Electronic soft starters
Electronic soft starters are available for large asynchronous motors. Such a soft starter reduces the voltage when switching on by means of phase control and slowly increases it up to the full mains voltage. However, the torque of asynchronous motors decreases quadratically at lower voltages, and the torque is also low at very low speeds. For this reason, such a soft start is only possible when there is no load or when the load is low. When the full voltage is reached, the electronics of some soft starters are bypassed by means of a relay or contactor in order to reduce the power loss. Soft starters are used, for example, for large fans and machines in industry. Starting with a 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 for electric motors
Soft start or frequency inverter?
Another method of achieving a soft start for asynchronous motors is to use a frequency inverter, with which the speed can be slowly increased to the full value using 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. to cope with large moments of inertia. The torque may briefly have higher values than during nominal operation so that starting under full load is also possible. This is important when starting compressors, for example, if they have to start up against pressure that is still present.
Another advantage of frequency inverters is the ability to vary the speed of the motor during operation and also to slowly reduce the speed or decelerate the motor in a controlled manner when switching off, depending on requirements.
Soft start of the SSCi series
The economical soft starter is designed in accordance with IEC/UL/KC standards and offers 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 through 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 triggers when the calculated motor temperature reaches 105%. No external motor protection is required when using an SSCi soft starter.