Soft Starters

When should a soft starter be used? Advanced features of soft starters

Currently, soft starters are quite commonly used to help motors operate smoothly right from startup. By using this device, motor damage is minimized, and it protects the electrical system in factories, workshops, or industrial areas from voltage drops caused by sudden current surges. Thanks to its function of allowing the current to ramp up gradually to the rated level, a soft starter helps the entire system operate more safely, especially in places where many high‑power devices run simultaneously.

So what exactly is a soft starter? What components does it include? How does it work and where is it applied in practice? All of this will be explained in detail below.

What is a soft starter?

 When a motor starts, whether small or large, the input current often jumps 5 to 9 times higher than the current during stable operation. Such a sudden increase can overload the local electrical system, causing voltage drops, sudden power outages, and putting high stress on the motor, which affects its durability and lifespan.

A soft starter, also called a “soft starter”, was created to handle this situation. This device supports a smoother motor start by adjusting the current to rise gradually up to the rated value. That way, the electrical system in a factory or workshop operates more steadily, reducing risk and preventing equipment damage.

Some obvious benefits of a soft starter include the ability to adjust torque according to each operational stage, reduce pressure on both the electrical system and the motor, which contributes to maintenance cost savings. Additionally, soft starters allow motors to stop smoothly, preventing the water hammer phenomenon often occurring in pumps when stopping abruptly—something traditional start methods like star‑delta or direct on‑line cannot handle.

When should a soft starter be used?

A soft starter is suitable for use in certain situations with a motor. First and foremost, the device can only be used with one motor per installation. It is particularly useful when assistance is needed during motor startup or shutdown, as well as when adjusting speed during startup. However, a soft starter cannot reverse the motor’s direction.

This device also helps protect the motor from unexpected incidents such as overcurrent, overvoltage, phase failure, overload, or short circuit. In environments where grid voltage drops must be limited, using a soft starter helps stabilize the electrical system. Compared to investing in a frequency converter, a soft starter is a more cost‑effective solution initially, while also extending motor life and reducing ongoing maintenance expenses.

Key characteristics of soft starters

A soft starter can control the motor’s stopping process gently. With this device, the voltage supplied to the motor is reduced gradually over a period of 1 to 20 seconds, instead of being cut off suddenly. In the case of sudden power loss, the motor can still coast to a stop using its rotational inertia, helping protect both the equipment and the system.

Furthermore, soft starters help save energy when the motor runs under no load or light load. By reducing the motor voltage down to the U0 level, the device lowers power consumption and reduces current through the motor, thereby contributing to lower operating costs.

Advanced features

Soft starters are not only for smooth starts—they also come with specialized features that optimize performance under various conditions. One critical feature is overload protection. The device can detect overcurrent and activate a rotor lock function to stop the motor from running further, minimizing damage risk.

In addition, a soft starter can precisely control torque thanks to voltage control matching the actual load. This prevents sudden current changes that could destabilize the power system.

The device also works well in harsh environments with high humidity, chemicals, exhaust gases, or waste that can clog the motor. In heavy load situations or environments with mud and debris, the soft starter’s Kick Start feature helps the motor start more effectively.

Operating principle

The soft starter’s operation is based on controlling the incoming voltage to the motor during startup and shutdown. Specifically, the device adjusts the RMS value of the voltage. Since motor torque is proportional to the square of the voltage, adjusting the voltage allows control over torque, which in turn regulates motor speed and current during startup.

Inside the device, each soft starter includes three pairs of thyristors (SCRs) connected in parallel and reverse. When the device is in standby or off mode, these thyristors completely block current flow. When it starts, the thyristors gradually open by controlling the firing angle of the semiconductor valves. This allows current to flow gradually rather than all at once, supplying the motor smoothly and preventing voltage drop on the grid.

The firing angle of these semiconductor valves determines the voltage applied to the motor at each moment. When the valves fully open, the voltage reaches the rated level, which occurs when the motor reaches its maximum speed and operates steadily.

Once the motor reaches its designated steady speed, the soft starter’s contactor bypass function automatically closes. At this point, the entire current flows through the contactor without passing through the thyristors, reducing energy loss and limiting unnecessary heat generation in the device.

Applications of soft starters

 Today soft starters are used widely in many fields thanks to their ability to support effective and stable motor operation. They are commonly used with systems that have large fans or machinery with high inertia. Examples include air handling fans, compressors, conveyor belts, elevator systems, industrial mixers, and textile production lines.

In fluid handling and wastewater processing, soft starters are a common choice for pump systems. From water supply pumps, wastewater treatment pumps, sludge and slurry pumps to food‑grade pumps, the device ensures smooth motor operation and avoids surge damage during startup. It is also used in feed extruders, waste agitators, and other equipment in harsh working conditions.

Another notable application is in motors requiring rapid speed changes or long periods of idle operation. These include motors used in material handling, those attached to gearboxes and conveyors, crushers, presses, and many other mechanical systems in industrial production.

We hope this overview of soft starters has provided helpful insights—from how they work, their features, to real-world applications. If you're looking for a suitable solution for your production system or would like more detailed advice, don’t hesitate to contact EMIN. Our team is always ready to support you through the EMIN Vietnam fanpage or via our nationwide distribution network.