What are Softstarters?

When induction motors are started a very heavy current is drawn.  This current, which can be 5 to 6 times the rated current of the machine, occurs due to lack of back emf in the windings.  These currents  cause sudden loading on power sources such as generators or transformers.  Besides, such heavy currents also cause voltage dips and a momentary drop in frequency in generators which can trip other equipment connected to the power system.

Softstarters are equipment which limit the initial start current of the motors by controlling the voltage.  This is done by means of thyristors.  This "soft" starting prevents the occurence of the high inrush currents.  The downside of using softstarters is that the reduced voltage applied on the stator windings can result in increased slip causing heating of the rotor. 

Another advantage of the softstarter is that it can be used to gradually increase the torque.  In loads such as conveyor belts involving chains and other transmission gear, this avoids jerks which over time can cause wear on the machinery.  The voltage and the torque are gradually increased to the rated value.

While stopping a motor, the softstarter usually provides the "softstop" function which involves doing the starting process in reverse.  The voltage is gradually reduced to zero this gradually stops the system avoiding sudden jerky motions due to system inertia.