Soft-Starter is a Solid State device used for starting of motor . Soft- Starters temporarily reduce the load and torque on motor during starting of motor. This will leads to reduced mechanical stress on the motor shaft as well as electrical stress on Power cables and whole electrical systems. In Typical Star-Delta Starter Starting current is about 3-6 times rated current of motor but using Soft-Starters this staring current can be reduced to 2-4 times motor rated current. This is ultimately reduces stress on electrical system which will leads to smooth running of electrical systems. In that Soft-Starter reduced voltage is applied during starting and that reduced voltage will reduce Starting torque. if 50% voltage is applied to a motor, it will produce 25% of it's available torque at that point.
Starting time also feed into soft-Starter parameters , when motor get accelerated Soft-Starter get bypassed by soft-starter through bypass contactor provided inside Soft-Starter. Bypass contactor rating is according to Soft-Starter rated current.
Electrical soft starters can reduces the torque by temporarily reducing the voltage and current I/P to connected motor.
There are two circuits by which soft-starter is connected to motor:-
1. In-line Circuit known as Standard Circuit
2. Inside Delta Circuit.
1. Inline Circuit or Standard Circuit
Circuit by which Soft-Starter is connected to motor is shown below:-
From above circuit it has been clear that Soft-Starter is connected in series with the line voltage applied to the motor. This circuit is known as Soft-Starter within inline circuit.
2. Inside Delta Circuit
There is another circuit by which Soft-Starter is connected is known as Inside delta circuit in which is shown below:-
In that circuit two cables which are going to motor one of them will be connected directly to I/P supply and other cable will be connected through soft-Starter.
One Specialty of that circuit is that Small Soft-Starter can be used for bigger motors. e.g. you have 60KW soft-Starter then that Soft-Starter can be used for motors having rating 90X1.732=103.92 i.e. can be used for 100 KW motors. As phase currents get divided in 2 circuits. But One should remember that in that 60KW soft-Starter 100 KW motor setting parameters should be feed.