft starter when you only need controlled starting at a fixed running speed, and a VFD when process variable speed is required.
Conclusion
Soft starter selection is fundamentally a topology decision, not a brand decision. The four SCR configurations — single-phase, two-phase, three-phase, and inside-delta — combined with the choice between open-loop voltage ramp, closed-loop current limit, and torque control, define whether the device will deliver decades of reliable service or fail within the warranty period.
Brand reputation matters, but only after topology and AC-53 duty class are correctly specified.
For procurement managers, the cost-optimization opportunity is real but narrow. Inside-delta connection saves 30–40% on controller cost for large motors. Bypass configurations reduce panel cooling requirements. Standardizing on a single brand family across a facility reduces spare parts inventory. But none of these savings are worth pursuing if the basic topology is wrong for the application.
For the complete selection methodology including site survey checklists, harmonic considerations, and commissioning procedures, see our comprehensive guide on selecting soft starters for industrial motors. For the operational fundamentals at the SCR level, our complete engineering guide on how soft starters work covers firing-angle control and thermal modeling. Get the topology right first, then the brand decision becomes straightforward.