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What is soft-starting? | Methods of soft-starting, Benefits and Applications of soft-starting

Soft Starting, also known as motor starting or gentle starting, is a method that gradually increases the voltage and torque applied to an electric motor during startup. This technique aims to reduce the inrush current and mechanical stress on the motor and connected equipment, resulting in a smoother and more controlled start within a soft start system.

Methods for Soft Starting

To achieve a successful soft start, various techniques and devices are employed to control the motor's starting characteristics. The following are some common methods used for soft starting:

Reduced Voltage Starters

Reduced voltage starters limit the voltage applied to the motor during startup, effectively reducing the inrush current. Several types of reduced voltage starters are commonly used:

  • Auto Transformer Starters
  • Star Delta Starters
  • Part Winding Starters

Solid State Soft Starters

Solid state soft starters employ semiconductor components such as thyristors or silicon-controlled rectifiers (SCRs) to gradually increase the voltage and torque applied to the motor. These devices offer precise control over the starting characteristics and can be adjusted to meet specific requirements.

Variable Frequency Drives (VFDs)

VFDs are sophisticated motor control devices that not only provide soft starting but also offer speed control and other advanced features. They work by converting the incoming AC power to DC and then back to AC (to some extent) at the desired frequency and voltage. This allows for precise control over motor speed and torque during startup and operation, offering multiple benefits of soft starting.

Benefits of Soft Starting

Soft starting brings several advantages for electric motors and the systems they operate within. Let's explore some of these benefits:

Reduced Mechanical Stress

Soft starting minimizes the mechanical stress on the motor and connected equipment by gradually increasing torque and minimizing sudden torque transience. This results in a longer equipment lifespan and reduced maintenance requirements.

Lower Inrush Current

One significant advantage of soft starting is the significant reduction in inrush current during motor startup. By lowering the inrush current, soft starting helps prevent voltage dips, minimizes power supply disturbances, and avoids nuisance tripping of circuit breakers.

Improved Power Quality

Soft starting plays a crucial role in improving power quality by reducing voltage dips and current spikes, thereby reducing disturbances in the electrical system. This is particularly important in facilities with sensitive equipment or where power quality issues can affect other processes that require controlled acceleration.

Controlled Acceleration

Soft starting allows for a controlled acceleration of the motor, which is highly beneficial in applications where rapid acceleration could cause product spillage, belt slipping, or other operational issues.

Energy Savings

Soft starting contributes to energy savings by reducing the mechanical and electrical stress on the motor. Additionally, some soft starters and VFDs offer energy optimization features such as automatic adjustment of motor speed and torque, further enhancing energy efficiency.

Applications of Soft Starting


Overall, soft starting provides a range of benefits by minimizing the impact of motor startup on electrical systems and connected equipment. It offers improved motor control, energy efficiency, and equipment protection, making it a valuable technique in many industrial and commercial settings. Implementing soft starting methods not only ensures a smoother and controlled startup for electric motors but also enhances their overall performance and longevity.

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