Can a VFD be replaced by soft starter?

A soft starter is NOT a replacement for a VFD (variable frequency drive) if you have to start a high inertia load. A soft starter works by reducing the RMS value of the voltage supplied to the motor by delaying the firing of the output SCR's (or IGBT's etc.) until later in the half cycle after the zero crossing. To all intents and purposes, it is an oversized lamp dimmer that will reduce the motor starting torque (saves the mechanical drive) and reduces the starter current (avoids angering the utility). However, if the ramp time is set too long, the motor will overheat because it is still trying to accelerate to the synchronous speed set by the line frequency.

With a VFD, the motor need only accelerate to the synchronous speed of the VFD output, while being able to produce full rated starting torque; so it the output frequency is slowly increased, the motor will come up to speed gently and without damage to mechanical or electrical systems.

With high inertia loads, a motor fed by a soft starter will usually sit with a locked rotor until the drive output voltage ramps high enough to provide the current required to produce the torque to get it turning. Overload tripping & motor burnout are common. With a VFD, the motor can be brought up to speed over several MINUTES without harming it.