The soft start cabinet is designed to minimize the voltage drop that occurs during a hard start (also known as direct-on-line starting) of an electric motor. This helps prevent disturbances in the shared electrical network and avoids affecting other connected devices. It also reduces the inrush current, which can cause excessive temperature rise in the motor and shorten its lifespan. Additionally, it minimizes mechanical stress caused by sudden starts, which can accelerate wear on components like shafts and gears. The inrush current may also generate electromagnetic interference, disrupting the operation of sensitive electrical instruments.
With a soft start cabinet, the motor can start and stop more smoothly, reducing idle time and increasing operational efficiency, which ultimately leads to energy savings. Soft starting methods are generally categorized into two types: stepped and continuous. Stepped soft starters have discrete adjustment levels, while continuous ones allow for smooth and precise control. Traditional soft starters often use stepped methods, such as star-delta switching, autotransformer, or reactor-based systems.
On the other hand, continuous regulation soft starters include liquid resistance types with electrolyte limits, thyristor-based (SCR) systems, and magnetic saturation reactors. These provide smoother control and better performance. Another type of soft start solution is the variable frequency drive (VFD), which is considered an ideal option due to its ability to maintain high starting torque while limiting current. However, its higher cost is a major barrier to widespread adoption. Many users purchase VFDs primarily for speed control, so they are often not classified strictly as soft starters.
In the motor’s stator circuit, a soft start can be achieved using a power device that limits the initial current, commonly referred to as a buck or current-limited soft start. This method is a key component in most soft starter cabinets. Another important category is the high-voltage soft starter cabinet, which is used in applications requiring higher voltage levels. In our next article, we will explore the details of high-voltage soft starter cabinets and their advantages in industrial settings.
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