Electronic soft starters

Electronic soft starters are available for large asynchronous motors. Such a soft starter reduces the voltage during startup using phase-angle control and gradually increases it to the full mains voltage. However, with asynchronous motors, the motor's torque decreases quadratically at lower voltages, and the torque is also low at very low speeds. Therefore, such a soft start is only possible under no-load or light load conditions. In some soft starters, the electronics are switched off upon reaching the full voltage. Relays or contactors are bypassed to reduce power loss. Soft starters are used, for example, for large fans and industrial machinery. Starting at rated load is also possible with these devices. Soft starters often replace the previously widespread star-delta starters, which increase the motor voltage in two stages.

Soft start or frequency converter?

Another method for achieving a soft start with asynchronous motors is the use of a frequency converter, which allows the speed to be gradually increased to its full value using a programmable ramp function. Unlike voltage reduction with a soft starter, when operated with a frequency converter, the motor already has high torque at low speeds. Therefore, the speed can be increased very slowly, for example, to overcome large moments of inertia. The torque can also briefly exceed the rated operating value, enabling starting under full load. This is important, for example, when starting compressors against residual pressure. A further advantage of frequency converters is the ability to vary the motor speed during operation and, depending on requirements, to gradually reduce the speed or decelerate the motor in a controlled manner when switching off.

Soft start of the SSCi series

The energy-efficient soft starter is designed according to IEC/UL/KC standards and offers motor protection functions that ensure stable operation of the application. The SSC series soft starter features an integrated bypass and can be mounted horizontally on a DIN rail. The SSC series allows for quick fault diagnosis via two LEDs located on the front of the device. The SSCi model has built-in thermal motor protection. The motor current is continuously monitored, and the expected temperature is calculated based on the monitored current. The SSCi trips when the calculated motor temperature reaches 105%. When using an SSCi soft starter, no external motor protection is required.