Electronic soft starters
There are electronic soft starters for large asynchronous motors. Such a soft starter reduces the voltage when switched on using phase control and slowly increases it to full mains voltage. However, with asynchronous motors, the torque of the motor decreases squarely at lower voltages, and the torque is also low at very low speeds. Therefore, such a soft start is only possible in no-load condition or at low load. When full voltage is reached, in some soft starters the electronics are bridged using a relay or contactor to reduce power loss. Soft starters are used, for example, for large fans and machines in industry. Starting with nominal load is also possible here. Soft starters often replace the previously widely used star-delta switches, in which the motor voltage is increased in two stages.
Starting methods of electric motors
Soft starter or frequency inverter?
Another method of achieving soft starting in asynchronous motors is to use a frequency inverter, which can slowly increase the speed to full value with a programmable ramp function. In contrast to reducing the voltage with a soft starter, the motor has a high torque even at low speeds when operated with a frequency inverter. The speed can therefore also be increased very slowly, e.g. B. to cope with large moments of inertia. The torque can briefly have higher values than in nominal operation, so that starting under full load is also possible. This is important when starting compressors when they have to start against the pressure that is still present. Another advantage of frequency inverters is the ability to vary the speed of the motor during operation and, depending on requirements, to slowly reduce the speed or brake the motor in a controlled manner when switching off.
Soft starter of the SSCi series
The economical soft starter is designed in accordance with IEC/UL/KC standards and provides motor protection functions that ensure stable operation of the application. The SSC series soft starter has an integrated bypass and can be mounted horizontally on a DIN rail. The SSC series allows for quick fault diagnosis using two LEDs located on the front of the device. The SSCi model has built-in thermal motor protection. The motor current is constantly monitored and the expected temperature is calculated based on the monitored current. The SSCi trips when the calculated engine temperature reaches 105%. When using an SSCi soft starter, no external motor protection is required.