What is a soft starter?
A soft starter is an electronic motor starting device that gradually raises the voltage applied to a three-phase induction motor during start-up, so that inrush current and starting torque stay limited and the motor reaches full speed without a shock. Where a motor started direct-on-line (DOL) draws six to eight times its rated current, a soft starter typically keeps this to two to four times. Many soft starters also let the motor coast down in a controlled way when stopping. That makes the soft starter the low-cost middle ground between a simple motor starter and a variable-frequency drive.
A soft starter only controls starting and stopping; in normal operation the motor simply runs on the full supply at a fixed speed.
🧠 How does a soft starter work?
Each controlled phase contains two thyristors (SCRs) in anti-parallel. By firing them a little earlier in every half-cycle (phase-angle control), the effective motor voltage rises from a start level, for example 30%, to 100%. Because the torque of an induction motor scales roughly with the square of the voltage, 50% voltage yields only about 25% of the normal starting torque.
A modern soft starter offers several control modes:
- Voltage ramp - the voltage rises linearly over a set time; simple, but torque does not follow the voltage linearly
- Current limit - when a set current is reached (for example 3.5 × rated), the soft starter holds the voltage until the current falls
- Torque control - the soft starter calculates torque from voltage and current and follows a predefined torque curve; this gives the softest mechanical start and stop
- Kick start - a short voltage pulse at the beginning to overcome breakaway torque, for instance on a conveyor stuck under load
Once the motor is up to speed, a bypass contactor shorts out the thyristors. Most modern devices have this bypass built in. Conducting thyristors have a voltage drop of roughly 1 to 2 V and therefore run hot; with a bypass these losses disappear and the control cabinet has far less heat to dissipate.
🔧 Which designs and connection methods exist?
| Feature | Variant | What it means |
|---|---|---|
| Controlled phases | Two-phase | Cheaper and more compact; the uncontrolled phase causes some current imbalance |
| Controlled phases | Three-phase | Symmetrical current, better for heavy starts and torque control |
| Bypass | Without bypass (AC-53a) | Thyristors carry current continuously; more heat, but rated for continuous duty |
| Bypass | With bypass (AC-53b) | Thyristors only conduct during ramp-up and ramp-down |
| Connection | In-line | Three wires to the motor; soft starter sized for line current |
| Connection | Inside-delta | Soft starter inside the delta of the windings; carries 58% (1/√3) of line current, needs six motor leads and a line contactor |
The utilisation categories AC-53a and AC-53b come from IEC 60947-4-2, the product standard for AC semiconductor motor controllers and starters. The current version is edition 4.0 from 2020, consolidated with amendment 1 into edition 4.1 in 2024. In Europe the adopted version is EN IEC 60947-4-2:2023. The standard also defines a rating code: AC-53a: 3.5-30: 99-1 means a current limit of 3.5 × rated current, a 30-second start, a 99% on-load factor and one start per hour. For AC-53b the last figure is the minimum off-time between two starts in seconds.
🔍 What is the difference between DOL, star-delta, soft starter and VFD?
| Starting method | Inrush current | Starting torque | Speed control | Typical application |
|---|---|---|---|---|
| DOL | 6–8 × rated (higher for high-efficiency motors) | Full, a multiple of rated torque | No | Small motors, simple duties |
| Star-Delta | approx. 1/3 of DOL | approx. 1/3 of DOL, with a current surge at changeover | No | Pumps and fans that start unloaded |
| Soft starter | Adjustable, approx. 2–4 × rated | Adjustable, rising smoothly | No, only ramp-up and ramp-down | Pumps, fans, compressors, conveyors |
| Variable Frequency Drive | ≤ approx. 1.5 × rated | High torque from standstill possible | Yes, stepless | Processes with varying demand |
A variable-frequency drive is technically the most flexible, but it is more expensive, larger and produces harmonics continuously. A soft starter only distorts the supply briefly during ramp-up and ramp-down; once bypassed it contributes no harmonics at all. So choose a soft starter whenever a fixed speed is acceptable and only the start is the problem.
🏭 Where are soft starters used?
- Pumps - a soft stop with torque control prevents water hammer in pipelines, typically at water board pumping stations and drinking water pumping plants
- Fans and blowers - large flywheel masses come up to speed without a current peak and without slipping V-belts
- Compressors and conveyors - less wear on couplings, gearboxes and belts
- Weak grids or standby power - a smaller generator or transformer is sufficient because the voltage dip during start-up is smaller
Soft starters usually sit as a functional unit in an MCC or control cabinet, together with a circuit breaker or fuses for short-circuit protection. Most soft starters include their own electronic overload protection with a selectable trip class (10, 20 or 30), plus monitoring for phase loss, undervoltage, underload, excessive start time and locked rotor.
