Soft starters are widely used in industry to protect motors and motor-driven equipment. However, it is important to size soft starters correctly for each application, otherwise the benefits can be lost, says Jaco Brits at WEG Africa.
As just one example, soft starters are commonly used in pump applications to reduce mechanical stress and ensure efficient motor operation.
According to Brits, Projects and Technical Manager for Drives and Controls at WEG Africa, a soft starter protects the motor and the load against mechanical shocks or jolts on startup. It does this, he explains, by controlling the voltage that is applied to the motor during startup, 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,” he says. “If you under-size the soft starter, it may not reduce the starting current adequately or be able to accelerate the load – resulting in damage to the motor, electrical components or power supply infrastructure.”
Over-sizing a soft starter brings its own problems. Apart from not being cost efficient, it may provide less precise control over motor acceleration and deceleration.
“Accurate sizing ensures that customers are not overspending on a unit larger than necessary. The correct size will deliver a balance between initial investment and long-term operational efficiency,” Brits says.
He highlights that the most important factors to consider are the specifications of the motor that the soft starter will be controlling 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 higher 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.
He points out, too, that 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 able to provide the necessary torque control during motor startup to achieve smooth acceleration and prevent mechanical stress on the motor and connected equipment.
“Loads vary widely between applications,” Brits 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 specific needs for overload protection. And other types of motor protection may also be considered,” Brits notes, “such as phase imbalance detection, undercurrent protection and over-temperature protection.”
The soft starter also needs to be compatible with the control systems in place – including the programmable logic controllers (PLCs) or supervisory control and data acquisition (SCADA) systems.
“To support customers in understanding more about soft starter technology and sizing, WEG Africa offers a range of resources and training opportunities,” says Brits. “Additionally, our technical team is on hand to examine customers’ applications and answer questions about size selection, installation and troubleshooting.”
For more information visit: www.weg.net
