What is a soft starter?

A soft starter is an electronic motor starter that gradually increases the voltage delivered to an AC motor as it starts. A well-matched starter can reduce the current surge and mechanical shock compared with starting the same motor directly across the line. Certain models can also soften a stop. Pumps, fans and conveyors are common candidates, but the required starting torque is the deciding constraint.

It does not vary the supply frequency to control running speed. When the motor needs adjustable speed throughout production, consider a variable frequency drive (VFD). For the underlying motor and starter characteristics, see ABB's Softstarter Handbook.

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How it works  ·  Current and torque  ·  Voltage ramp demo  ·  Bypass and stopping  ·  Sizing and setup  ·  Soft starter vs VFD  ·  FAQ  ·  Related guides

How does a soft starter work?

Power semiconductor switches, usually thyristors (SCRs), control how much of each AC half-cycle reaches the motor during starting. The motor's effective terminal voltage rises from an initial level toward the full line voltage. The utility supply itself is not ramped up; the starter changes the voltage applied to the motor. The AC frequency remains the line frequency. Some models add current limiting or a calculated torque ramp.

A selected voltage ramp time is not a promise that the shaft will reach full speed in precisely that many seconds. Acceleration depends on the motor's torque-speed curve, the process load and inertia, and any active current limit. This is a different control problem from setting a VFD acceleration ramp.

The current and torque trade-off

Reduced starting voltage can lower the initial current, but it also reduces available motor torque. For a basic induction-motor comparison at a given speed, torque is approximately proportional to the square of the applied voltage: at 70% voltage, about 49% of the corresponding full-voltage torque may remain. This is an illustration, not a universal setting or a prediction of final speed. ABB's handbook gives the related example of roughly 25% torque at half voltage.

Picture a loaded conveyor that needs substantial breakaway torque. If initial voltage or the current limit is set too low, the motor may hum and draw current without accelerating. Extending the ramp alone can then increase heating or cause a protection trip. A properly selected starter may reduce a supply-voltage dip, but it cannot guarantee that every weak supply or high-torque load will start.

Explore the voltage ramp

The original interactive chart is retained below. The blue trace shows a programmed motor-voltage ramp; the green trace is a schematic speed response. Actual speed cannot be calculated from the voltage percentage alone: a motor with inadequate torque may never accelerate. In this particular demo, Stop pauses the chart and Reset clears it. The voltage target slider is for illustration, not a commissioning recommendation.

Bypass operation and soft stopping

Once the motor has started, a model with a bypass contactor can carry normal running current around the SCRs. This reduces starter losses and heat; it does not necessarily switch off the unit's monitoring or protective functions. Bypass can be built in, external or absent, depending on the product. Verify the manufacturer's circuit and overload-protection arrangement before installation. ABB's PSE specifications list bypass, current limiting and electronic protection as separate features.

A suitable soft-stop function reduces voltage or controls torque during shutdown. This may ease pressure transients in a pumping system, depending on the pipework, pump and check valve. It is not the same as powered braking of a high-inertia load or holding a vertical axis. Compare the requirements with coast to stop and dynamic braking before selecting hardware.

What should you check when selecting one?

Motor: rated current, supply voltage, connection method and actual starting current. Machine: loaded or unloaded starts, initial torque demand, inertia and acceptable acceleration time. Duty: starts per hour, cabinet temperature and manufacturer thermal rating. Protection: short-circuit coordination, overload protection, bypass arrangement and the required stop behavior. A motor's kW rating alone does not answer these questions.

During commissioning, record when the shaft first turns, its actual acceleration time, motor current and any trips. Adjust initial voltage, ramp and current limit together and within the specific product manual. If the motor stays at the current limit without reaching speed, re-check the load torque and the selected starter rather than repeatedly lengthening the ramp.

Should you use a soft starter or a VFD?

If the machine runs at full line speed and only needs gentler transitions, a correctly rated soft starter can be a straightforward solution. If the process must continuously adjust speed, flow, pressure or torque, specify a VFD. A heavily loaded machine may need more low-speed torque than reduced-voltage starting can provide; a VFD still needs to be sized for duty and overload requirements. Schneider Electric's conveyor comparison gives practical examples.

Neither choice guarantees an energy saving. Bypass can reduce losses in a starter at full speed. A VFD can save substantial energy where, for example, a centrifugal fan or pump truly needs less flow and can run slower. Savings depend on the actual duty cycle and system curve. A soft starter does not keep the motor at a continuously selected reduced speed.

Frequently asked questions

Can a soft starter regulate motor speed all day?
No. It mainly manages the starting and, on compatible models, stopping transition. Choose a suitable VFD for continuous speed regulation.

Will 50% voltage produce 50% speed?
No. Motor speed does not follow that percentage rule. The reduction can leave too little starting torque for the motor to move at all.

Does bypass disable overload protection?
That depends on the starter and circuit. Check where overload protection operates in the manufacturer's bypass wiring diagram; a bypass is not an isolator.

Does a 10-second ramp mean a 10-second motor start?
Not necessarily. The selected ramp describes the control command. Load torque, inertia, current limiting and motor characteristics determine actual acceleration.

Related OtomasyonAVM guides

Starting and torque: VFD guide  ·  Acceleration/deceleration time  ·  Current limiting  ·  Torque boost  ·  Static and dynamic friction

Running the machine: S-curve ramps  ·  Speed vs torque control  ·  PID with a VFD  ·  Heavy-duty drive  ·  Choosing a VFD

Stopping: Coast stop  ·  Dynamic braking

Türkçe sürüm: Softstarter (yumuşak yolverici) nedir?