How the soft starter works

GW MB10F 0.8A 1000V rectifier bridge

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Industrial Router Crystal 3.2*2.5mm 3225 26M (26.000MHZ) 12PF 10PPM 20PPM 30PPM

Photocoupler

Guide: A soft starter is a modern motor control system that integrates smooth starting, soft stopping, energy saving under light load, and multiple protection features. Let’s explore how a soft starter works and why it’s essential in industrial applications.

1. Working Principle of the Soft Starter – An Overview

A soft starter is an advanced device used to control the operation of electric motors. It combines functions such as soft start, soft stop, energy efficiency during low load, and various protective mechanisms. In many countries, it's referred to as a "Soft Starter." The core component is a set of three-phase thyristors connected in parallel between the power supply and the motor, along with an electronic control system. By adjusting the conduction angle of these thyristors, the voltage supplied to the motor can be controlled, allowing for different operational modes and performance optimization.

2. Working Principle of the Soft Starter – Startup Modes

The soft starter is designed to provide smooth motor startup and reduce mechanical and electrical stress. It typically supports several startup methods, including:

• (1) Ramp Constant Current Start
• (2) Ramp Boost Start
• (3) Step Start
• (4) Pulse Shock Start

Each method is suitable for different applications, ensuring optimal performance based on the motor’s requirements. For example, the ramp constant current start gradually increases the current, while the pulse shock start delivers a quick initial boost to overcome inertia.

3. How the Soft Starter Works

The soft starter uses three-phase thyristors connected in a full-bridge configuration between the power supply and the motor’s stator. This setup allows precise control over the voltage applied to the motor. When the motor starts, the thyristor output voltage increases gradually, allowing the motor to accelerate smoothly without sudden current spikes. This reduces the risk of tripping circuit breakers due to excessive starting current.

Once the motor reaches its rated speed, the soft starter automatically switches to a bypass contactor, which provides full voltage directly to the motor. This helps reduce heat generation in the thyristors and extends the life of the system. Additionally, this switch prevents harmonic distortion in the power grid, improving overall efficiency.

The soft starter also offers a soft stop function, which gradually reduces the voltage and slows down the motor before it comes to a complete stop. This prevents torque shocks that could damage equipment or cause mechanical stress. The voltage curves for both soft start and soft stop are shown below.

For further reading, you may find these topics helpful:

• 1. Single-chip AT89C51 motor soft starter
• 2. Principles of soft start and practical examples
• 3. Application of motor soft starters in mining engineering