what is the future of soft starter in motor control field, if we can use variable frequency drive instead of soft starter?

Alive & Well, especially if you do not need to vary the Speed, but want to save Money .

The question could be; What is the Future of Direct-Line starters? Soft Start technology has proved to be an adequate sustainable alternative to Direct-Line start. Especially the newer S/S with HP rated contractors that allow for emergency bypass. For many applications VFDs create more challenges than solutions. (Ie: shaft currents, harmonics, reflective wave, etc) The Soft Starter will likely serve the industry for many years to come.

Yes, there will be enough space for soft starter to be alive in future according to the exact need of motor control. Because so many applications do not need to vary the speed but only to soft start and soft stop the motor. And there will be no harmonics conflict to the grid with soft starter.

Soft starter has come to stay in the industry. The advantages are many. In many applications, I have changed my customers' controls to soft starters and we now have less problems. Even in medium voltage applications. The new technology now is DRIVE START. This is noarrowing the space for drives in high torque starting applications.

Mr. SEBASTINE ANEKE is right. You make soft starter clear in front of us. Even ac drive has striking priority for energy saving in fan and pump application. Soft starter still have wide application in current industry condition.

We know Soft Starters have a very important part to play in Motor Control. At the moment they are a better solution for a motor that doesn't require or can be benefited by a VFD as the technology is cheaper. At full speed they are usually more economical than a VFD would be to run. A smart Soft Starter has many good features built into them and we see them being around for a long time to come. The important question is when or when not to use them..

Soft starters are a good way to save energy as compared to DOL as they temporarily reduce electric current surge of the motor during start up because of this it also extends the lifespan of the system..

We have found soft start technology is very important in rural applications where the distribution system is stretched. My company's Written-Pole single-phase motor controls use a a soft start to reduce start current to 3-4 times running current in our high horse-power (up to a hundred HP) single-phase motors. We have even had an application where a 50HP motor of ours was used where they had three-phase power, simply due to the low starting current and power factor.

Hello... many years ago when I first got involved in the Industry of control systems I used to build motor control systems. The core of the business was an in-house, cheap thyristor controlled soft starter and we supplied Gozuk VF AC drives and the usual contactor starters.

Now as a young man I always thought that the days of the soft starter where numbered my naïve reasoning was that as soon as AC drives competed price wise they would be gone and as people got more in tune with the technology of AC drives they would go for the drives. But here I am twenty years later and they are still out there.
Just reading some of the comments I see that some of the old arguments still stand. Although I don’t come across them as often as I used to, mainly because I am more involved with automation software and integration of systems.
I have noticed that the technology appears to have hardly budged from where is used to be all those years ago. Maybe it’s just like the old needle and thread ….you just can’t beat a good idea.

I think we will see more of a move back towards soft starters. I have seen in recent years a move towards VSD's for every application - even ones that do not require speed control. Engineers have been I presume, specifying VSD's "just in case". Not enough consideration has been given to the adverse effects of excessive cable length, harmonics and general serviceability.
As these issues become more and more recognised, I think there will be a realisation that engineering design cannot continue to use VSD's as a complete solution.