A soft starter is a device used with AC electric motors to temporarily reduce the load and torque in the powertrain and electrical current surge of the motor during startup. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network, extending the lifespan of the system.
It can consist of mechanical or electrical devices, or a combination of both. Mechanical soft starters include clutches and several types of couplings using a fluid, magnetic forces, or steel shot to transmit torque, similar to other forms of torque limiter. Electrical soft starters can be any control system that reduces the torque by temporarily reducing the voltage or current input, or a device that temporarily alters how the motor is connected in the electric circuit.
Electrical soft starters can use solid state devices to control the current flow and therefore the voltage applied to the motor. They can be connected in series with the line voltage applied to the motor, or can be connected inside the delta loop of a delta-connected motor, controlling the voltage applied to each winding. Solid state soft starters can control one or more phases of the voltage applied to the induction motor with the best results achieved by three-phase control. Typically, the voltage is controlled by reverse-parallel-connected silicon-controlled rectifiers (thyristors), but in some circumstances with three-phase control, the control elements can be a reverse-parallel-connected SCR and diode.
Another way to limit motor starting current is a series reactor. If an air core is used for the series reactor then a very efficient and reliable soft starter can be designed which is suitable for all type of 3 phase induction motor ranging from 25 kW 415 V to 30 MW 11 kV. Using an air core series reactor soft starter is very common practice for applications like pump, compressor, fan etc. Usually high starting torque applications do not use this method.
A soft starter continuously controls the three-phase motor’s voltage supply during the start-up phase. This way, the motor is adjusted to the machine’s load behavior. Mechanical operating equipment is accelerated in a gentle manner. Service life, operating behavior and work flows are positively influenced.