A soft starter is generally used in applications where there is a large inrush of current that could damage a motor while a VFD controls and can vary the speed of a motor.
A typical three-phase soft starter uses six Thyristors or Silicon Controlled Rectifiers, oriented in an anti-parallel configuration to start the electric motors smoothly.
When an internal pulse is applied to the gate, it allows current to flow from Anode to Cathode which then sends current out to our motor. When the internal pulses do not apply to the gate, the SCRs (Silicon Controlled Rectifier) are in the OFF state and therefore they restrict the current to the motor.
These internal pulses limit the applied voltage to the motor slowing down inrush current. The pulses are sent based on ramp time so the current will be slowly applied to the motor. The motor, attached to the soft starter, will start up at a nice smooth current and top out at the preset maximum speed. The motor will stay at that speed until we stop the motor where the soft starter will ramp down the motor in a very similar way as the ramp up.
The Rectifier acts like Diodes, takes the incoming AC voltage and changes it to DC voltage. Next, the Filter uses Capacitors to clean the DC voltage making it a smoother incoming power. Finally, the Inverter uses transistors to convert the DC voltage and sends the motor a frequency in Hertz. This frequency drives the motor to a specific RPM. We can set the ramp up and down times just like in a soft starter.
In closing, a VFD and a Soft Starter can do similar functions when it comes to ramp up or down a motor. The main difference between the two is that a VFD can vary the speed of a motor while a soft starter only controls the starting and stopping of that motor. When faced with an application, price and size are in the favor of a soft starter. A VFD is the better choice if speed control is required. The good news is that if an existing application has a soft starter in place and speed control is later determined to be a requirement, a VFD can easily replace a soft starter. The opposite is true as well, a soft starter can replace a VFD.
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