Soft Starter

A motor soft starter is a device used with AC electric motors to temporarily reduce the load and torque in the power train of the motor during start up. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamics’ stresses on the attached power cables and electrical distribution network, extending the lifespan of the system.

Motor soft starters 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 utilize 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 (Δ) 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 (thrusters), but in some circumstances with three-phase control, the control elements can be a reverse-parallel-connected SCR and diode.

Another traditional and the most reliable way to soft start the motor with reduced current is with the help of series reactor. This method is called as series reactor soft starter, which refers to the IEC standard 60289. If an air core is used for designing 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 [ synchronous / asynchronous ] 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 technology for the starting device. Very famous types of such starters are Harmonics free (due to air core) series reactor soft starter.