Soft starters are used in mining and industrial applications to help protect motors and motor-driven equipment, however, the correct size selection of the units is key to gaining the benefits of improved uptime and greater lifespan of equipment.

WEG Africa projects and technical manager: drives and controls Jaco Brits explains that a soft starter protects the motor and the load against mechanical shocks or jolts by controlling the voltage that is applied to the motor during startup, thereby reducing the starting current and allowing the gradual acceleration of the load up to the rated speed.

“Properly sizing the soft starter ensures that it can handle the specific motor and load to which it is connected,” says Brits. “If you under-size the softstarter, it may not adequately reduce the starting current or be able to accelerate the load – resulting in damage to the motor, electrical components or power supply infrastructure.”

Over-sizing a soft starter can also have negative effects as it may exercise less precise control over motor acceleration and deceleration and it is also not cost-efficient.

“Accurate sizing ensures that you are not overspending on a unit larger than necessary,” he points out. “The correct size will deliver a balance between initial investment and long term operational efficiency.”

Brits highlights that the most important factors to consider are the specifications of the motor to be controlled by the soft starter as well as the characteristics of the driven load.  The motor’s power rating directly affects the starting current required during motor startup, with higher power motors requiring larger starting currents.

“Motors with different power ratings operate at varying current levels, so the soft starter must be compatible with the motor to ensure proper operation,” he says.

The power rating often correlates with torque requirements, especially in applications with high torque demands such as conveyors or crushers. In these applications, the soft starter must be capable of providing the necessary torque control during motor startup to achieve smooth acceleration and prevent mechanical stress on the motor and connected equipment.

“Loads will vary widely between applications,” he explains. “Crushers and fans, for instance, may demand enhanced torque control capabilities, while other applications have relatively constant torque demands – like pumps or blowers – which may require soft starters optimised for efficient energy usage rather than enhanced torque control.”

Some soft starters offer adjustable torque profiles or advanced algorithms to optimise torque delivery based on the load type. Another aspect that varies from one application to the next is the acceleration and deceleration profiles.

“The user might also have some specific needs for overload protection,” says Brits. “Other types of motor protection may also be considered, such as phase imbalance detection, undercurrent protection and over-temperature protection.”

The soft starter must also be compatible with the control systems in place, such as programmable logic controllers or supervisory control and data acquisition systems.

“To support customers in understanding more about soft starter technology and sizing, WEG Africa offers a range of resources and training opportunities,” notes Brits. “Our technical team is on hand to examine customers’ applications and answer questions about size selection, installation and troubleshooting.”