Manufacturers include Siemens (SIRIUS 3RW), ABB (PSR, PSE, PSTX) and Schneider Electric (Altivar Soft Starter ATS).
🧭 How do you select and size a soft starter?
- Determine the load - light (centrifugal pump, fan without large inertia) or heavy (mill, loaded conveyor, fan with a large flywheel). Heavy starts need a higher current limit and a longer ramp.
- Check the start time - as a guide, an unloaded motor starts with a soft starter in about 1 second, a load with a small flywheel in around 6 seconds and a heavy flywheel load in 8 to 12 seconds.
- Count the starts per hour - frequent starts heat the thyristors; check the AC-53 rating code and choose a larger size if needed.
- Choose connection and bypass - inside-delta can allow a smaller device for large motors; a bypass limits heat generation.
- Consider the environment - altitude and ambient temperature above the datasheet value require derating.
- Define the communication - which status and diagnostic data do you want in the PLC or SCADA?
For example, a soft starter with torque control on a sewage pumping station pump can let the motor ramp down over 10 to 20 seconds, so the non-return valve closes gently instead of slamming shut.
🔐 What does a soft starter mean for OT and cybersecurity?
Basic soft starters are hard-wired via digital IO: start, stop, running feedback and fault. More and more devices, however, have a network connection. Siemens’ SIRIUS 3RW55, for example, takes plug-in modules for ProfiNET, Profibus, IP and Modbus, and offers functions such as pump cleaning and condition monitoring; its failsafe versions include integrated Safe Torque Off for functional safety, on its own up to SIL 1 / PL c and up to SIL 3 / PL e with an added safety relay and safety contactor. Current, motor temperature and number of starts then become usable for predictive maintenance.
This also turns the soft starter into an OT asset with an attack surface:
- Remote parameters - a changed ramp time, current limit or trip class can overheat a motor or cause water hammer
- Network Segmentation - place fieldbus and drive networks in their own zone according to IEC 62443, never on the office network
- Access Control - restrict who may change parameters with engineering software and disable unused services
- Asset Inventory - record the type, firmware and communication module of every soft starter
For critical installations a soft starter can be combined with redundancy, for example a second pump or a DOL emergency mode via the bypass contactor.
❓ Frequently asked questions
Does a soft starter save energy?
A soft starter saves virtually no energy during operation, because after start-up the motor runs at full supply voltage and fixed speed. Its benefits are lower inrush current, smaller voltage dips and less mechanical wear. If you want to save energy on pumps and fans with varying demand, choose a variable-frequency drive instead.
Can a soft starter brake a motor?
A soft starter does not normally brake a motor actively; it lets the motor coast down in a controlled way by reducing the voltage gradually. That actually extends the stopping time, which prevents water hammer in pump systems. Some soft starters also offer a separate DC braking function to shorten the run-down of high-inertia loads.
Does a soft starter cause harmonics?
A soft starter only causes harmonics during ramp-up and ramp-down, when the thyristors chop the voltage. Once the bypass contactor has closed, the motor runs directly on the supply and the soft starter adds nothing. A variable-frequency drive, by contrast, produces harmonics continuously.
What is the difference between AC-53a and AC-53b?
AC-53a is the IEC 60947-4-2 utilisation category for a soft starter that carries the motor current continuously through its thyristors. AC-53b applies to a soft starter with a bypass, where the thyristors only conduct during starting and stopping. The rating code that follows states the current limit, start time and permitted starts or off-time.
Can every motor be started with a soft starter?
A soft starter suits almost any three-phase squirrel-cage motor, as long as the load still gets enough torque at reduced voltage. Loads with a high breakaway torque may stall at low voltage, in which case a kick start or a variable-frequency drive is needed. For an inside-delta connection the motor must have six leads and be wound for delta at the supply voltage.
How do you connect a soft starter to a PLC?
You connect a soft starter to a PLC via digital inputs and outputs for start, stop and fault, or via a fieldbus such as PROFINET, PROFIBUS, EtherNet/IP or Modbus. A fieldbus connection provides extra diagnostics, such as motor current, thermal load and fault codes. Secure such a network connection like any other OT device, with segmentation and access control.
📌 In summary
A soft starter uses thyristors to raise the motor voltage gradually, cutting inrush current to roughly 2–4 × rated and removing the mechanical shock, without the cost and complexity of a variable-frequency drive. Use it for fixed-speed pumps, fans, compressors and conveyors, size it for load and starts per hour according to IEC 60947-4-2, and treat networked units as fully fledged OT assets.