soft stater (Static frequency converter)

A soft starter is not a frequency converter. It works by ramping up the voltage (at nominal mains frequency). An inverter varies the frequency as well as the volts but is used for variable-speed operation. Inverter is better for soft start than a soft starter but wouldn't normally be installed just for that, as a soft starter is much cheaper.

Cheers........Codey

Motor Soft starters can consist of mechanical or electrical devices, or a combination of both. Mechanical soft starters include clutches and several types of coupling using a fluid, magnetic forces, or steel shot to transmit torque, similar to other forms of tourque 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 electrical.

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 can be connected inside the delta (Δ) loop of a delta connected, 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 SCRs like bridge rectifire circuit, but in some circumstances with three-phase control, the control elements can be a reverse-parallel-connected SCR and Diodes.

Actually, the use of the term Static Frequency Converter can mean different things depending on the industry. Context is important.

SFCs can be really large power conversion systems for entire facilities that convert one frequency to another. For example an SFC might be installed on a facility designed to be semi-portable but built for 50Hz operation, then sent to a country that uses 60Hz.

An SFC is also something used in the transportation industry to provide a variable frequency power rail from a fixed frequency utility source.

In the large motor industry, an SFC can be used to describe a type of drive called a Line Commutated Inverter (LCI) that is used primarily to accelerate a motor, not necesscessarily run it at varying speeds. So in that sense, they are often referred to as "Soft Starters". That would usually imply a very very large motor, such as 10,000HP. I have also heard the term used to describe what were called "Cycloconverter Drives" that performed the same basic function, but did it in a different way.

In all those cases though, the term "static" is just refering to the fact that there are no movng parts; "static" as opposed to "rotary" converters. The soft starter part is usually a term assigned to motor controllers, so most lkely the user in your case was referring to the last description I mentioned.

There is an item on this subject in Wikipedia (other on-line encyclopaediae are available).