How Does a Soft Starter Work — Working Principles & Applications

Introduction

A soft starter is an electronic device used to control the acceleration and deceleration of electric motors. It regulates the voltage supplied to the motor during startup, gradually increasing it from zero to the desired speed. Unlike traditional direct-on-line (DOL) starters that apply full voltage to the motor instantaneously, soft starters ensure a controlled and adjustable startup process.

Working Principles of a Soft Starter

One of the primary functions of a soft starter is to gradually ramp up the voltage applied to the motor. This soft acceleration helps reduce the mechanical stress on the motor’s windings and shaft, preventing sudden jerks or shocks during the startup process. By smoothly increasing the voltage, the soft starter allows the motor to reach its full speed without excessive current draw.

Smooth Transition to Full Speed

Once the motor reaches its desired speed, the soft starter ensures a seamless transition to full voltage operation. It monitors the motor’s speed and adjusts the voltage supply accordingly to maintain a steady rotation. This feature eliminates sudden jolts or vibrations that could occur during the transition from startup to normal operation.

Types of Soft Starters

Soft starters are available in different configurations, each offering unique features and benefits. The three common types of soft starters are:

Primary Resistance Soft Starter

The primary resistance soft starter employs a series of resistors connected in line with the motor windings. During startup, these resistors limit the initial current flow, gradually reducing their resistance as the motor gains speed. While this type of soft starter provides a simple and cost-effective solution, it dissipates energy in the resistors, resulting in energy losses.

Autotransformer Soft Starter

Autotransformer soft starters use tapped windings on an autotransformer to control the voltage applied to the motor. By adjusting the tap position, the soft starter regulates the voltage level and provides a smooth startup. Autotransformer soft starters are known for their energy efficiency and compact design, making them suitable for a wide range of applications.

Solid-State Soft Starter

Solid-state soft starters utilize power electronics to control the motor’s voltage during startup. They employ semiconductor devices like thyristors or silicon-controlled rectifiers (SCRs) to adjust the voltage supply smoothly. Solid-state soft starters offer precise control over the acceleration and deceleration of the motor, making them highly versatile and efficient.

Variable Frequency Drives (VFDs)

While soft starters are effective in controlling the motor’s starting current and torque, variable frequency drives (VFDs) offer additional benefits, including speed control and energy savings.

Conclusion

Kimo Instruments, a renowned manufacturer of precision measurement instruments, has made significant contributions to the field of soft starter technology. Soft starters provide an efficient and reliable solution for controlling the starting current and torque of electric motors. By gradually ramping up the voltage and offering precise control over the acceleration process, soft starters minimize mechanical stress, enhance motor performance, and extend the lifespan of motor-driven systems.