Here is a good place to start.

http://en.wikipedia.org/wiki/Motor_soft_starter

3. Is there other starter that i can use for the replacement of softstarter?

Yes, a variable speed drive (also called variable frequency drive).

http://en.wikipedia.org/wiki/Variable_speed_drive

http://en.wikipedia.org/wiki/Variable-frequency_drive

in answer to 1. Yes a VSD, as a replacement for a DOL, however there are "softstart units that limit the in-rush current. Why a VSD? in many cases if the supply, usually generators cannot handle the start current of larger motors. 2. less strain on the supply system, on the motors & equipment attached to it, couplings, shafts, mountings, lowers the start-up vibration and prolongs the life of the equipment, especially the motor windings.

Do you mean a start cap? see link: http://en.wikipedia.org/wiki/Motor_capacitor

Soft start is good to commence turning of not loaded motors that have a clutch to connect to load at higher rpm. Either wise you would rather want to use a frequency manipulation that raises frequency electricity from 5 Hz up to 400 Hz, thus starting up current never exceeds max allowed current. It is expensive, but economic as its installation is less expensive and also saves electricity.

I have installed many of these as I also invented, patented and produced these about 30 years ago

Independent inventor

I can't totally agree with you there. The purpose of a soft start is to limit the in-rush current, which will not only be as per motor manu's spec's but also be relative to the load. That is, if a motor is connected to a load, be it pump or any type of machine requiring an electric motor as the prime driving source, then that motor will draw current as required to get itself and whatever is attached to it up to its designed RPM (relative to the supply Hz). A VFD limits the in-rush current by ramping up the Hz slowly, the motor RPM increase accordingly and therefore the current. Remember most motor will not start until you reach 40Hz and should not exceed 70Hz unless designed for higher frequencies/RPM. A reactive type soft start, a wound reactor with a method for removing the higher taps out of the circuit only limits so much in-rush current. Gearboxes/clutches only allow the motor to start without load, then DUMP the load onto the motor, doing as much harm as starting without a soft start and the motor connected directly to the load. Furthermore VFD's also limit the harmonic distortion associated with DOL starters. I am sure that you have had success with your invention and I have no wish to doubt your theories, but it is good to debate.

When you say that most motors will not start under 40 Hz. maybe you reffer specifically to those motors requiring a built in start-up system, like a start winding with a centrifugal switch or a start capacitor; but all three phase motors are able to start at frequencies even below the 5 Hz. if you have the VFD for them.

Yahlasit