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A VSD is a variable speed drive. A VSD can run at different speeds getting a manual or digital input reference. Its starting function is similar to that of a Soft Starter, it starts up with a time/speed ramp according to the load of the motor and inertia factors. Essentially a soft starter was designed to take over the harsh method of star delta staring, providing a smooth acceleration of motor speed with a voltage or/and frequency increment value. The stopping function is similar but only in an decremented value.The torque of a soft starter is also controlled by a power factor function. A Soft starter normally runs at its rated speed and is not variable like a VSD.
Any AC induction motor when started Direct on Line (DOL) is in effect a short circuited transformer. This is ok for low power motors with low inertia loads but does cause a very high starting current and it isn't good to continuously start a motor like this. Most are limited to 5 or 6 full loads starts per hour using this method.
Soft starting is just that. It soft starts the motor and so enables high power/high inertia loads to be started. It does this by limiting the torque and ramping the motor up to synchronous speed (i.e .50/60Hz)
An Inverter is more complex as it facilities variable speed and normally incorporates special functions such in built PID loops.
The power electronics in them is the same but the front end controls and software are different. That is why soft starters are cheaper than inverters.
Hope this helps.
Soft Starters start motors by limiting the large initial inrush current associated with motor start up and provide a gentle ramp up to full speed. A Soft Starter is only used for starting and not for stopping.
Frequency Converter Motor Starting (Variable Frequency Drives) controls a motor's starting by changing the frequency of the power. It also uses less inrush current. It can also control the speed of a motor by adjusting the frequency.
The IGBTs act as switches to provide a series of DC pulses to the motor. Since most AC motor frequency controls are for 3-phase motors, there are six IGBTs, two for each phase. One IGBT connects each motor terminal to the positive side of the DC supply and one connects each motor terminal to the negative side of the DC supply. In that way, each terminal to terminal or line to line voltage can be either positive or negative. By controlling the switching sequence of the IGBTs, the control provides a simulated 3-phase sine voltage with frequency and voltage control. The waveform is composed of DC pulses and doesn't look too much like a sine wave, but the effective value is a reasonably good simulation of a sine wave.
Each IGBT has a diode in parallel but positioned to conduct current in the opposite direction. The "anti-parallel" diodes conduct the portion of the motor current waveform that lags the voltage.
